an evaluation of tillage radish (raphanus sativus l.) to

Graduate Theses and Dissertations Iowa State University Capstones, Theses and Dissertations

2021

An evaluation of tillage radish (Raphanus sativus L.) to alleviate An evaluation of tillage radish (Raphanus sativus L.) to alleviate

post-construction soil compaction and germination potential post-construction soil compaction and germination potential

under varying environmental conditions under varying environmental conditions

Marcus David Jansen Iowa State University

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An evaluation of tillage radish (Raphanus sativus L.) to

alleviate post-construction soil compaction and germination potential under varying

environmental conditions

by

Marcus David Jansen

A thesis submitted to the graduate faculty

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

Major: Horticulture

Program of Study Committee:

Grant L. Thompson, Major Professor

Ajay Nair

Marshall McDaniel

The student author, whose presentation of the scholarship herein was approved by the program

of study committee, is solely responsible for the content of this thesis. The Graduate College will

ensure this thesis is globally accessible and will not permit alterations after a degree is conferred.

Iowa State University

Ames, Iowa

2021

Copyright © Marcus David Jansen, 2021. All rights reserved.

ii

DEDICATION

This thesis is dedicated to my late grandfather, John “Dave” Duncan. You instilled in me

a love for the land and shared a passion for trees. Like the trees, your roots were strong, your life

was abundant, and your legacy will continue to grow.

iii

TABLE OF CONTENTS

Page

LIST OF FIGURES .........................................................................................................................v

LIST OF TABLES ......................................................................................................................... vi

ACKNOWLEDGMENTS ............................................................................................................ vii

ABSTRACT ................................................................................................................................. viii

CHAPTER 1. GENERAL INTRODUCTION ................................................................................1 Introduction................................................................................................................................ 1 Effects of Compaction on Soil ................................................................................................... 2 Effects of Compaction on Plant Development .......................................................................... 5 Causes of Compaction ............................................................................................................... 8 Conclusion ............................................................................................................................... 11 References................................................................................................................................ 11

CHAPTER 2. EVALUATION OF TILLAGE RADISH COVER CROP TO ALLEVIATE SOIL

COMPACTION AND SUBSEQUENT ESTABLISHMENT OF A MODEL ORNAMENTAL

PERENNIAL .................................................................................................................................17 Abstract .................................................................................................................................... 17 Introduction.............................................................................................................................. 18 Methods and Materials ............................................................................................................ 20

Phase 1 – Radish establishment.......................................................................................... 21 Phase 2 – Radish termination ............................................................................................. 22 Phase 3 – Coral bells establishment ................................................................................... 23

Results...................................................................................................................................... 24 Radish growth and harvest ................................................................................................. 24 Coral bells growth and harvest following radish termination ............................................ 25

Discussion ................................................................................................................................ 30 Radish growth and harvest ................................................................................................. 30 Coral bells growth and harvest ........................................................................................... 31 Radish termination effects on coral bells ........................................................................... 32 Radish allelopathy nutrient unavailability .......................................................................... 33 Limitations and future research .......................................................................................... 34

Conclusion ............................................................................................................................... 36 Acknowledgements.................................................................................................................. 37 References................................................................................................................................ 37 Appendix A. Tillage Radish Aboveground Growth at Harvest ............................................... 41 Appendix B. Tillage Radish Belowground Growth at Harvest ............................................... 41 Appendix C. Coral Bells Aboveground Growth at Harvest .................................................... 42

iv

CHAPTER 3. EVALUATION OF GERMINATION PERCENTAGE OF FIVE TILLAGE

RADISH CULTIVARS AT VARYING AIR TEMPERTURES AND WATER AMOUNTS ....43 Abstract .................................................................................................................................... 43 Introduction.............................................................................................................................. 44 Methods & Materials ............................................................................................................... 46

Treatments .......................................................................................................................... 47 Blotter Paper Germination Trial ......................................................................................... 47 Media Germination Trial .................................................................................................... 48

Results...................................................................................................................................... 50 Discussion ................................................................................................................................ 58

Air Temperature ................................................................................................................. 58 Water .................................................................................................................................. 59 Cultivar ............................................................................................................................... 60 Blotter Paper vs. Media Substrate ...................................................................................... 61 Application ......................................................................................................................... 62 Limitations and Future Research ........................................................................................ 63

Conclusion ............................................................................................................................... 64 Acknowledgements.................................................................................................................. 65 References................................................................................................................................ 65

CHAPTER 4. GENERAL CONCLUSIONS .................................................................................69 General Discussion .................................................................................................................. 69 Recommendations for Future Research ................................................................................... 72 References................................................................................................................................ 74

v

LIST OF FIGURES

Page

Chapter 1

Figure 1. Major interactions between soil properties and plant root function at the root-soil

interface in a compacted soil. ..................................................................................... 8

Chapter 2

Figure 1. (A) Mean calculated ellipsoidal volume (cm3) of radishes (Raphanus sativus L.

‘Nitro’) at each compaction level across the 8 weeks following seeding in

containers in the greenhouse. ................................................................................... 27

Figure 2. Mean calculated ellipsoidal volume (cm3) of coral bells (Heuchera micrantha var.

diversifolia ‘Palace Purple’) at each compaction level across the 8 weeks

following plug transplanting in containers in the greenhouse for the (A) cold

termination radish (CTR) treatment, (C) herbicide termination radish (HTR)

treatment, and (E) no radish control (NRC) treatment. ............................................ 29

Figure 3. Aboveground growth of tillage radishes (Raphanus sativus L. ‘Nitro’) in the

Harvested Radish (HR) treatment at week (56 days) arranged with increasing

soil compaction level bulk density from left to right: (CL 1) 0.96 g*cm-3, (CL

2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4) 1.44 g*cm-3, and (CL 5)1.6 g*cm-3. .... 41

Figure 4. Belowground growth of tillage radishes (Raphanus sativus L. ‘Nitro’) in the

Harvested Radish (HR) treatment at week (56 days) arranged with increasing

soil compaction level bulk density from left to right: (CL 1) 0.96 g*cm-3, (CL

2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4) 1.44 g*cm-3, and (CL 5)1.6 g*cm-3. .... 41

Figure 5. Aboveground growth of coral bells (Heuchera micrantha var. diversifolia ‘Palace

Purple’) in the (A) cold termination radish (CTR) treatment, (B) herbicide

termination radish (HTR) treatment, and (C) no radish control (NRC) treatment

at week (56 days) arranged with increasing soil compaction level bulk density

from left to right: (CL 1) 0.96 g*cm-3, (CL 2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3,

(CL4) 1.44 g*cm-3, and (CL 5)1.6 g*cm-3................................................................ 42

Chapter 3

Figure 1. Mean percent germination of radish cultivars across tested air temperatures (C )

throughout the blotter paper trial in the growth chambers. ...................................... 55

Figure 2. Mean percent germination of radish cultivars across tested water amounts (mL )

throughout the blotter paper trial in the growth chambers. ...................................... 57

Figure 3. Regression model of percent germination between radish cultivars in the blotter

paper germination trial and the jiffy tray media germination trial. .......................... 58

vi

LIST OF TABLES

Page

Chapter 2

Table 1. Summary of planting treatment groups showing abbreviations and actions during

the phases 1, 2, and 3 in a greenhouse. ..................................................................... 24

Table 2. Analysis of variance table for radish germination, aboveground radish growth,

radish leaf area, radish aboveground biomass, and radish belowground biomass

in radish growth phase (phase 1) in a greenhouse. ................................................... 26

Table 3. Analysis of variance table for coral bells aboveground growth, volume at harvest,

leaf area, and aboveground biomass between growth planting treatment groups

in the coral bells growth phase (phase 3) in a greenhouse. ...................................... 28

Chapter 3

Table 1. Mean air temperatures (C and F) and rainfall (mL per month and per day) for the

months of the growing season (April – November) in Polk County, Iowa form

1990-2019. ................................................................................................................ 49

Table 2. Analysis of variance table for radish germination within and between treatments

throughout the blotter paper trial in the growth chambers and between

treatments within specific cultivars. ......................................................................... 51

Table 3. Tukey test comparison between cultivars within the air temperature treatments in

the blotter paper trial in the growth chambers. ......................................................... 53

Table 4. Tukey test comparison between cultivars with the water amount treatments in the

blotter paper trial in the growth chambers. ............................................................... 55

vii

ACKNOWLEDGMENTS

I express my gratitude towards my advisor and committee chair, Dr. Grant Thompson for

his mentorship, collaboration, and investment in me as his first graduate student. I also extend

my appreciation towards my committee members, Dr. Ajay Nair and Dr. Marshall McDaniel for

their contributions to this thesis along with their guidance throughout the course of this research.

Furthermore, I recognize Dr. Diana Cochran for her support through my start as a graduate

student.

Additionally, I thank my fellow researchers in the Sustainable Landscapes and

Management Lab, Abigail Enos, Cody McKune, Connor Evers, Elizabeth Hurley-Blewett, Emily

Meader, and Justin Wigdahl for their assistance with the execution of my studies and data

collection. I acknowledge Pete Lawlor for his guidance and expertise with the work that I

conducted in the greenhouse. I also share appreciation for the community of faculty and staff in

the Department of Horticulture and the Horticulture Research Station for their continued support

throughout my time as a student at Iowa State University. Lastly, I profess my gratefulness

towards my family, friends, and fellow graduate students for their encouragement, fellowship,

and memories throughout this experience.

viii

ABSTRACT

Tillage radish (Raphanus sativus L.) has been utilized as a cover crop for alleviating soil

compaction and scavenging nutrients in agricultural crop productions systems. Recognizing the

improvements of compacted soils from a tillage radish cover crop in agricultural cropping

systems and the need for compaction remediation in post-construction urban soils where

landscape plants will be established, we sought to test the potential of tillage radish for post-

construction applications. Unlike the more predictable seasonal periods of sewing, terminating,

and incorporating a tillage radish cover crop in an annual agricultural crop rotation, the timing

and environmental conditions of building and landscape construction is more variable, which

may have an effect on cover crop radish seed germination.

In the first study, a simulated landscape system in a controlled environment was used to

evaluate the performance of tillage radish (Raphanus sativus L. ‘Nitro’) at five soil compaction

levels and the impacts of that cover crop on the growth of the following establishment of a model

herbaceous perennial – coral bells (Heuchera micrantha var. diversifolia (Rydb.) Rosend.,

Buttters & Lakela ‘Purple palace’). Radishes produced comparable amounts of mean

aboveground and belowground dry biomass through a range of compacted soils but showed

reduced mean biomass at the highest soil compaction level. Coral bells following the no radish

control treatment produced more aboveground dry mean biomass than coral bells following the

cold termination radish treatment, which is speculated to be a result of allelopathic effects and/or

nutrient unavailability. The findings of this study provide evidence that tillage radishes have the

capacity to grow successfully at a range of soil bulk densities, but more work is needed better

understand the management considerations for adapting agricultural cover crops, such as tillage

radish, for use in ornamental landscapes and in post-construction soils.

ix

The aim of the second study was to test the effects of temperature and soil water

availability on germination of five tillage radish cultivars. We used a growth chamber to evaluate

germination rates of these cultivars at eight air temperatures (8 – 38 ºC) crossed factorially with

four soil water contents (25 – 55ml). We found that air temperature significantly affected the

germination percentage across all cultivars, and the optimal range for germination was 23-33 ℃.

Water rates significantly affected the germination of Nitro, Smart, and Badger radish cultivars,

suggesting that irrigation management during the germination of these cultivars may be crucial

to their success. In our study conditions, the Nitro radish cultivar has the highest overall mean

germination percentage and proved to be successful across a range of air temperatures and water

treatment combinations. The findings of this study have potential value in providing estimated

germination rates of these cultivars across a range of environmental conditions, including sub-

optimal air temperatures and water levels.

1

CHAPTER 1. GENERAL INTRODUCTION

Introduction

Soil compaction continues to be one of the leading threats to soil health (McGarry, 2003)

and plant performance, impacting the many industries across agriculture that rely on soils to

support plant growth. A broad field of research spanning managed landscapes rom row-crop

fields to turf grass systems has developed around understanding the phenomenon of soil

compaction, identifying the causes, and quantifying the effect on plant growth and production

(Nawaz, Bourrié and Trolard, 2013; Hu et al., 2021). The influence of soil compaction on soil

properties and crop growth are complex (Batey, 1990). Reviews have highlighted the range of

research addressing the modeling of soil compaction (O’Sullivan and Simota, 1995; Lipiec and

Hatano, 2003) along with the physical (Soane, Dickson and Campbell, 1982; Horn et al., 1995),

biological (Frey et al., 2009; Pupin, Freddi and Nahas, 2009), and chemical (De Neve and

Hofman, 2000; Tamene et al., 2019) properties of soil affected by forces resulting in compaction.

With a growing understanding of soil compaction, research has continued to identify and

quantify the causes of compaction (Alakukku et al., 2003; Nawaz, Bourrié and Trolard, 2013;

Shah et al., 2017; Keller et al., 2019), effects on crop growth (Drewry, Cameron and Buchan,

2001; Unger and Kaspar, 1994; Nawaz, Bourrié and Trolard, 2013), generalized impacts on the

environment (Batey, 2009; Palmer and Smith, 2013; Schjønning et al., 2015; Hu et al., 2021),

and economic costs of soil compaction (Graves et al., 2015). This review will explore the

fundamentals of this issue by defining and quantifying soil compaction by outlining the wholistic

effects of compaction on physical soil properties. It will also address the physiological reactions

of plants in a compacted soil, examining the response of root (belowground) development and

the subsequent effect on shoot (aboveground) growth and the success or yield of plants. Lastly,

2

connections will be made from these fundamental principles to real-world applications,

discussing the major causes and implications of soil compaction.

Effects of Compaction on Soil

Soil compaction occurs when soil particles are compressed together by external forces,

decreasing the pore space between these particles and creating a denser media. (Soil Science

Society of America, 2008). Soil compaction occurs in topsoil (Bouwman and Arts, 2000), but is

most detrimental to subsoils (Jones, Spoor and Thomasson, 2003). Soil compaction is often

quantified in terms of bulk density or resistance to penetration (Passioura, 2002).

Bulk density is defined as the mass of particles divided by the total volume they occupy

(Soil Science Society of America, 2008). Soil bulk density is determined by dividing the dry

weight of soil by the total volume of the sample, but is also estimated with a variety of direct and

indirect methods (Al-Shammary et al., 2018) and is typically expressed in units of grams per

cubic centimeter (gcm-3, Blake, 1965). Compaction increases soil bulk density and impacts

nearly all other soil physical properties (Shah et al., 2017). In a typical loam soil, bulk density is

usually ~1.3 g cm-3 and root growth is severely impaired or stops when soil bulk density reaches

the range of 1.5-1.7 g cm-3 (Jin et al., 2017). Soil texture alters the range of values at which soil

bulk density becomes root limiting. (Jones, 1983; Pierce et al., 1983; Unger and Kaspar, 1994).

Soils with a higher clay content become more inhibitory to root growth when compacted, thus

are problematic at lower soil bulk densities, whereas sandy soils can maintain root growth at

higher soil bulk densities (Atwell, 1993). Different from soil bulk density, the resistance of soil

to penetration by an applied force provides a relative measure of the difficulty of roots to grow in

compacted soils. This metric can be measured with a penetrometer or more specifically, a cone

3

penetrometer (Herrick and Jones, 2002; Mome Filho et al., 2014), and is measured in

megapascals (MPa). Soil penetration resistance and the depth at which threshold pressures are

reached has become a standard measurement within soil management and soil physics research

(Medeiros et al., 2011, 2013; Mome Filho et al., 2014), because of the ease and efficiency of

testing (Busscher, Frederick and Bauer, 2000), although results may be more variable than soil

bulk density. Research by Chaney, Hodgson and Braim (1985) suggests that compaction causes a

significant increase in penetration resistance. While soil bulk density provides a direct

representation of the physical compactness of the soil, penetration resistance has shown to be a

strong predictor of the ability of roots to frow in compacted soils (Busscher and Bauer, 2003;

Otto et al., 2011; Bécel, Vercambre and Pagès, 2012).

A related factor when considering compaction, is soil porosity. Compacted soils show a

decrease in soil porosity or a change in the amount, size, and connectedness of pores that may

hold water or air (Dexter, 1988). Changes in soil porosity from compaction results in decreased

pore size and connectivity, restricting the permeability of air and water into and out of the soil

(Richard et al., 2001). Decrease in macropores can result in the development of anoxic

conditions (Correa et al., 2019), interfering with crop growth and development. This reduction in

pore space also has a negative impact on the water availability of the soil (Katou, Miyaji and

Kubota, 1987; Dexter, 2004). Alteration in pore size distribution due to compaction results in

increased runoff, decreased infiltration, and higher erosion losses (Shah et al., 2017). Along with

this chain of negative effects of compaction on soil physical properties, soil aggregate stability

index and soil hydraulic conductivity are also reduced (Potocka and Szymanowska-Pulka, 2018).

Aggregate stability refers to the ability of soil aggregates to resist disintegration when disruptive

forces are applied (Shah et al., 2017). Disturbed soils lose stability to resist further compaction,

4

causing the structure of soil aggregates to collapse and further degrade. Hydraulic conductivity

describes the ease with which water can move through pore spaces or fractures with respect to

the intrinsic permeability of the material and the degree of saturation (Soil Science Society of

America, 2008). Hydraulic conductivity is highly responsive to soil degradation resulting from

compaction (Whalley, Dumitru and Dexter, 1995) and consequently reduced porosity (Matthews

et al., 2010). Furthermore, hydraulic conductivity varies with different soil depths even within

the same soil profiles, typically declining with depth (Nakano and Miyazaki, 2005). With the

reduced pore space of compacted soils, water less able to move downwards in the soil and

waterlogging can become an issue (Batey, 2009). A compacted soil can locally become an

anaerobic environment due to the reduction in gas diffusion through a soil (Hamamoto et al.,

2012). A compacted soil subject to waterlogged and reduced gas exchange resulting in anaerobic

conditions has substantial ramifications. Anoxia impedes aerobic microbial mineralization and

encourages anaerobic denitrification activity. The anerobic microbial activity leads to reduced

nitrogen availability in the soil.

Other factors intertwined with compaction effects include the amount of soil organic

matter and soil water content. When soil organic matter decreases, there is a loss of aggregate

stability and thus structural stability, resulting in a higher susceptibility to compaction (Casanova

et al., 2013). Increased soil mechanical resistance under a variety of water potential levels is also

a consequence of reduced organic matter (To and Kay, 2005). Soil organic matter has been

identified as a key contributor to the formation of soil pores and soil aggregates, consequently

affecting soil gas exchange (Hamamoto et al., 2012). Soil strength increases with decreasing soil

water content (Bengough et al., 2011). Therefore, in dryer soils root growth can be limited by a

combination of increased resistance to root penetration and water deficiency (Kolb, Legué and

5

Bogeat-Triboulot, 2017). While we may conceptualize soil compaction as a singular problem,

this issue has many contributing factors and results in numerous changes to soil physical

properties that result in cascading outcomes for soil chemistry, biology, and overall soil health.

Effects of Compaction on Plant Development

Soil compaction lowers plant performance by altering and largely hindering belowground

root growth and consequently reducing aboveground plant growth (Figure 1). Root growth is

often slowed by a combination of soil physical stresses, including mechanical impedance, water

stress, and oxygen deficiency. The stress on the plant may vary significantly depending on the

location of the root in the soil profile, the prevailing soil water conditions, and the degree to

which the soil has been compacted (Bengough et al., 2006). In a compacted soil, roots are

continuously exposed to mechanical pressure and this often results in morphological

modifications. The most notable of these root changes are in the overall architecture of the root

system and shape of particular roots. These changes are often accompanied by modifications of

the cell structure and cell morphology (Potocka and Szymanowska-Pulka, 2018). Root system

architecture (RSA) refers to the specific arrangement of root components, encompassing the

overall form of the root system and shape of particular roots. The development of a particular

root system architecture pattern results from the processes of root tip extension, lateral root

formation, and root tropism (Correa et al., 2019). Considering the general influence of root

system architecture on the plant’s exploration of soil for water and nutrients, detriments to proper

architectural development resulting from compaction can have deleterious outcomes for the

overall plant. A review conducted by Potocka and Szymanowska-Pulka (2018) summarizes the

specific effects of mechanical stress on root systems architecture across many plant species.

6

The primary influence of soil compaction or soil strength on root system architecture is a

decrease in total root length (Pfeifer et al., 2014) parallel to an increase in root diameter (Popova

et al., 2016). At a bulk density of ~1.5-1.7 gcm-3 (Pierce et al., 1983) or a penetrometer

resistance of ~ 5MPa (Bengough et al., 2011), shorter and thicker roots have been observed. This

increase in root diameter in response to compacted soils is a mechanism that the plant uses to

push through substrates with higher penetration resistance at the same penetration pressure

imposed by root extension (Popova et al., 2016). Roots must exert a pressure in length and girth

in order to displace soil particles, overcome friction, and elongate through the soil. If a plant is

able to maintain a larger root system with more roots in compacted soil, it would result in greater

soil exploration than a plant with a severely stunted root system with fewer roots. However, the

potential for root penetration and growth into a compacted soil layer is also dependent on how

plastic the root angles and diameter are. The angle at which the roots grows also determines the

direction of root elongation and the soil volume the root system can occupy for water and

nutrient uptake. Roots in a compacted soil tend to forge a shallower growth angle as the

mechanical force and strength of the soil deters them from growing down at a steeper angle.

When roots hit a compacted layer of soil such as a hardpan, there can be different outcomes on

growth: roots may avoid the deleterious effects of compaction by inducing a more lateral growth

direction; roots may penetrate the hardpan and grown down deeper into the soil profile; or root

growth may stop completely (Clark, Whalley and Barraclough, 2003). Root growth response to a

compacted layer is variable and depends on species and situational conditions, but ultimately

plasticity of root diameter and steepness of angle in response to encountering higher density soils

results in the capacity of a root system to cope with hardpan layers. The property of root

tortuosity, or waviness of growth along the root length, is also influenced by soil compaction. An

7

increase in root tortuosity is a mechanism that plants use to avoid soil frictional resistance to root

tip penetration and increase the potential for exploring larger volumes of soil (Popova et al.,

2016).

Plants root-shoot ratios change as a function of plant size and stage of development.

Typically, younger and smaller plants have a higher ratio of roots to shoots. With more biomass

belowground, younger plants are more susceptible to stunting and less capable of transferring

energy when growing in compacted soils. The root system architecture in compacted soil will

often respond to the restriction of apical roots from compaction by increasing lateral root growth

that is less subject to compaction with depth. Compensatory growth may be a strategy of

adaptive plasticity to counter the limited function of a root system impeded by compaction by

growing less in areas with high soil strength and growing more where soil strength is lower

(Correa et al., 2019). Besides directly interfering with root proliferation, soil compaction also

causes numerous indirect effects on aboveground plant growth. Lower tissue nutrient

concentration has been observed in plants growing in compacted soils. This is likely due to a

combination of reduced nutrient availability in compacted soils and reduced root exploration in

the soil. Low yields under severely compacted soils are correlated with low concentrations of

nitrogen, phosphorus, and potassium (Arvidsson, 1999). Some research has found that in

compacted soils, reduced water uptake resulted in reduced stomatal conductance and higher

accumulation of abscisic acid in roots (Tardieu et al., 1992). Research by Young et al., (1997)

attributed the reduction in leaf appearance rate to a hormonal signal generated by impeded roots.

Reduced photosynthesis from stomatal or non-stomatal inhibition can also be attributed to

conditions of compacted soil. Research suggest that negative effect on photosynthesis, water

8

relations, and shoot growth caused by soil compaction could be closely related to the sensitivity

of root system architecture with high mechanical impedance of soil (Tubeileh et al., 2003).

Figure 1. Major interactions between soil properties and plant root function at the root-soil

interface in a compacted soil.Arrows indicate the influence of one property on another and

circles indicate a combined influence. Adapted from Correa et al., 2017.

Causes of Compaction

Soil compaction reduces agriculture productivity and a major concern for growers.

Intensive agricultural practices are beginning to be recognized as the cause of major soil

structure degradation (Palmer and Smith, 2013; Tamene et al., 2019), though compacted soils

can also occur under natural conditions without human or animal involvement (Potocka and

Szymanowska-Pulka, 2018). High mechanical load, less crop diversity, intensive grazing, and

irrigation methods contribute to soil compaction. Soil compaction is further magnified when

9

these management practices are accompanied by low soil organic matter, animal traffic, engine

vibrations, and tillage at high moisture contents (Batey, 2009). Efforts have been made to

quantify these effects of soil compaction and to analyze spatial and temporal relationship

between extent of compaction and its causes (Nawaz, Bourrié and Trolard, 2013).

Many agricultural and industrial operations require the use of heavy machinery for

farming practices or construction activities. Mechanically induced soil compaction has been

identified and characterized (Smith, Johnston and Lorentz, 1997), and research has also

addressed the interactions and compounded effects of a variety of factors including soil physical

properties, wheeling, the number of passes, specific farming practices, soil structure, soil water

status, and crop rotations (Hamza and Anderson, 2005). The severity and implications of

compaction can be affected by the size and weight of the machinery used, an issue that is

becoming more prevalent with the increase in industrialized agriculture and the use of larger

farming and construction implements (Keller et al., 2019). Axel load has been pinpointed as a

major cause of compaction, making it the focus of many studies, and a reliable measure to gauge

compaction potential. Ground contact pressure can be determined by axle load divided by the

surface area of contact between machine and soil. Ground pressure contributes to top soil

compaction, while high axle load leads to subsoil compaction (Botta et al., 1999). Different soil

textures result in variation in compaction and compaction resistance to different pressure

magnitudes (Ellies Sch. et al., 2000). Vibration due to heavy mechanical implements can

compact soils effectively at higher moisture contents. Beyond simple axel load, vibrations

actually impose additional impact and high intensity pressure on soil particles. The speed of a

implement together with vibration intensity can cause significant soil compaction. The threat of

10

heavy machinery certainly is a multi-faceted issue with many points of contention to be

considered (Soane, Dickson and Campbell, 1982).

Livestock production is a fundamental component of agriculture and certainly provides

its own contributions to soil compaction. Soil type and soil moisture are factors that determine

the magnitude of soil compaction as a result of livestock grazing. For example, fine-textured

soils are more vulnerable to trampling by grazing animals than coarse-textured soil (Batey,

2009). Furthermore, dry soils experience less trampling action due to higher aggregate stability

index, while moist soils are more vulnerable to compaction (Mosaddeghi et al., 2000). Similar to

the issue of axel load in machinery, livestock concentrate a substantial weight on a small surface

area with their hooves. Compaction by animals proves to be more destructive than machinery.

Implement tires are typically wider than an animal’s hoof, thus there is a decreased ground

pressure with an increase contact surface area.

While water is one of the most essential resources for plant growth, it can also contribute

to soil compaction. Soil moisture content is the most influential factor of the soil itself that that

increases compaction potential, as soil moisture reduces soil strength and allows soil particles to

slip and move more easily (Lipiec and Hatano, 2003). Due to this, soil water content becomes

crucial during soil tillage to reduce the potential for compaction. The effect of soil moisture

content is strongest in the subsoil, while the effect of rain impact is more detrimental in the

surface of topsoil. The direct impacts raindrops can disperse soil particles by breaking the soil

surface crust. Through this dispersal, fine particles become separated from soil clods, which

aided by the movement of infiltrating water settle down into soil pores make a hard layer of

compacted soil. Soil compaction thus results from many contributing and interrelated factors.

While many of these factors cannot be avoided, completely managed, or entirely mitigated,

11

identifying limits and relative importance of these various factors may help in a holistic approach

at reducing soil compaction.

Conclusion

Soil compaction continues to be one of the leading contributors to soil degradation,

warranting the need for further research. Numerous factors ranging from soil moisture content to

mechanical load are responsible for soil compaction threatening soil health via directly or

indirectly modifying soil physical, chemical, and biological properties. Beyond soil properties,

compaction reduces plant performance by negatively influencing the growth and the

development of plants. Stunted growth, leaf discoloration, reduced plant height, and shallow root

system are predominant morphological effects of soil compaction. Reduced nutrient uptake,

reduced leaf gas exchange, carbon assimilation, and less translocation of photosynthates are all

detrimental effects that round out the negative ramifications of soil compaction. While

sustainable practices such as alternative tillage systems, use of cover crops, and remediation of

soil are gaining in popularity and proving effective, compaction continues to be global problem

across many sectors. Through continuous research there is potential enhance the understanding

and interrelationship of these factors towards developing a holistic approach for reducing or

mitigating soil compaction.

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19.

17

CHAPTER 2. EVALUATION OF TILLAGE RADISH COVER CROP TO

ALLEVIATE SOIL COMPACTION AND SUBSEQUENT ESTABLISHMENT OF A

MODEL ORNAMENTAL PERENNIAL

Marcus D. Jansen1 and Grant L. Thompson1

1Iowa State University Department of Horticulture

Modified from a manuscript to be submitted to HortTechnology (2021)

Abstract

Recognizing the successful reduction of compaction from a tillage radish cover crop in

agricultural cropping systems and the need for compaction remediation in post-construction

urban soils where landscape plants will be established, we sought to test the potential of tillage

radish (Raphanus sativus L.) for such uses. This study used a simulated landscape system in a

controlled environment to evaluate the performance of a tillage radish (Raphanus sativus L.

‘Nitro’) cover crop at five soil compaction levels and the impacts of that tillage radish cover crop

on the growth of the following establishment of a model herbaceous perennial – coral bells

(Heuchera micrantha var. diversifolia ‘Purple palace’). We found that tillage radish germination

was unaffected by soil compaction level. Radishes produced comparable amounts of mean

biomass through a range of compacted soils (15.7-19.9g) but showed reduced mean biomass at

the highest soil compaction level (3.1g). Coral bells following an herbicide termination radish

treatment where stunted, likely due to herbicide residues in the soil. Coral bells following the no

radish control treatment produced more mean biomass (9.2g) than coral bells following the cold

termination radish treatment (7.8g). We speculate that allelopathic effects and nutrient

unavailability may have contributed to the reduced amount of growth following the cold

termination treatment. The findings of this study confirmed that tillage radishes have the capacity

to grow successfully at a range of soil bulk densities, but more work is needed better understand

18

the managements consideration for adapting agricultural cover crops for use in ornamental

landscapes.

Introduction

Soil compaction is a major issue for post-construction urban soils and can negatively

affect the establishment and growth of ornamental landscape plants. Residential and commercial

construction process, often intentionally compact soils for foundations, pavement, and other

structures (Strom, Nathan and Woland, 2013) and the general operation of heavy machinery

utilized for construction can unintentionally compact site soil, including those areas that will

become landscaping or lawns (Randrup and Dralle, 1997). Substantial effort has addressed soil

compaction and its impacts on plant growth and development (Batey, 2009). Soil compaction is a

result of soil particles being compressed together by external forces, decreasing the pore space

between particles and creating a denser growing media(Soil Science Society of America,

2008)(Soil Science Society of America, 2008)(Soil Science Society of America, 2008)(Soil

Science Society of America, 2008)(Soil Science Society of America, 2008)(Soil Science Society

of America, 2008)(Soil Science Society of America, 2008)(Soil Science Society of America,

2008). Soil compaction is often quantified in terms of bulk density and resistance to penetration

(Passioura, 2002). Reduced poor space inhibits the movement of air and water within the soil

profile (Dexter, 2004) and can affect both gravitational water and plant unavailable water, thus

affecting plant growth. Soil compaction has shown to deter root exploration and establishment of

landscape plants since increased soil bulk density results in fewer, smaller, and less well-

connected soil pores for roots to inhabit (Kozlowski, 1999; Day, Seiler and Persaud, 2000). The

negative impact of soil disruption on urban plants has been recognized, and there has been a push

for urban soil remediation to improve plant growth (Sloan et al., 2012). Some soil rehabilitation

19

or amendment practices including top soil replacement (Layman et al., 2016), mechanical tillage

or physical fracturing (Somerville, May and Livesley, 2018), and compost incorporation (Sax et

al., 2017; McGrath et al., 2020; Somerville et al., 2020) have shown to accelerate establishment

and improved success of urban landscape plants.

Agricultural soils are better studied than urban soils and may provide insights for means

of mitigating and managing deleterious soil conditions, such as compaction, that are suitable for

direct use or use with modification in urban and constructed landscapes. Soil compaction has

also proven to be a substantial concern in agricultural systems due to use of heavy machinery,

working wet fields, intensive cropping, short crop rotations, grazing, and inappropriate soil

management (Hamza and Anderson, 2005). Cover crops have been widely utilized as a soil

management practice in conventional and organic agricultural systems for numerous benefits

including soil stabilization, weed suppression, and nutrient management. (Fageria, Baligar and

Bailey, 2005; Justes, 2017). Cover crop species with extensive tap roots have been shown to be

effective at reducing soil compaction, scavenging nutrients from deep in the soil and bringing

them into the root zone, and providing root channels for subsequent crops, such that the term

“bio-drillers” is sometimes applied to these tap rooted cover crops (Williams and Weil, 2004;

Chen and Weil, 2010). Within the Brassica plant family, Raphanus sativus L. var. longipinnatus

(Daikon radish) along with comparable varieties (tillage radish, forage radish, oilseed radish) and

other radish cultivars are valued for significantly higher soil penetrating capacity (Chen and

Weil, 2010) and capacity for nutrient accumulation (Toom et al., 2019).

20

The urban application of cover crops and short-term vegetative cover have been used for

erosion control and weed suppression on exposed soil along roadways and other construction

areas (By and Busby, 2014). There is less published research about the potential use of cover

crops for amending compacted landscape soils. Recognizing the successful reduction of

compaction from a tillage radish cover crop in agricultural cropping systems and the need for

compaction remediation in post-construction urban soils where landscape plants will be

established, we sought to test the potential of tillage radish for such uses. Our objectives in this

study were to quantify the growth of tillage radishes at varying levels of landscape soil

compaction and measure the subsequent establishment of a model perennial plant following the

tillage radish cover crop.

Methods and Materials

The study was initiated on December 8, 2019 and concluded on April 30, 2020 and was

conducted in three phases: (1) radish establishment; (2) radish termination; and (3) coral bell

establishment. The experiment consisted of four planting treatments (Table 1), three of which

were planted with Raphanus sativus L. ‘Nitro’ (tillage radish, Green Cover Seed, Bladen, NE)

seeds and one of which remained as an unplanted control. Within each planting treatment, there

were five soil compaction levels and five replicates of each for a full factorial design (4

treatments * 5 experimental soil compaction treatments * 5 replicates = 100 total experimental

units). At the end of phase 1, radishes in the harvested radish (HR) treatment group were

destructively harvested. All remaining units proceeded to phase 2 where the cold termination

radish (CTR) and the no radish control (NRC) treatment groups were terminated with a

simulated winter and the herbicide termination radish (HTR) treatment group was chemically

terminated. In phase 3 the CTR, HTR, and NRC treatment sets were planted with Heuchera

21

micrantha var. diversifolia (Rydb.) Rosend., Butters, & Lakela ‘Palace Purple’ (coral bells)

plugs as a model landscape perennial to evaluate establishment subsequent to tillage radish use

or the unplanted control. Palace Purple coral bells was chosen because of its sustained

popularity, rosette and mounding growth form, ease of growth, and prior selection as a Plant of

the Year (Perennial Plant Association, 1991). Throughout the experiment, containers were

randomized by replicate group (planting treatment * soil compaction treatment) on the

greenhouse benches.

Phase 1 – Radish establishment

Field soil was collected from the Iowa State University Horticulture Research Station

(Ames, Iowa, USA). The soil type was a Clarion series, which is a fine-loamy, Typic Hapludoll

with a slope of 2-6%. Field soil was sieved through 500 µm mesh to remove large soil

aggregates and other debris. Cylindrical polyvinyl chloride (PVC) columns with an interior

height of 208 mm and radius 75 mm were fitted with bottoms with an 18 mm drainage hole and

used as containers for the experiment to simulate the soil compaction effects of typical

construction practices. Each PVC container was divided into a compacted sublayer (138mm

deep) and top-dressed with a noncompacted layer of the same soil (70mm deep), representing

respread topsoil. To achieve the desired range of soil bulk densities amongst the compaction

sublayer experimental treatment groups, different masses of soil were compacted with a hand

tamper into the set volume of the lower portion of the PVC containers. The levels of were

compacted to bulk densities of 0.96 g*cm-3 (Compaction Level 1), 1.11 g*cm-3 (Compaction

Level 2), 1.28 g*cm-3 (Compaction Level 3), 1.44 g*cm-3 (Compaction Level 4), and 1.6 g*cm-3

(Compaction Level 5). Three seeds of Raphanus sativus L. ‘Nitro’ (radish) were planted at a

depth of 2cm and 10cm apart from one another in a triangular pattern centered in each container.

22

Containers were grown in the greenhouse with a 12 hour day length and a constant temperature

of 21 5°C. An automated (Model 24600, Orbit Irrigation Products LLC, North Salt Lake, UT)

drip irrigation system with pressure compensating spray stakes (Netafim USA, 3.2 GPH) were

run for 60 seconds every other day at a rate of 300mL to each container. Radish germination was

counted when the first true leaf appeared above the soil. Containers were monitored daily at the

same time until the seeds were completely germinated. On day 15 the containers were thinned,

leaving the single strongest seedling representative in each container. Dimensional

measurements of the radish plants were taken at the same time each week. Weekly, a ruler and

digital caliper were used to measure aboveground radish height and bidirectional width (widest

point then perpendicular). The calculated ellipsoidal aboveground volume (Thorne et al., 2002)

of plants was calculated from the dimensional measurements(2/3𝜋H (A/2 x B/2), H=height,

A=width 1, B=width 2). After 56 days of growth, plants from the HR treatment group were

harvested. Aboveground and belowground biomass was calculated from fresh and dry weights.

Leaf count and leaf area (cm2) (LI-3100C; LI-COR Biosciences, Lincoln, NE) were also

documented.

Phase 2 – Radish termination

Following the radish establishment phase, plants from the CTR treatment group were

terminated with a cold period and plants from the HTR treatment group were terminated with a

chemical herbicide. Cold and herbicide termination methods were utilized as they are the

standard for cover crop radish termination in farming practice. The simulated cold winter period

was created utilizing a large storage freezer to chill the plants at -20 5C for 30 days.

Replicates in the CTR and NRC treatment groups were returned to the greenhouse for 5 days

following the cold period to thaw the soil and radish root before transplanting of the coral bells.

23

Plants from the HTR treatment group were sprayed with RoundUp® glyphosate herbicide per

label instructions (Monsanto Company, St. Louis, MO, glyphosate isopropylamine salt) and

remained in the greenhouse for 35 days before installation of the coral bells. At the end of Phase

2, foliage was removed from the terminated radishes in the CTR and HTR treatment groups and

the tap root was left to deteriorate in the soil.

Phase 3 – Coral bells establishment

Heuchera micrantha var. diversifolia ‘Purple palace’ (coral bells) plugs in 50-cell trays

were obtained from a wholesale greenhouse (Swift Greenhouses, Inc., Gilman, IA). Plugs were

installed in the uncompacted soil layer in the containers of the CTR, HTR, and NRC treatment

groups. An automated (Model 24600, Orbit Irrigation Products LLC, North Salt Lake, UT) drip

irrigation system with pressure compensating spray stakes (Netafim USA, 3.2 GPH) were run for

60 seconds every third day at a rate of 300 mL to each container. Dimensional measurements of

the coral bells were taken at the same time each week to document plant growth. Weekly height

and bidirectional width measurements were taken to track coral bell growth and determine the

aboveground ellipsoidal volume using the same formula as for the radishes. After 52 days of

growth, plants from the CTR, HTR, and NRC treatment groups were harvested. Above-ground

biomass was calculated from fresh and dry weights. Leaf count and leaf area (leaf area meter

model) were also documented. Regression analysis was utilized to identify significance at P <

0.05 between treatment. Means were separated by Tukey’s significant difference test at P < 0.05.

24

Table 1. Summary of planting treatment groups showing abbreviations and actions during the

phases 1, 2, and 3 in a greenhouse.

Treatment Group Abbreviation Phase 1 Phase 2 Phase 3

Harvested Radish HR radish harvested

--- ---

Cold Termination

Radish

CTR not harvested cold treatment coral bell

harvested

Herbicide Termination

Radish

HTR not harvested herbicide treatment coral bell

harvested

No Radish Control NRC not harvested cold treatment coral bell

harvested

Results

Radish growth and harvest

Radish seed germination was unaffected by compaction (P = 0.91) or treatment group (P

= 0.63) (Table 2). Following germination, there were differences in radish growth over time or at

harvest between all compaction levels (CL) (Figure 1, Table 2). There were significant

interactions of treatment * compaction level (P = 0.0007) and compaction * week (P < 0.0001),

along with the main effects of treatment, compaction level, and week of growth (all P < 0.001)

for the calculated aboveground volumetric size of the radishes. Radishes grown at a soil bulk

density of 1.6 g*cm-3 (CL 5) were the most negatively affected in calculated volumetric size

followed by radishes grown at 1.44 g*cm-3 (CL 4, Figure 1A). Radishes grown soil bulk density

of 0.96 g*cm-3,1.11 g*cm-3, and 1.28 g*cm-3 (CL 1, 2, and 3, respectively) produced significantly

greater calculated volumes than the higher compaction levels and were not significantly different

from each other. At harvest, radishes grown at CL 1 (bulk density: 0.96 g*cm-3), CL 2

(1.11 g*cm-3), CL 3 (1.28 g*cm-3), and CL 4 (1.44 g*cm-3) had produced comparable amounts of

aboveground biomass and belowground biomass, while radishes grown at a higher soil bulk

25

density of (CL5: 1.6 g*cm-3) produced less aboveground and belowground biomass (Figure 1C,

1D and Appendix 1 and 2). More variation was observed in leaf area at harvest. Radishes grown

at a CL 3 (1.28 g*cm-3) produced the greatest leaf area at harvest (mean SE:451.51 28.7

cm-2). Radishes grown at a soil bulk density of CL 5 (1.6 g*cm-3) produced the least leaf area at

harvest (79.98 15.0 cm-2, Figure 1B).

Coral bells growth and harvest following radish termination

There were treatment group and compaction resulted in drastic differences in Heuchera

micrantha var. diversifolia ‘Purple palace’ (coral bells) growth after transplant (Table 2).

Positive volumetric growth of coral bells was observed with the no radish control (NTC)

treatment and the cold termination radish (CTR) treatment, but little growth was observed with

the herbicide termination radish (HTR) treatment (Figure 2A, 2C, and 2E). Stunted growth in the

HTR treatment is speculated to be a result of residual herbicide effects, therefore subsequent

comparisons will predominantly be made between the NTC and CTR treatments. Calculated

volume at harvest of coral bells in the NTC treatment (1665.6 ± 154.4 cm3) was 34.1% greater

than that of coral bells in the CTR treatment (1097.6 ± 83.3 cm3, Figure 2B). Leaf area at harvest

of coral bells in the NTC treatment (460.1 ± 31.7 g*cm-2) was 20.0% greater than that of coral

bells in the CTR treatment (368.1 ± 24.9 g*cm-2, Figure 2D). Aboveground dry biomass at

harvest of coral bells in the NTC treatment (9.3 ± 0.6 g) was 15.5% greater than that of coral

bells in the CTR treatment (7.9 ± 0.6 g, Figure 2F, Appendix 3). For coral bells in the NRC

treatment, there were significant interactions with compaction level for calculated volume at

harvest (P = 0.0001), leaf area (P < 0.0001), and aboveground dry biomass (P < 0.0001, Table 3).

For coral bells in the CTR treatment, there was a significant interaction with compaction level

26

for calculated volume at harvest (P = 0.0002), but there were not significant interactions with

compactions level for leaf area (P = 0.1780) and aboveground dry biomass (P = 0.0562). For

coral bells in the HTR treatment, there was a significant interaction with compaction level for

calculated volume at harvest (P = 0.0135) and leaf area (P = 0.0239), but there were not

significant interactions with compactions level for aboveground dry biomass (P = 0.1970).

Aboveground dry biomass increased with declining soil bulk density for coral bells in the NRC

treatment: CL5 (5.26 g), CL4(7.23 g), CL3(9.90 g), CL2(11.75 g), CL1(12.32 g). Aboveground

biomass did not follow this same trend for coral bells in the CTR treatment: CL4(5.07 g),

CL5(7.05 g), CL3(8.45 g), CL3(9.14 g), CL1(9.57 g).

Table 2. Analysis of variance table for radish germination, aboveground radish growth, radish

leaf area, radish aboveground biomass, and radish belowground biomass in radish growth phase

(phase 1) in a greenhouse.

Growth Response Model Term n DF F p-value

Radish Germination (days to

germination)

compaction 5 4 0.25 0.9099

treatment 3 2 0.46 0.6335

treatment *compaction 15 8 1.31 0.2579

Aboveground Radish Growth

(cm3)*

compaction 5 4 103.68 <.0001

treatment 3 2 10.43 <.0001

week 8 7 93.20 <.0001

treatment*compaction 15 8 3.47 0.0007

treatment*week 24 14 1.18 0.2842

compaction*week 40 28 4.86 <.0001

treatment*compaction*week 120 56 0.47 0.9996

27

Table 2 (Continued)


Radish Leaf Area (g*cm-2) compaction 5 4 32.07 <.0001

Radish Aboveground Biomass (g) compaction 5 4 15.60 <.0001

Radish Belowground Biomass (g) compaction 5 4 6.40 0.0017

*calculated ellipsoidal volume

Figure 1. (A) Mean calculated ellipsoidal volume (cm3) of radishes (Raphanus sativus L. ‘Nitro’)

at each compaction level across the 8 weeks following seeding in containers in the

greenhouse.Data was pooled across the replicates in all planting treatment groups with radishes.

Each symbol represents the mean of 15 replicates. Error bars represent the standard errors of the

mean of the 15 replicates. (B) Leaf area (g*cm-2), (C) aboveground biomass (g), and (D)

belowground biomass (g) of radishes at each compaction level at harvest. Each box represents

the mean of 5 replicates. Error bars represent the standard errors of the mean of the 5 replicates.

Week

Cal

cula

ted v

olu

me

(cm

3)

Lea

f ar

ea (

g*cm

-2)

500

400

300

200

100

0

1 2 3 4 5Compaction level



Ab

ov

egro

und

bio

mas

s (g

)

25

20

15

10

5

0

15

10

5

0

A B

C D

Bel

ow

gro

un

d b

iom

ass

(g)

28

Table 3. Analysis of variance table for coral bells aboveground growth, volume at harvest, leaf

area, and aboveground biomass between growth planting treatment groups in the coral bells

growth phase (phase 3) in a greenhouse.


Heuchera Aboveground Growth (cm3)

Cold Termination Radish (CTR) compaction 5 4 22.19 <.0001

week 8 7 83.26 <.0001


Herbicide Termination Radish (HTR) compaction 5 4 16.21 <.0001

week 8 7 1.07 0.3883

compaction*week 40 28 1.26 0.1922

No Radish Control (NRC) compaction 5 4 32.20 <.0001

week 8 7 85.28 <.0001


Coral Bells Volume at Harvest (cm3)

Cold Termination Radish (CTR) compaction 5 4 9.33 0.0002

Herbicide Termination Radish (HTR) compaction 5 4 4.12 0.0135

No Radish Control (NRC) compaction 5 4 10.41 0.0001

Coral Bells Leaf Area (g*cm-2)




Coral Bells Aboveground Biomass (g)




*calculated ellipsoidal volume

29

Figure 2. Mean calculated ellipsoidal volume (cm3) of coral bells (Heuchera micrantha var.

diversifolia ‘Palace Purple’) at each compaction level across the 8 weeks following plug

transplanting in containers in the greenhouse for the (A) cold termination radish (CTR)

treatment, (C) herbicide termination radish (HTR) treatment, and (E) no radish control (NRC)

treatment. Each symbol represents the mean of 5 replicates. Error bars represent the standard

Week

Cal

cula

ted v

olu

me

(cm

3)

Week

Cal

cula

ted

vo

lum

e (c

m3)

Week

Cal

cula

ted

volu

me

(cm

3)

CTR HTR NRC

Planting treatment group

CTR HTR NRC


CTR HTR NRC


3000

2000

1000

0

Cal

cula

ted

volu

me

(cm

3)

Lea

f ar

ea (

g*

cm-2

)

800

600

400

200

0

15.0

10.0

7.5

5.0

2.5

0.0

12.5

Abo

veg

rou

nd b

iom

ass

(g)

A B

C D

E F

30

errors of the mean of the 5 replicates. (B), Calculated volume at harvest (cm3, ellipsoidal

volume), (D) Leaf area (g*cm-2), and (F) aboveground biomass (g) of coral bells at each

compaction level at harvest. Each box represents the mean of 5 replicates. Error bars represent

the standard errors of the mean of the 5 replicates.

Discussion

Radish growth and harvest

We observed that the radish seed germination was not significantly different among the

treatment groups with nearly all seeds germinating within 7 to 8 days, while radish seed has been

known to germinate in as little as 3 days in ideal conditions (Jacobs, 2012). The similarity in

germination was expected as all treatments received the simulated uncompacted topsoil layer

(70mm deep) in which the seeds germinated. A reduction in the calculated volumetric growth

was observed during the radish growth phase at higher soil bulk densities (CL 4: 1.44 g*cm-3 and

CL 5: 1.6 g*cm-3), which is consistent with previous work suggesting that root development and

overall plant growth is negatively impacted when soil reaches a bulk density above 1.5 g*cm-3

(Jin et al., 2017). In this simulated system, radishes at lower compaction levels (CL 1: 0.96

g*cm-3, CL 2: 1.11 g*cm-3, CL 3: 1.28 g*cm-3) reached peak growth approximately 5 weeks after

sowing (35 days), while radishes in a fall to winter cover crop rotation typically reach peak

growth in eight to twelve weeks (56-84 days) (Lawley, Weil and Teasdale, 2011). The rate of

development for tillage radish is influenced by day length, so the reduced time to maturity in this

experiment was likely influenced by the twelve-hour day length. At harvest, we found that

radishes grown at CL 1 (0.96 g*cm-3), CL 2 (1.11 g*cm-3), CL 3 (1.28 g*cm-3), and CL 4 (1.44

g*cm-3) all produced similar rates of aboveground and belowground dried biomass. This finding

agrees with previous research (Williams and Weil, 2004; Chen and Weil, 2010, 2011)

concluding that a tillage radish cover crop has the capacity to grow successfully and produce a

31

sufficient tap root at a range of soil bulk densities. We found that our range of aboveground

biomass (3.1 - 19.9 g) from high to low levels of compaction in this greenhouse container study

was lower than but somewhat comparable the range of aboveground biomass (18.8 - 27.2g) from

high to low levels of compaction in a field study (Chen and Weil, 2010), a difference likely

influenced by the environmental conditions of the study and the duration of the crop time.

Coral bells growth and harvest

It was evident early in the coral bells growth phase that the plugs transplanted in the

herbicide radish termination (HTR) treatment were uniformly stunted and were likely negatively

impacted by the herbicide termination treatment. Therefore, the growth and harvest data from

this treatment does not provide a suitable comparison to the other treatments. Coral bells

following the no radish control (NRC) treatment produced more growth (calculated aboveground

volume, leaf area, and above dry ground biomass) than coral bells following the cold radish

termination (CTR) treatment. This finding was contrary to our expectations, as we expected the

tillage radish cover crop to increase the growth of the subsequent coral bells. The unexpected

underperformance of the coral bells in the cold terminated radish treatment containers compared

to the no-radish control This outcome may have been associated with reported allelopathy

properties of tillage radish, nutrient removal with the aboveground radish biomass following

termination, or nutrients retained in the radish root that were not decomposing at a rate suitable

for coral bells update, which are not mutually exclusive and require further research to

substantiate or disentangle. These will be discussed in greater detail below.

32

Radish termination effects on coral bells

If a tillage radish cover crop is to be adopted as a potential soil amendment practice for

landscape soils, then termination and carry over effects of the cover crop must be addressed.

Radish and other Brassica cover crops must be terminated to keep them from progressing to

flower and seed set and to initiate decomposition before the planting of the following crop

(Ferrell et al., 2018; Askew et al., 2019). Cold temperatures over winter (i.e. winter-kill) and

herbicide applications are both standard methods for terminating a radish cover crop (Jacobs,

2012; Oliveira, Butts and Werle, 2019). In this greenhouse study, we attempted to simulate these

methods of termination, which may have also inadvertently negatively impacted the subsequent

coral bells growth.

We suspect that residual herbicide is what stunted the coral bells in the HRT treatment

group, since coral bells in this treatment group put on minimal growth while coral bells in the

other treatments did. Despite following the manufacturer’s recommended application rate and

residual period, the herbicide may have persisted in the PVC containers and reacted differently

than it would have in a field setting. Soil microbes have the capacity to degrade glyphosate

herbicide in aerobic and anerobic conditions but reduced microbial activity can result in

glyphosate accumulation in the soil (Daniele & Federico, 2017). Productive farm field soil was

used for this study that was presumed to have adequate microbial activity. Following the

glyphosate application, the herbicide terminated radish (HTR) treatment group remained in the

controlled greenhouse conditions for 30 days before planting the coral bell plugs. These

containers were not watered until a week before the coral bells plugs were transplanted, so there

is a possibility that dry soil did not allow for sufficient microbial activity necessary to breakdown

the glyphosate (la Cecilia and Maggi, 2018; Kanissery et al., 2019). Residual herbicide may have

then caused the observed stunting of coral bells in the chemical termination treatment. Dry

33

conditions and compaction do occur under field conditions and herbicide residual periods

account for adverse conditions, therefore, more investigation is needed under field conditions to

determine if herbicide termination of tillage radish prior to ornamental plant installation is a

viable practice. Due to the stunting of the coral bells, it is challenging to tell if the radishes

themselves would have had an effect on the coral bells transplants.

Radish allelopathy nutrient unavailability

The herbicide termination treatment aside, the coral bells transplanted into the cold

terminated radish (CRT) treatment containers grew less than those in the no-radish control

(NRC) treatment. We speculate that reported allelopathic effects of radish or nutrient

unavailability, or a combination of those causes, may have contributed to reduced coral bells

growth following the cold termination treatment.

Allelopathy is one possible factor that reduced coral bells growth, since tillage radishes

and other Brassica species have shown to have allelopathic effects on other plant species

(Rehman et al., 2013). All Brassica species contain glucosinolates, which are hydrolyzed to

isothiocyanates (ITC), thiocyanates, and nitriles in the soil (Boydston & Hang, 1995). There are

numerous examples of ITC’s inhibition of plant growth or germination (Evenari, 1949; Bell and

Muller, 1973; Teasdale, Taylorson and Taylorson2, 1986). While much work has addressed the

use of allelopathic effects of Brassica for weed suppression (Norsworhty, 2003; Uremis et al.,

2009; Rehman et al., 2019), Lawley, Teasdale and Weil (2012) found that winter radish cover

crops inhibit weed growth by competition and canopy establishment and not through allelopathic

mechanisms. Despite research on the disputed allelopathic effects of Brassica family members

for weed control, less research has been conducted on Raphanus sativus allelopathy affecting

subsequent crops instead of weeds (Uygur et al., 1970).

34

Another possible cause of the reduced coral bells growth after tillage radish in this study

is linked to nutrient availability. The deep taproot and ability to accumulate biomass rapidly

allow tillage radish to accumulate nutrients that are deeper in the soil and bring them towards the

soil surface, within the rootzone of other plants (Dean and Weil, 2009a; Hirsh et al., 2021). It is

possible that radishes in the cold termination treatment (CRT) scavenged nutrients from the soil

and incorporating them into the radish biomass, thus making nutrients unavailable to the

subsequent coral bells and this did not occur in the no-radish control (NRC) treatment. Unlike

weed seeds that may use nitrogen availability as a signal to germinate (Lawley, Teasdale and

Weil, 2012), decomposing radish root biomass may not have supplied sufficient nitrogen for

coral bells growth since the radish roots were not tilled in after termination and did not have a

sustained winter period as under field conditions to begin decomposition. In the short duration of

the termination period (35 days) and since the tillage radish was not incorporated into the soil

prior to the coral bells planting, it is possible that scavenged nutrients by the radish were not

made available through decomposition at a rate that would have made those nutrients available to

the coral bells. Additionally, radish aboveground biomass in the CRT treatment was removed for

this study and not supplemented, which represents a net export of nutrients that were not

available for the coral bells compared to the NRC treatment that experienced no nutrient loss or

uptake prior to the transplanting of the coral bells.

Limitations and future research

This preliminary study testing tillage radish at various compaction levels and subsequent

model ornamental plant establishment exposed some limitations that should be addressed in

future research. The use of a controlled greenhouse environment for the study allowed for

35

conducting the experimental soil compaction treatments but imposed some challenges in

mimicking field conditions and management practices better suited to field research. Since the

tillage radishes were successful at all but the highest compaction levels, future research is needed

into how tillage radish termination practices at any time of year, using chemical products, or

relying on winter-kill can be combined with a subsequently installed ornamental landscape.

Specifically, with regards to incorporating or not incorporating the terminated tillage radish into

the soil before adding landscape plants; the availability and timing of nutrients with biomass

decomposition; and residual effects of radish termination practices. Removal of aboveground

radish growth removed nutrients that were not supplemented in the radish treatment vs. the no

radish control. There are management practices used for the implementation of a tillage radish

cover crop in an agricultural crop system that would not be feasible in a perennial ornamental

landscape. With regards to establishing landscape perennials, the great variety in herbaceous and

woody plants precludes the cropping system refinements as in agricultural settings, thus even

with more research into subsequent establishment, there may remain the potential for tillage

radish to negatively affect the establishment of landscape plants. In an agricultural production

system, a tillage radish cover crop is used to compliment a monocultural annual cash crop such a

corn, soybeans, small grains, or vegetables; is most typically planted in the fall season following

cash crop harvest; and can be implemented once or more per year and over a period of many

years. Due to the semi-permanent nature of perennial landscapes, a tillage radish cover crop can

only be implemented once before the installation of that landscape, thus some benefits gained

from long-term use of cover crops may not be possible in ornamental landscapes. Additionally,

unlike relatively predictable seasonal planting and harvest periods for agricultural settings, urban

and landscape construction projects may be completed at nearly any point during the year. Thus,

36

tillage radish performance across varying seasonal conditions should be studied. Using cover

crops in mixes has shown to be a valuable strategy in agricultural systems (Blesh, 2018), and

could be another area of research for tillage radish to be used in conjunction with other cover

crop species which may result in multifunctional outcomes to aid in the establishment of

perennial ornamental landscapes.

Conclusion

This study used a simulated landscape system in a controlled environment to evaluate the

performance of a tillage radish cover crop at a variety of soil compaction levels and the impacts

of that tillage radish cover crop on the growth of the following establishment of a model

herbaceous perennial – coral bells. We found that tillage radish germination was unaffected by

soil compaction level. Radishes produced comparable amounts of growth through a range of

compacted soils but showed reduced growth at the highest soil compaction level. Coral bells

following an herbicide termination radish treatment where stunted, likely due to herbicide

residues in the soil. Coral bells following the no radish control treatment produced more growth

than coral bells following the cold termination radish treatment. We speculate that allelopathic

effects and nutrient unavailability may have contributed to the reduced amount of growth

following the cold termination treatment. Further research is needed to better understand the

effects of a tillage radish cover crop on the performance of a variety of ornamental landscape

plants. We recognize an opportunity for future research to address the managements

consideration for adapting agricultural cover crops for use in ornamental landscapes.

Understanding the planting and termination methods for a tillage radish cover crop, potential of a

tillage radish cover crop at various environmental conditions, the effects of a tillage radish cover

37

crop on perennial landscape plants, and combination effects of tillage radishes in cover crop

mixes are all areas for consideration for the future of using cover crops in ornamental landscapes.

Acknowledgements

The authors gratefully acknowledge Connor Evers, Emily Meader, and Pete Lawlor for

greenhouse and laboratory assistance; Alex Lindsey for statistical consultation; Dr. Ajay Nair

and Dr. Marshall McDaniel for guidance and project design; support from the Iowa State

University Horticulture Research Station; and funding from the Iowa State University

Department of Horticulture. This research was also funded by NIFA Hatch grant IOW03657, “In

search of sustainable landscape and horticultural production systems.” The authors appreciate

the generosity of green Cover Seed for donating seed for this research project. Data and results

are independent and not influenced by the material support from green Cover Seed. Mention of

trade names in this publication does not imply endorsement by Iowa State University of products

named, nor criticism of similar products not named.

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Brassica Species on Johnsongrass [Sorghum Halepense (L.) Pers.].” African Journal of

Biotechnology 8 (15).

Uygur, F. N., F. Köseli, A. Çınar, and W. Koch. 1970. The Allelopathic Effect of Raphanus

Sativus L. Journal of Plant Diseases and Protection. E. Ulmer. 12: 259-264.

Williams, Stacey M., and Ray R. Weil. 2004. Crop Cover Root Channels May Alleviate Soil

Compaction Effects on Soybean Crop.” Soil Science Society of America Journal 68(4): 1403–

1409.

41

Appendix A. Tillage Radish Aboveground Growth at Harvest

Figure 3. Aboveground growth of tillage radishes (Raphanus sativus L. ‘Nitro’) in the Harvested

Radish (HR) treatment at week (56 days) arranged with increasing soil compaction level bulk

density from left to right: (CL 1) 0.96 g*cm-3, (CL 2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4)

1.44 g*cm-3, and (CL 5)1.6 g*cm-3.

Appendix B. Tillage Radish Belowground Growth at Harvest

Figure 4. Belowground growth of tillage radishes (Raphanus sativus L. ‘Nitro’) in the Harvested

Radish (HR) treatment at week (56 days) arranged with increasing soil compaction level bulk

42

density from left to right: (CL 1) 0.96 g*cm-3, (CL 2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4)

1.44 g*cm-3, and (CL 5)1.6 g*cm-3.

Appendix C. Coral Bells Aboveground Growth at Harvest

Figure 5. Aboveground growth of coral bells (Heuchera micrantha var. diversifolia ‘Palace

Purple’) in the (A) cold termination radish (CTR) treatment, (B) herbicide termination radish

(HTR) treatment, and (C) no radish control (NRC) treatment at week (56 days) arranged with

increasing soil compaction level bulk density from left to right: (CL 1) 0.96 g*cm-3, (CL 2)

1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4) 1.44 g*cm-3, and (CL 5)1.6 g*cm-3.

43

CHAPTER 3. EVALUATION OF GERMINATION PERCENTAGE OF FIVE

TILLAGE RADISH CULTIVARS AT VARYING AIR TEMPERTURES AND WATER

AMOUNTS

Marcus D. Jansen1 and Grant L. Thompson1

1Iowa State University Department of Horticulture

Modified from a manuscript to be submitted to HortTechnology (2021)

Abstract

Tillage radish (Raphanus sativus L.) has but utilized as a cover crop for alleviating soil

compaction and scavenging nutrients in agricultural crop productions systems and has potential

as a soil amendment practice to address compacted soils on landscape construction sites. Unlike

the more predictable seasonal periods of sewing, terminating, and incorporating a tillage radish

cover crop in an annual agronomic crop rotation, the timing and environmental condition of

building and landscape construction is more variable, which may have an effect on cover crop

radish seed germination. The aim of this study was to determine the percentages of germination

produced by different tillage radish cultivars at a range of temperatures and water conditions.

Our study utilized a germination box in growth chamber system and a soilless media jiffy tray in

growth chamber system to quantify percent germination of five cover crop radish cultivars across

eight air temperature and four water amounts. We found that air temperature significantly

affected germination percentage across all cultivars, and the optimal range for germination was

23-33℃ where the cultivars produced a mean germination range of 91.10 - 79.14%. Water

amount significantly affected Nitro, Smart, and Badger radish cultivars, suggesting that irrigation

management during the germination of these cultivars may be crucial to their success. In our

study conditions, Nitro radish cultivar has the highest overall mean germination percentage

(68.03%) and the highest germination percentage at many of the air temperature and water

44

treatment combinations. Germination percentages were not particularly comparable between the

blotter paper and soilless media substrate at the optimal air temperature ranges. The findings of

this study have potential value in providing estimated germination rates of these cultivars across

a range of conditions, including sub-optimal air temperatures and water levels.

Introduction

Soil compactions contributes to soil degradation and a major concern for agriculture

(Hamza and Anderson, 2005) and other uses across the world (Batey, 2009). Soil compaction

occurs when mechanical force is applied to the soil increasing soil bulk density and decreasing

soil porosity (Soil Science Society of America, 2008). Plant growth is negatively impacted when

reduced poor space suppresses the movement of air and water within the soil profile impacting

both gravitational and plant available water (Dexter, 2004). The breakdown of soil aggregates,

reduced pore spaces, and reduced pore space connectivity associated with soil compaction also

directly affects the ability of plant roots to explore the soil and grow (Bengough et al., 2006). In

agricultural production systems, research has substantially characterized mechanically induced

soil compaction (Shah et al., 2017). Furthermore, efforts have been made to understand

interactions and compound effects of a variety of factors including soil physical properties,

wheeling, number, of passes, production practices, soil structure, soil water status, and crop

rotations (Hamza and Anderson, 2005). In respond to the negative effects on crop yield (Nyéki et

al., 2017), the industry has considered solutions to soil compaction including tillage, crop

rotation, and cover crops. (Forte et al., 2018; Mirzavand and Moradi-Talebbeigi, 2020)

Raphanus sativus L. var. longipinnatus (Daikon radish) along with its comparable varieties

(tillage radish, forage radish, oilseed radish) and cultivars have proven their worth as a cover

crop species and are valued for having a substantial taproot with the capacity to penetrate

45

compacted soils (Williams and Weil, 2004; Chen and Weil, 2010) and accumulate nutrients from

deep within the soil profile to shallower depths where nutrients may be made available to other

plants (Toom et al., 2019). A tillage radish cover crop is typically planted by seed in the fall

(Charles et al., 2006) and terminated by winter kill or spring herbicide application (Dean and

Weil, 2009b; Chen and Weil, 2011). This soil amendment practice can be implemented annually

and provide compounded benefits from year to year in an annual cropping system. (Hodgdon et

al., 2016)

The deleterious effects of soil compaction and their causes in agricultural production are

largely similar to soil compaction resulting from construction activities associated with urban

development and built landscapes. In the landscape construction and installation process,

mitigation strategies are required to alleviate compaction for landscape plant establishment

(Sloan et al., 2012). Conventional practice utilized in the landscape management industry

includes tillage (Layman et al., 2010), soil fracturing (Somerville, May and Livesley, 2018), and

the incorporation of compost (Sax et al., 2017; McGrath et al., 2020; Somerville et al., 2020).

While there has been less published research on the use of cover crops for amending compacted

landscape soils, there is increasing interest in the potential for tillage radish to be used for

decompaction in post-construction urban soils since it can be easily seeded and would result in

less traffic and shipping costs compared to compost application. However, unlike the more

predictable seasonal periods of sewing, terminating, and incorporating a tillage radish cover crop

in an annual agronomic cropping system, the timing of building and landscape construction is

more variable. Thus, if tillage radish is to see wide-spread use in constructed landscapes to

remediate deleterious post-construction effects, a necessary initial step is needed to determine the

46

range of temperature and water conditions that will allow for acceptable percentages of tillage

radish seed germination. While much work has looked into germination of vegetable radishes

(Raphanus sativus L.) for edible consumption (Kordon, 2010; Bakhshandeh and

Gholamhossieni, 2019) and germination of other brassica species (Russo, Bruton and Sams,

2010; Alias et al., 2018), less work has addressed the germination of cover crop radishes

(Raphanus sativus L. var. longipinnatus) at a range of environmental conditions. Therefore, the

objective of this study is to determine the percentages of germination produced by different

tillage radish cultivars at a range of temperatures and water conditions.

Methods & Materials

This study was conducted to determine the variation of seed germination rates amongst

different cultivars of cover crop radish when grown at varying levels of moisture and air

temperature. Five cultivars of cover crop radish seed were tested: TapMaster Brand Radish

(Albert Lea Seeds, Albert Lea, MN), Tillage RadishⓇ (La Crosse Seed, LaCrosse, WI), Nitro

Radish (Green Cover Seed, Bladen, NE), Smart Radish (Green Cover Seed, Bladen, NE), and

Badger Brand Daikon Radish (Hood River Seed Company, Evansville, IN). These cover crop

radish cultivars were selected based on their popularity and availability amongst cover crop

growers in the Midwest. Sample seeds were obtained from the various manufacturers in the

summer of 2020 for this study. Following the protocol used by the Iowa State University Seed

Testing Lab under the guidelines of AOSA Rules for Testing Seed (Association of Official Seed

Analysts, 2017), with treatment modifications described below, radish seeds were placed on

blotter paper (AHL628-838880, Anchor Paper Co, St. Paul, MN) in clear plastic germination

47

boxes (26.67 x 15.72 x 3.97cm, K601 box, Flambeau, Middlefield, OH) and germinated in

controlled environment growth chambers (PGC 10;Percival Scientific, Perry, Iowa).

Treatments

Following established procedures of the Iowa State University Seed Testing Lab, it was

experimentally determined that the blotter paper saturation water volume was 35 ml per

container (0.08mL * cm-2). Two water volume treatments above and one below the blotter paper

saturation point were selected to represent germination under wetter and dryer conditions

respectively, resulting in four water rates (55, 45, 35, 25 mL per area of the container). Seven air

temperatures (8, 13, 18, 23, 28, 33, and 38℃) were tested and were simulated in growth

chambers; one temperature at a time. Ranges of variables were selected to replicate the variety of

environmental conditions that cover crop radish seeds would experience while being sown at

different times of the year. Historical environmental data was references to determine increments

that would be representative of conditions in Iowa (Table 1). Treatment combinations were

repeated across 4 replications. All cultivars were tested across all combination of water amount

and temperature (5 cultivars * 4 water amounts * 7 temperatures * 4 replicates = 560

experimental units)

Blotter Paper Germination Trial

Blotter paper in the germination boxes was hydrated with the respective water treatment

immediately prior to seeding. One hundred seeds were placed on the hydrated blotter paper using

a custom trip tray to fit the germination boxes. Seeds were organized into 5 rows of 20 seeds

with 12.3 mm spacing within row and 25.4 mm spacing between rows. Each germination box

was an experimental unit. Immediately following seed placement, germination boxes were

48

closed, sealed in clear plastic bags for moisture retention, and placed in the growth chambers.

Four identical growth chambers were used for this experiment with each growth chamber

representing one of the 4 temperature treatment repetitions. Experimental units were randomized

within each chamber. Growth chambers were set to a twelve-hour day length and temperature

was set based on the seven treatment temperatures. Light was supplied through fluorescent bulbs

at a target intensity of 250 molm-2s-1. Seeds were left to germination in the growth chamber

for seven days, and percent germination was manually counted on day seven. Identification of

seed germination status was influenced by AOSA guidelines (Association of Official Seed

Analysts, 2017). Seedlings that possessed both a radical and cotyledons were identified as

germinated. Seedlings that did not possess all structures needed for successful plant

establishment were identified as abnormal. Seedlings that had not produced any part of a

seedling were identified as dead seeds. Abnormal seedlings and dead seeds were combined into a

classification of ungerminated seeds for statistical analysis. Regression analysis was utilized to

identify significance at P < 0.05 between treatment. Means were separated by Tukey’s

significant difference test at P < 0.05.

Media Germination Trial

A secondary germination trail was executed with comparable methodology to the original

trial with the exception of soilless potting media as the substrate instead of blotter paper. This

trial was executed to observe the effect of these treatment combinations on germination rates

when subject to a media substrate that may more closely emulate the seed to soil contact and

water relations of field conditions. For this trial, only the most optimal air temperatures of 23, 28,

33℃ were tested. With the change in substate conditions, water amounts were amplified from

49

55, 45, 35, 25 mL per area of the container in the original trial to water amounts with comparable

water percentages in media. It was experimentally determined that the media saturation water

volume was 459 mL (0.16mL * cm-3). Two water volume treatments above and one below the

blotter paper saturation point were selected to represent germination under wetter and dryer

conditions respectively, resulting in four water rates (323, 459, 588, and 717 ml per volume of

container). Rectangular (35.6cm x 12.8cm x 6.4cm) growing trays (Jiffy tray 51, 505, Jiffy

Products, Zwijndrecht, Netherlands) were filled with a peat-based germination potting mix

media (Pro-Mix PGX with Biofungicide, Premier Tech Horticulture, Rivière-du-Loup, Quebec,

Canada). Media was hydrated with the respective water treatments prior to seeding. One hundred

seeds were sown at a depth of 2cm in the media in individual trays for each experimental unit.

Seeds were organized into 4 rows of 25 seeds with 13.1 mm spacing within row and 25.4 mm

spacing between rows. Seeded trays were sealed with plastic bags for moisture retention and

placed in the growth chambers. Chamber conditions and data collection procedures were

identical to the original trial.

Table 1. Mean air temperatures (C and F) and rainfall (mL per month and per day) for the

months of the growing season (April – November) in Polk County, Iowa form 1990-2019.

Month Mean

Temperature (C)

Mean

Temperature (F)

Mean Rainfall

per Month (mL)

Mean Rainfall

per Day (mL)

April 9.47 49.05 3900.88 130.03

May 15.71 60.28 5179.11 172.64

June 21.19 70.14 5707.87 190.26

July 23.04 73.47 4503.88 150.13

August 21.75 71.15 4470.62 149.02

September 17.83 64.10 3581.07 119.37

50

Table 1 (Continued)

Month Mean

Temperature (C)

Mean

Temperature (F)

Mean Rainfall

per Month (mL)

Mean Rainfall

per Day (mL)

October 10.80 51.44 2656.57 88.55

November 2.86 37.15 1813.71 60.46

*mean of environmental data for Polk County Iowa 1990-2019

Results

There were significant interactions of cultivar*air temperature (P < 0.001) and water*air

temperature (0.0004) along with the main effects of cultivar, water, and air temperature (all P <

0.001, Table 2). When averaged across all temperature treatments, water level treatments

resulted in significant differences for Badger (P = 0.0218), Nitro (P = 0.0008), and Smart (P =

0.0355) cultivars (Table 2). When averaged across all water level treatments, temperature

treatments resulted in significant differences for all cultivars (all P < 0.001, Table 1). Across all

air temperatures and water levels, Nitro produced the highest mean germination percentages

(68.03 2.65%), followed by Smart (64.63 2.87%), Badger (60.14 3.23%), Lacrosse (49.71

3.39%), and TapMaster (47.48 3.27%). Across all cultivars and water levels, the highest

mean germination was achieved at 28℃ (91.10 0.73%), 23℃ (87.49 1.17%), 33℃ (79.41

1.24%), 18℃ (62.60 2.71%), 13℃ (34.33 2.62%), 38℃ (33.91 3.46%), and 8℃ (17.15

1.99%). Across all cultivars and air temperatures, the highest mean germination was achieved at

the 45ml (66.19 2.70%) water level, followed by 55ml (65.19 2.65%), 35ml (55.94%

2.78%), and 25ml (44.67 2.88%). At the optimum air temperature (28℃), Badger (93.50%

1.24%) and Smart (92.56 1.67%) produced statistically comparable and higher percentages of

mean germination than Nitro (91.13 2.10% and Lacrosse (90.50 0.89%) produced

significantly lower percentages of mean germination; and TapMaster (87.81 1.77%) produced

51

the lowest percentage of mean germination (Table 3, Figure 1). At the least optimal air

temperature (8℃), Nitro (38.88 5.35%) produced the highest mean germination; Smart (22.44

2.59%) produced a significantly lower percentages of mean germination; and Badger (13.06

2.59%), Lacrosse (6.38 2.98%) TapMaster (5.00 0.98%) produced statistically comparable

percentage of mean germination that were the lowest amongst the cultivars (Table 3, Figure 1).

At the optimum water amount (45mL), all of the cultivars produced statistically different mean

germination percentages descending from Smart (79.43 4.23%), Nitro (71.11 5.26%),

Badger (68.89 5.93%), Lacrosse (57.21 6.89%), and TapMaster (54.29 6.63%, Table 4,

Figure 2). At the least optimum water amount (25mL), all of the cultivars had statistically

comparable mean germination percentages with Nitro (52.14 5.96%), Smart (51.96 5.77%),

Badger (44.93 6.90 %), Lacrosse (39.29 7.00%), and TapMaster (35.04 6.31% Table 4,

Figure 2). In the regression analysis between germination percentages of radish cultivars in the

blotter paper germination trial and the jiffy tray media germination trial, the coefficient of

determination (R2) was fairly low and comparable between cultivars with Badger (R2 = 0.2362),

Lacrosse (R2 = 0.2483), Nitro (R2 = 0.1672), Smart (R2 = 0.2645), and TapMaster (R2 = 0.2348,

Figure 3)

Table 2. Analysis of variance table for radish germination within and between treatments

throughout the blotter paper trial in the growth chambers and between treatments within specific

cultivars.


Radish Germination (%)

cultivar 5 4 48.17 <.0001

water 4 3 73.52 <.0001

air temperature 7 6 377.15 <.0001

52

Table 2 (Continued)


cultivar*water 20 12 0.73 0.7192

cultivar*air temperature 35 24 7.52 <.0001

water*air temperature 28 18 2.60 0.0004

cultivar*water* air

temperature

140 72 0.61 0.9943


Badger water 4 3 3.35 0.0218

Lacrosse water 4 3 1.29 0.2808

Nitro water 4 3 6.01 0.0008

Smart water 4 3 2.96 0.0355

TapMaster water 4 3 2.13 0.1005


Badger air temperature 7 6 48.38 <.0001

Lacrosse air temperature 7 6 87.67 <.0001

Nitro air temperature 7 6 28.75 <.0001

Smart air temperature 7 6 49.03 <.0001

TapMaster air temperature 7 6 84.42 <.0001

53

Table 3. Tukey test comparison between cultivars within the air temperature treatments in the

blotter paper trial in the growth chambers.

Treatment Cultivar Mean N Tukey Grouping

8C

Nitro 38.88 16 A

Smart 22.44 16 B

Badger 13.06 16 C

LaCrosse 6.38 16 C

TapMaster 5.00 16 C

13C

Nitro 60.56 16 A

Smart 50.13 16 B

Badger 30.38 16 C

LaCrosse 16.13 16 D

TapMaster 14.44 16 D

18C

Nitro 81.75 16 A

Smart 80.31 16 A

Badger 68.88 16 B

LaCrosse 44.31 16 C

TapMaster 37.75 16 C

23C

Nitro 91.75 16 A

Badger 90.94 16 A

Smart 90.56 16 AB

LaCrosse 84.13 16 BC


54

Table 3 (Continued)


28C

Badger 93.50 16 A

Smart 92.56 16 A

Nitro 91.13 16 AB

LaCrosse 90.50 16 AB

TapMaster 87.81 16 B

33C

Badger 81.00 16 A

LaCrosse 80.94 16 A

Smart 80.00 16 A

Nitro 77.56 16 A

TapMaster 77.56 16 A

38C

Badger 43.25 16 A

Smart 36.38 16 A

Nitro 34.56 16 A


LaCrosse 25.56 16 A

55

Figure 1. Mean percent germination of radish cultivars across tested air temperatures (C )

throughout the blotter paper trial in the growth chambers.Letters indicate Tukey test statistical

groups

Table 4. Tukey test comparison between cultivars with the water amount treatments in the blotter

paper trial in the growth chambers.


25ml

Smart 52.14 28 A

Nitro 51.96 28 A

Badger 44.93 28 A

LaCrosse 39.29 28 A


Air Temperature (ºC)

Cultivar: Badger Lacrosse Nitro Smart TapMaster

Ger

min

atio

n (

%)

a

b

c

c

c

a

b

c

d

d

a

a

b

c

c

a

a

ab

c

bc

a

ab

bab

a

aa

aa

a

aaa

aa

56

Table 4 (Continued)


35ml

Nitro 65.43 28 A

Smart 62.04 28 AB

Badger 57.64 28 AB

LaCrosse 49.21 28 AB


45ml

Nitro 79.43 28 A

Smart 71.11 28 AB

Badger 68.89 28 ABC

LaCrosse 57.21 28 BC


55ml

Nitro 75.29 28 A

Smart 73.21 28 A

Badger 69.11 28 AB


LaCrosse 53.11 28 B

57

Figure 2. Mean percent germination of radish cultivars across tested water amounts (mL )

throughout the blotter paper trial in the growth chambers.Letters indicate Tukey test statistical

groups

Water amount (mL)

Cultivar: Badger Lacrosse Nitro Smart TapMaster

Ger

min

atio

n (

%)

a

a

a

a

a

a

abc

c

bc

ab

ab

a

b

b

a

aa

a

b

ab

ab

ab

58

Figure 3. Regression model of percent germination between radish cultivars in the blotter paper

germination trial and the jiffy tray media germination trial.R2 models for each cultivar are

indicated

Discussion

Air Temperature

Air temperature was a highly significant factor affecting germination for all cultivars,

which aligns with findings from vegetable radish research (Bakhshandeh and Gholamhossieni,

2019). As a cool season crop, radishes are tolerant of lower temperatures (15-20C) during

growth (Gunay, 2005), but benefit from higher temperatures (20-28C) for seed germination.

Our study found 28C to be the optimum temperature for seed germination across the radish

cultivars, which is higher than that found by Bakhshandeh and Gholamhossieni, 2019 (21.9C)

but comparable to that found by Lindgren and Browning, 2011 (27C) with Raphanus sativus L.

y = 0.9335x + 0.2367

R² = 0.2362

y = 1.1836x - 22.663

R² = 0.2483

y = 0.4943x + 36.675

R² = 0.1672

y = 0.8941x + 2.8076R² = 0.2645

y = 0.9393x + 4.2792

R² = 0.2348

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100

Jiff

y t

ray

med

ia g

erm

inat

ion (

%)

Blotter paper germination (%)

Badger

LaCrosse

Nitro

Smart

TapMaster

Linear (Badger)

Linear (LaCrosse)

Linear (Nitro)

Linear (Smart)

TapMaster

Badger

LaCrosse

Nitro

Smart

TapMaster

59

This optimum temperatures is also similar to other Brassica germination studies concluding that

Brassica spp. has an optimum germination temperature of 25C (Alias et al., 2018) and Brassica

rapachinenis has an optimum germination temperature of 29C (Motsa et al., 2015). Within seed

germination research, effects of changes in soil temperature on germination are often determined

by studies that draw conclusions from changes in air temperature in a growth chamber without

soil. While daily soil temperate can be predicted from daily air temperature (Zheng, Hunt and

Running, 1993; Barman et al., 2017), soil temperature and air temperature do not change at the

same rate (Zhan et al., 2019). In our study, the blotter paper substrate temperature was more

likely similar to the air temperature due to its lower thermal mass when compared to that of soil

in field conditions. The blotter paper did not provide the same insulting capacity that a field soil

would, therefore air temperature may have had a stronger effect on germination in this study than

under typical field soil conditions. Additionally, the relatively small volume of the jiffy trays

would not provide the same insulating and temperature buffering capacity of a larger volume of

soil that would occur in field conditions. Germination trends may be observed based on studies

like ours, that manipulate air temperature, but may not directly reflect germination trends in the

field due to this different between air and soil temperature.

Water

Water level was less significant than air temperature in affecting germination. Research

has indicated a reduction in germination rate and germination percentage with a decrease in

water potential for edible vegetable radish seeds (Bakhshandeh and Gholamhossieni, 2019) and

other species (Atashi et al., 2015; Bakhshandeh and Gholamhossieni, 2018). While we did see an

overall similar trend amongst our water level treatments, there were some cultivars in our study

60

that showed non-significant responses to water across the ranges tested. Water amount was

significant for germination rates of Nitro, Smart, and Badger cultivars, indicating that irrigation

management during the germination of these cultivars may be important for their performance.

Under lower water conditions, all cultivars produced similar germination percentages. The

significant interaction between water and air temperature indicates that the effect of water was

not uniform amongst all temperatures. At optimum temperatures a change in water amount may

have less of an effect on germination, yet at the higher and lower temperature extremes an

increase in water has the potential to increase germination.

Cultivar

While germination of edible vegetable radishes cultivar have been studied extensively

(George and Evans, 1981; Martinez-Villaluenga et al., 2010), little published research has

compared cultivars of cover crop radishes. Within the germination environment of our study,

there were some prominent trends in cultivar performance. Within our study, Nitro had the

overall highest germination percentage and consistently had the highest germinations percentage

in many of the temperature and water treatment combinations. Nitro germinated well across a

range of air temperatures and had higher germination rates than the other cultivars at lower air

temperatures in particular from 8-18C (Nitro means: 38.88 – 81.75%, Bager means: 13.06 –

68.88%, Tapmaster means: 5.00 – 37.75% ). The ability of a cultivar to germinate in a range of

temperatures would be valuable in post construction landscapes since construction schedules

may begin early or end late in the season when temps are less predictable. Within more ideal air

temperatures (23-33C) all cultivar produced mean germination rates that were above 75% and

statically comparable to one other, so the difference between these cultivars may be negligible at

these air temperatures compared to cooler air temperatures. At 38C all cultivars produced

61

consistently lower germination rate (25.56 - 43.25%), which aligns with finding suggesting that

Raphanus sativus L. (edible vegetable radish) shows insufficient levels of germination above

35C air temperatures (Lindgren and Browning, 2011; Bakhshandeh and Gholamhossieni, 2019).

Germination percentages may not be identical in field conditions, but we can suggest that trends

between cultivars may be comparable.

Blotter Paper vs. Media Substrate

The trial using soilless media substrate in Jiffy trays was conducted to compare radish

germination at the optimal germination temperatures of 23, 28, and 33℃ found in the blotter

paper trial. While the blotter paper method is a standard seed testing protocol (Association of

Official Seed Analysts, 2017) to estimate field germination, this trial allowed us to consider

germination rates in a soilless media that more closely replicates the seed-to-soil contact of field

soil conditions. When comparing germination results between the blotter paper and Jiffy tray

trial, there was not good correlation between the germination methods (R2 = 0.17 – 0.26).

However, within each trial method, the relative differences in germination between the cultivars

was consistent. This indicates that while numerically different and non-comparable germination

results were found in each trial, the general outcome of cultivar performance was consistent.

Overall, lower germination rates were observed in the soilless media trial, which may have been

influenced by the nature of the substate and the depth of the media. On the blotter paper the

seeds remained in contact with the wet paper, thus were better able to hydrate. Conversely, in the

soilless media, gravity pulled water away from the surface where seeds were sown. We observed

the bottom of the Jiffy trays were wetter than the top of the media, supporting this explanation.

Additionally, germination rates responded similarly to changes in water amount for both testing

62

trials. While some comparable trends were observed in air temperature between the testing trials,

testing a wider range of air temperature in the soilless media substate would allow for a stronger

comparison of that trend.

Application

Estimated germination rates of these cultivars has been identified by the producers and

distributers of these radish cultivars at optimal conditions within an agricultural cropping system.

The findings of this study have potential value in providing estimated germination rates of these

cultivars across a range of conditions, including sub-optimal air temperatures and water levels.

From these findings, management recommendations could be for using these radish cultivars in

sub-optimal conditions, such as landscape contractors looking to reduce site soil compaction by

using tillage radish. Understanding the expected germination rate of a radish cultivar at a certain

combination of air temperature and water amount can assist in management decisions to produce

an acceptable cover crop germination rate. Outside of temperatures ranging from 18-28℃,

higher water rates improved germination for Nitro, Smart, and Badger cultivars compared to

lower water amounts. Yet even with supplemental water, mean germination in these cultivars

was between 26 – 97.25%, showing that while water helped improve germination at low

temperatures, the air temperature had the stronger negative effect on germination. Nitro cultivar

had the best performance across a range of air temperatures, though the amount of water did

matter too. Higher water amounts improved Nitro radishes germination by 46% at 8℃ and by

34% at 38℃, suggesting that supplemental water may be necessary for adequate germination at

these temperature extremes. Our results show, for some tillage radish cultivars, temporary

supplemental irrigation during the germination and establishment may be beneficial during sub-

63

optimal temperature conditions. Under ideal temperatures (23-28℃) lower water amounts

achieved acceptable germination rates and additional water did not substantially improve

germination, suggesting that for the tillage radish cultivars tested, the need for higher soil

moisture was less under optimal conditions. Therefore, the potential need for supplemental

irrigation for adequate germination is reduced when air temperature is within the optimum range

for these cultivars. This study indicates that temperature and water differences in germination

exist among cultivars and that experimentation may be needed to identify cultivars that would

work best under local conditions and where supplemental management such as irrigation may be

needed to obtain desirable results.

Limitations and Future Research

This preliminary study of tillage radish cultivar germination at varying levels of air

temperature and water levels yielded promising results and identified some limitations that

should be addressed in future research. Our methods followed the standard procedure of counting

germination percentage after seven days (Association of Official Seed Analysts, 2017), but

higher germination percentages may have been observed if a longer germination window were

tested. Further research is needed to observe rate of germination of these cultivars, beyond

percent germination at seven days. Actual field conditions, including variations in temperature,

moisture, and other sources of variability that were not included in this controlled environment

experiment, may result in different tillage radish germination and growth. Such differences

would need to be field tested for future application. Furthermore, this study only observed

percent seed germination. While proper seed germination is necessary and a crucial component

in the development of a radish plant, a high germination percentage does not guarantee superior

growth and cover crop success. The threshold of desired germination rate and respective

64

management decisions may vary amongst growers of tillage radishes. Conclusion can be made

about the germination rates of these radish cultivars at certain air temperatures and water levels,

but further research is needed to better understand how the seedlings of these cultivars put on

growth and perform as a cover crop under these environmental conditions. Quantifying growth

and biomass production of these cover crop cultivars and making comparison to percent

germination could be another area of research helping to better understand the overall

performance of these cultivars at varying environmental conditions.

Conclusion

These studies utilized a germination box in growth chamber system and a soilless media

jiffy tray in growth chamber system to quantify percent germination of five cover crop radish

cultivars at varying combinations of air temperature and water amount. We found that air

temperature significantly affected germination percentage across all cultivars, and the optimal

range for germination was 23-33℃. Water amount significantly affected Nitro, Smart, and

Badger radish cultivars, suggesting that irrigation management during the germination of these

cultivars may be crucial to their success. In our study conditions, Nitro radish cultivar has the

highest overall germination percentage and the highest germination percentage at many of the air

temperature and water treatment combinations. Germination percentages were not particularly

comparable between the blotter paper and soilless media substrate at the optimal air temperature

ranges. Providing estimated germination rates of these cultivars across a range of conditions,

including sub-optimal air temperatures and water levels is a potential application of these

findings. More research is needed to better understand the effects of germination percentage on

tillage radish growth and cover crop success in reducing compaction of developed landscape

soils.

65

Acknowledgements

The authors gratefully acknowledge Elizabeth Hurley-Blewett, Abigail Enos, Connor

Evers, Cody McKune, and Pete Lawlor for greenhouse and laboratory assistance; Alex Lindsey

for statistical consultation; Dr. Ajay Nair and Dr. Marshall McDaniel for guidance and project

design; support from the Iowa State University Horticulture Research Station; and funding from

the Iowa State University Department of Horticulture. This research was also funded by NIFA

Hatch grant IOW03657, “In search of sustainable landscape and horticultural production

systems.” The authors appreciate the generosity of Albert Lea Seeds, Green Cover Seed, Hood

River Seed Company, and La Crosse Seed for donating seed for this research project. Data and

results are independent and not influenced by the material support from Albert Lea Seeds, Green

Cover Seed, Hood River Seed Company, and La Crosse Seed. Mention of trade names in this

publication does not imply endorsement by Iowa State University of products named, nor

criticism of similar products not named.

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CHAPTER 4. GENERAL CONCLUSIONS

General Discussion

Compaction continues to be a chief cause of soil degradation all over the world

(McGarry, 2003). This challenge has led to a substantial body of research targeted at

understanding the phenomenon of soil compaction, identifying the causes, and quantifying the

effects on plant growth and production (Unger and Kaspar, 1994; Batey, 2009; Hu et al., 2021).

Due to the major economic (Oskoui and Voorhees, 1991; Graves et al., 2015) and environmental

(O’Sullivan and Simota, 1995; Hu et al., 2021) costs of compacted soils on agricultural crop

production, much work has gone into identifying the major causes of soil compaction and

potential remediation practices for crop production systems (Hamza and Anderson, 2005; Kumar

et al., 2018). Cover crops have been widely utilized as a soil management practice in

conventional and organic agricultural systems for numerous benefits including soil stabilization,

weed suppression, and nutrient management (Fageria, Baligar and Bailey, 2005; Justes, 2017).

Cover crop species with extensive tap roots have shown to be effective at reducing soil

compaction, scavenging nutrients, and providing root channels for subsequent crops, such that

the term “bio-drillers” is sometimes applied to these tap rooted cover crops (Williams and Weil,

2004). Amongst these cover crop species, tillage radish (Raphanus sativus L.) and other cover

crop radish selections such as Daikon radish (Raphanus sativus L. var. longipinnatus) have

grown in popularity (Gruver, Weil and Lawley, 2016), valued for significantly higher soil

penetrating capacity (Chen and Weil, 2010) and nutrient accumulation ability (Toom et al.,

2019).

The causes of compaction and their detrimental effects in agricultural production are

comparable to soil compaction resulting from construction activities associated with urban

70

development and built landscapes. Soil compaction has shown to deter root exploration and

establishment of landscape plants since increased soil bulk density results in fewer, smaller, and

less well-connected soil pores for roots to inhabit (Kozlowski, 1999; Day, Seiler and Persaud,

2000). The negative impacts of soil disruption and the need for urban soil remediation to

improve plant growth have been identified (Sloan et al., 2012). Tillage (Layman et al., 2010),

soil fracturing (Somerville, May and Livesley, 2018), and compost incorporation (Sax et al.,

2017; McGrath et al., 2020; Somerville et al., 2020) are conventional remediation practices

utilized in the landscape management industry. While cover crops or temporary vegetative cover

has been utilized for erosion control or weed suppression on the exposed soil along roadsides or

construction sites (By and Busby, 2014), less published research has addressed the use of cover

crops for amending compacted landscape soils. Recognizing the successful reduction of

compaction from a tillage radish cover crop in agricultural cropping systems and the need for

compaction remediation in post-construction urban soils where landscape plants will be

established, we sought to test the potential of tillage radish for such uses.

The negative effects of soil compaction on agricultural crop production and landscape

plants establishment are both well documented. Management of a tillage radish cover crop in

agricultural cropping systems has been well studied and practiced, while management of a tillage

radish cover crop in landscape construction systems has yet to be defined. The difference in

cover crop implementation between these two systems is influenced by dissimilarity in the

seasonality of the practices and the nature of the subsequent “cash crop”. The period of sewing,

terminating, and incorporating of a tillage radish cover crop in an agricultural cropping system

falls in a relatively predictable pattern and can be implemented on an annual basis in rotation

with the annual cash crop. The timing of construction activites is more variable, therefore the

71

implementation of a tillage radish cover crop could be needed at any time during the growing

season and may only happen once before a semi-permanent perennial landscape is installed.

Recognizing the major difference between these two systems, we are beginning to explore

potential management practices of a tillage radish cover crop for compaction remediation on

landscape sites.

Our research found that ‘Nitro’ tillage radish produced comparable amounts of root and

shoot growth across a wide range of soil bulk densities (0.96 – 1.44g*cm-3) with significant

reduction in growth at a soil bulk density of (1.6g*cm-3). This result suggests that a tillage radish

cover crop can be effect in many compacted soils situations but may be limited by extremely

compacted soil which may require mechanical means of decompaction. Additionally, we

discovered instances where a tillage radish cover crop has the potential to reduce growth of the

following model landscape plant (coral bells) following certain cover crop termination methods.

Standard cover crop termination and incorporation practices prior to the planting of a perennial

landscape plant may be different than those prior to the planting of an agricultural crop, and we

have yet to determine those standards. Acknowledging the variety of environmental condition at

which a tillage radish cover crop would be sewn throughout the landscape construction and

installation season, we sought to identify the germination rates of five cover crop radish cultivars

across a range of air temperatures and water amounts. We found that air temperature

significantly affected germination percentage across all cultivars, and the optimal range for

germination was 23-33℃. Water amount significantly affected some of the top performing

cultivars, suggesting that irrigation management during the germination of these cultivars may be

crucial to their success. Amongst the cultivars, ‘Nitro’ radish had the highest overall germination

percentage and was the most successful across a range of air temperatures and water treatment

72

combinations. From these findings, we could potentially provide estimated germination rates of

these cultivars across a range of conditions, including sub-optimal air temperatures and water

levels. While our research has begun to explore the potential of a tillage radish cover crop and

the necessary management practices for alleviation of soil compaction on landscape construction

sites, continued work is needed to develop a holistic understanding of this soil amendment

practice and its applications for the landscape industry.

Recommendations for Future Research

Our preliminary study testing tillage radish at various compaction levels and subsequent

model ornamental plant establishment did allows us to draw some promising conclusions but

also exposed some limitations that should be addressed in future research. The use of a

controlled greenhouse environment for the study allowed for the simulation of the experimental

soil compaction treatments but imposed some challenges in mimicking field conditions and

management practices better suited to field research. There may be value in documenting case

studies with landscape contractors to observe the performance of a tillage radish cover crop in

field soils and the effect on perennial plants in an actual landscape. We saw first-hand the

negative effects that a cover crop can cause under certain management practices, so future

research is needed to determine effective cover crop termination and incorporation strategies for

landscape systems. There is a need to specifically address incorporating or not incorporating the

terminated tillage radish into the soil before adding landscape plants, the availability and timing

of nutrients with biomass decomposition, and residual effects of radish termination practices.

Coral bells (Heuchera micrantha var. diversifolia ‘Purple palace’) were used as a model plant in

this study and observed across eight weeks of growth, but landscapes are often comprised of a

diverse pallet of herbaceous and woody plant species and can be long-lived once well

73

established. Future studies are needed to observe the effects of a tillage radish cover crop on

other popular landscape species and the long-term effect of this practice on perennial plants in

order to establish more generalizable responses of plant growth and establishment. Using cover

crops in mixes has shown to be a valuable strategy in agricultural systems (Blesh, 2018), and

could be another area of research in developed landscape systems. Tillage radishes used in

conjunction with other cover crop species may result in multifunctional outcomes to aid in the

establishment of perennial ornamental landscapes.

We did find some promising results in a preliminary study quantifying the germination of

tillage radish cultivars at varying environmental conditions, but there is substantial opportunity

for further research to address the management practices need for the germination and successful

growth of a tillage radish cover crop at diverse conditions throughout the year. Our study

observed gemmation percentages of radish seeds at seven days, but in an applied landscape

setting seeds would continue to germinate past seven days with potentially higher germination

percentages. Further research is needed to observe rates of germination of these cultivars beyond

germination at seven days and to translate germination to actual radish growth and development.

Field conditions, including variations in temperature, moisture, and other sources of variability

that were not included in this controlled environment experiment, may result in different tillage

radish germination and growth. Such differences would need to be field tested for future

application. Our study was also limited by the fact that we only observed germination percentage

and did not regard the future growth and success of those radish plants. While we can make

generalized conclusions from our study about the germination rates of these radish cultivars at

certain air temperatures and water levels, further research is needed to better understand how the

seedlings of these cultivars put on growth and perform as a cover crop under these environmental

74

conditions. Developing a well-rounded understanding of cover crop radish seed germination and

subsequent landscape performance at varying environmental conditions will contribute to

outlining the practices to managing a tillage radish cover crop for compaction remediation.

The agriculture industry has served as a guiding example for how tillage radish and other

bio-drilling cover crops can be used to reduce the stress of soil compaction. There is potential to

also take advantage of this practice on landscape construction sites, and future research has the

capacity to help make it possible. Future research will also need to integrate the agricultural and

horticultural aspects of cover crops and perennial landscapes with the realities and timing of

construction activities for this to be widely adopted by the building and construction trade. This

research has taken some of the initial steps towards identifying the versatility of tillage radish

and addressing the manage practices needed to support practitioners in the landscape industry.

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