structure, growth, and development of cool-season grasses

8/3/2019 Structure, Growth, and Development of Cool-Season Grasses

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The Cool-Season Turfgrasses: BasicStructures, Growth and Development

Department of Crop and Soil Sciences - Cooperative Extension

Prepared by Peter Landschoot, professor of turfgrass science

Turfgrasses are narrow-leaved grass species that form a uniform, long-lived groundcover that can tolerate traffic and low mowing heights (usually two inches or below).Only a few grass species produce acceptable turf in the northern U.S. These grassesare referred to as the cool-season turfgrasses. (Warm-season turfgrasses includespecies that are best adapted to southern areas of the U.S. and are not discussed inthis publication.) This publication covers the basic structures of grass plants, how theygrow and develop, and how to identify the different species of cool-season turfgrasses.

Contents

Introduction

The Basic Structures of Grass PlantsGrowth and Development

Introduction

The study of cool-season turfgrasses begins with learning the basic structures of grassplants and how they develop from seed to mature plants. Once you understand howturfgrasses growand develop, it is easier see how they function as long-livedcommunities and how they tolerate traffic, mowing, and other problems.
http://turfgrassmanagement.psu.edu/cool_season_turfgrasses_part1.cfm#GROWTHhttp://turfgrassmanagement.psu.edu/cool_season_turfgrasses_part1.cfm#STRUCTUREShttp://turfgrassmanagement.psu.edu/cool_season_turfgrasses_part1.cfm#INTROhttp://turfgrassmanagement.psu.edu/cool_season_turfgrasses_part1.cfm#GROWTHhttp://turfgrassmanagement.psu.edu/cool_season_turfgrasses_part1.cfm#STRUCTUREShttp://turfgrassmanagement.psu.edu/cool_season_turfgrasses_part1.cfm#INTRO


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Fig. 1. Diagram of a mature grass plant.

The Basic Structures of Grass Plants

A mature, unmowed grass plant iscomposed of leaves, roots, stems, anda seed head. The diagram of a grassplant in Fig. 1 shows these basicstructures. Keep in mind that somegrass species do not have all thestructures shown and that mowedgrasses typically lack flower stems andseed heads.

Leaves

A grass leaf is divided into three parts:the blade, sheath, and collar region(Fig. 2). The blade is long and narrowand grows more or less horizontallyaway from the main shoot. The sheathis the portion of the leaf thatenvelopes the shoot or stem. Thecollar region is located where the

blade and sheath meet and may ormay not have structures called thecollar, ligule, and auricle (Fig. 3). Thesmooth area on the back side of theleaf where the blade and sheath meetis the collar. It is usually a lightercolor than the blade and may continueacross the width of the leaf or bedivided in half by a large mid vein. Aligule is a thin piece of tissue that extends just above the top of the leaf sheath and can

vary in size and shape. An auricle is another small piece of leaf tissue that grows fromthe collar and can also vary in size and shape.


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Fig. 2. Diagram of a grass leaf,including blade, collar region, andsheath.

Fig. 3. The collar regionof a grass leaf with liguleand auricle.

Roots

Roots: Roots are the below-ground part of a grass plant that anchor it in the soil andtake-up water and nutrients. Turfgrass roots are fibrous, branching, and very slender.There are two types of root systems in grasses, the primary and the secondary. Thedifference between the two will be explained later in this section.

Stems

Three types of stems occur in grasses; the crown, horizontal stems (rhizomes andstolons), and the flower stem. Although the crown is a stem, it does not look like theother stem types found in grasses. It is very small (just a fraction of an inch long),

white, and completely enclosed by leaf sheaths. The crown is located in a protectedposition between the roots and shoot near the soil surface.

Horizontal stems begin to form in the crown and develop into rhizomes or stolons.Rhizomes grow below ground for a short distance, then rise to the soil surface to formnew shoots. In some grass species, rhizomes produce growing points (often referred toas nodes) which give rise to roots and shoots forming new or daughter plants.Rhizomes are usually white. Stolons grow above-ground and form nodes which give riseto new plants. Stolons are green and can creep over other grasses and bare spots inlawns, often forming circular patches.

Flower stems are also formed in the crown, usually in late spring or early summer in


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Fig. 4. A spikelet, thebasic unit of the seedheads.

most cool-season grasses. Typically, they are not seen in turf since they are mowed offbefore they reach maturity. On unmowed grass, flower stems grow vertically and giverise to seed heads.

Seed Heads

The seed head is the flowering part of the grass plant. Thebasic unit of the seed head is called the spikelet. A spikelet is

made up of grass flowers, the small stalks that support them,and bracts (small, papery leaves that cover the flowers) (Fig.4). There are three types of seed heads based on thearrangement of the spikelets; panicle, spike, and raceme (Fig.5). In the panicle type, the spikelets are borne on branchesthat are arranged along the central or main stem. The mainstem is basically an extension of the flower stem. Kentucky

bluegrass is a turfgrass with a panicle-type seed head. Thespike-type seed head has spikelets that are borne directly onthe main stem. Perennial ryegrass is a turfgrass with a spike-

type seed head. In the raceme type, spikelets are borne onvery short branches along a main stem. True raceme seedheads are rare in grasses and none of the cool-seasonturfgrasses produce them. Crabgrass, a common annual grass

weed, has a modified spike-like raceme.


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Fig. 5. Three types of turfgrass seed head; panicle, spike and receme.

Fig. 6. Diagram of grassseed.

Growth and Development

Grass Seed and Seed Germination

The seed of grass is really a dried fruit called a caryopsis(Fig. 6). The caryopsis is made-up mainly of the embryoand endosperm. The embryo contains the beginnings of the

leaves, growing points, and roots of the grass plant. Theendosperm makes up the bulk of the caryopsis andcontains the food (primarily starch) required by thedeveloping plant as it germinates and grows. The entirecaryopsis is surrounded by the pericarp, sometimesreferred to as the ovary wall (Fig. 7). The caryopsis andpericarp are enclosed by two papery structures called thelemma and palea.

The basic requirements for germination of turfgrass seed

are adequate moisture, favorable temperatures, andoxygen. The first step in seed germination is absorption of

water (sometimes referred to as imbibition). The rate atwhich grass seed absorbs water depends on the amount ofwater present and the permeability of the seed. As water isabsorbed, the seed swells. Shortly thereafter, enzymesproduced by the embryo break down the endosperm andconvert the starch into carbohydrates. Carbohydrates can

be used directly by the embryo and developing seedling for energy and growth.


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Fig. 7. Cross-section of caryopsis including the embryo,endosperm, and pericarp.

Table 1. Optimum temperatures for seed germination of cool-seasonturfgrasses.

Species Temperatures (F)

Kentucky bluegrass 59 - 86

Rough bluegrass 68 - 86

Chewings fescue 69 - 77

Creeping red fescue 59 - 77

Sheep fescue 59 - 77

Tall fescue 68 - 86

Perennial ryegrass 68 - 86

Annual ryegrass 68 - 86


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Fig. 8. Germinating grass seed with radicle and firstleaf breaking through seed coat.

The first evidence that the seedhas germinated occurs whenthe embryonic root or radicle

breaks through the seed coat(Fig. 8). Soon after, the firstleaf emerges from the seed. Atthis point germination hasoccurred and the plant is

considered a seedling.

Leaf Growth andDevelopment

The first true leaf to emergefrom the seed duringgermination is enclosed withina protective structure called the coleoptile. Soon after germination, the coleoptile andfirst leaf begin to elongate and grow towards the soil surface. The coleoptile stops

growing just after it reaches the soil surface, but the leaf continues to elongate andbreaks through the coleoptile sheath (Fig. 9). As the leaf expands and elongates itbegins to produce its own food through a process called photosynthesis. Soon after thefirst leaf emerges, the developing seedling produces a second leaf from the growingpoint or node enclosed in the coleoptile. All succeeding leaves follow the same route -emerging from the growing point and growing upward within the folds of the olderleaves. Eventually, the coleoptile withers away and is no longer visible.


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Fig. 9. Turfgrass seedling.

e grow ng po nt t at g ves r se toleaves on mature turfgrass plants is atthe tip of the crown and is called thestem apex. This structure looks like asmall dome with ridges risingalternately from each side (Fig. 10).These ridges are the beginnings of thenew leaves. As a leaf begins todevelop, it encloses the entire stemapex. This leaf continues to elongateand expand and eventually forms afully-developed leaf with a blade,sheath, and collar region. The fact thatgrass leaves begin to grow from thestem apex located at the base of theplant is the main reason why grass can

be mowed without sustaining seriousinjury. Growth continues from the

base of the leaf after a portion of the

leaf blade is mowed off.

New leaves are produced from otherridges on the stem apex and emergefrom the folds of the older leaves.Thus, the oldest leaves are on theoutside of the plant and the youngestare located in the center of the plant.Turfgrass leaves live for a period oftime then die and are replaced by new

ones. Under favorable environmentalconditions, the number of leaves perplant remains the same as new leavesreplace those that die.

The rate of leaf growth is dependenton many factors includingtemperature, moisture, nutrition, and to some extent, daylength. Optimumtemperatures for leaf growth among the cool-season turfgrasses range from 60 to 75F.Leaf growth increases with increasing daylength as long as temperatures are within the

optimum range and moisture is adequate. Application of nitrogen fertilizer can greatlyincrease leaf growth if moisture and temperature are not limiting.


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Fig. 10. Stem apex of grass plant. The ridges are the beginnings of new leaves. Asa leaf begins to develop, it encloses the stem apex.

Root Growth and Development

Soon after the radicle emerges from the seed, the first true roots develop from theembryo. These roots are called primary roots and begin taking-up water and nutrientsfrom the soil when they are fully developed. Although the primary roots continue tofunction for up to a year after germination, water and nutrient uptake is graduallytaken over by the secondary roots (sometimes referred to as adventitious roots) which

become more numerous as the grass plant matures. Secondary roots are produced fromnodes in the crown or from nodes on horizontal stems.


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Fig. 11. Diagram of the root tip includingmeristem, root cap, region of cellelongation, and root hairs.

ur grass roots are very erent romleaves and stems (Fig. 11). The growingpoint or meristem is located at the tip ofthe root. This is where all new root cellsare produced. The meristem is protectedfrom the abrasive effects of the soil by astructure called the root cap. In the area

just behind the meristem, new cells growmostly in length. This area is called theregion of cell elongation. Behind theregion of cell elongation, cells begin todevelop into tissues that absorb water andnutrients. Among these tissues are roothairs - tiny hair-like outgrowths that growfrom the root surface into the surroundingsoil. The primary function of root hairs is

water and nutrient uptake. Root hairsnumber in the billions for a fully-developed root system and can greatly

increase the amount of soil the rootscontact. Water and nutrients aretransported from root hairs to the interiorof the root where special conductingtissues move water and nutrients to theleaves and shoots.

Turfgrass root growth is affected mainly bysoil temperature, moisture, and oxygen.The optimum temperatures for root

growth of cool-season grasses are lowerthan those for shoot growth. Although the optimum temperature range for rootingdiffers somewhat among turfgrass species, most cool-season turfgrasses produce the

best root growth at soil temperatures between 50 and 65F. When temperatures reach90F in the surface inch of soil, Kentucky bluegrass root growth is greatly reduced.Roots of cool-season grasses can grow at soil temperatures below 50F, but growthslows dramatically as temperatures approach freezing (32F). Root growth is greatestfor cool-season grasses during spring and fall and much reduced during the summerand winter months.

Turfgrasses take-up water from the soil through their root system. The amount of waterthe roots absorb will depend primarily on the number of roots, the depth of rooting,and the amount of water in the soil. Since the rooting depth of cool-season grasses isusually between 2 and 6 inches, most water absorption initially occurs near the soilsurface. As the surface water is depleted, roots begin using up water deeper in the soil.

A well-developed and actively-growing root system can take advantage of this deepersoil moisture as surface moisture is depleted in dry periods. Contrary to popular belief,roots do not seek out water, instead they grow more vigorously and proliferate where

water is available.


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Fig. 12. Rhizomes of Kentucky bluegrass.

Turfgrass roots need an adequate supply of oxygen for normal growth anddevelopment. Severely compacted soils have limited supplies of oxygen and will notsupport good root growth even when favorable temperatures and moisture levels arepresent. Too much water will also deplete the soil of oxygen and cause deterioration ofturfgrass roots. Soils with loose, crumbly structure and good drainage are ideal for rootgrowth and development.

Other factors that have an effect on root growth and development are soil pH,fertilization practices, salt concentrations, herbicides, diseases, and insects. These will

be discussed in other sections of this manual.

Stem Development and Tillering

Of the three stem types mentioned previously, the crown is the most important. It givesrise to leaves, secondary roots, and other stems. Because new leaf growth occurs at the

base of the plant, grass plants can tolerate mowing and some other types of minorinjury to leaf blades. However, crowns can be damaged by mowers when blades are settoo low. When this happens, plants are severely damaged and new leaf growth isunlikely.

Since new secondary roots areproduced from the crown, some of theexisting root system can be damaged

without killing the plant providedthat the root-initiating portion of thecrown is not injured. Sod producersroutinely sever a portion of the grassroot system with sod harvesters, thentransport the sod to a new location.

The newly-laid sod generates a newroot system from secondary rootsformed in the crown.

Rhizomes and stolons begin to growfrom nodes in the crown and breakthrough the surrounding leaf sheathsto spread laterally. Rhizomes ofKentucky bluegrass and creeping red

fescue grow beneath the soil surface and then turn up towards the soil surface to form

new shoots (Fig. 12). Some other grasses (mostly warm-season grasses and weedgrasses) have long rhizomes that produce nodes that can branch and produce shootsand roots, forming new plants.

Rhizomes are a desirable trait in turfgrasses because they allow plants to send newshoots into areas that are thin or damaged by traffic, drought, and/or disease. Kentucky

bluegrass is the premier sod grass in the northern U.S. because its rhizomes allow turfto knit and hold together as the sod is cut, rolled, and lifted. Kentucky bluegrass is adesirable species for use in athletic fields because its rhizomes provide superior footing


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Fig. 13. Stolons of creeping bentgrass.

Fig. 14. Two tillers developing from thecrown of a grass plant.

for athletes.

Stolons grow along the soilsurface and can creep overestablished turf (Fig. 13). Newshoots are produced from nodesor from tips of the stolon as itturns upward. Although thestoloniferous cool-seasonturfgrasses, rough bluegrass andcreeping bentgrass, are desirablefor some applications, they can be

very troublesome weeds if mixedwith other lawn grasses since theyform light-colored, circularpatches as they creep over themore desirable turfgrasses.

Tillers are shoots that develop

from crown tissues and grow vertically within the sheaths that surround the crown (Fig.14). Mature tillers produce leaves, stems, and root systems; thus, they can functionindependently of the mother plant. Tillers increase the shoot density of lawns byreplacing shoots that die in winter and summer. Individual tillers live for about a yearand formation of new tillers is stimulated by cool temperatures, short daylengths,moderately low mowing heights, and high mowing frequencies. Peak tiller formationoccurs in early spring and fall. Turfgrass stands are long-lived because dying shoots areconstantly being replaced by new tillers. This process is so gradual that the transition isunnoticeable.

Carbohydrates - The Real Plant Food

Lawn fertilizers are often marketed as plantfood. Although most people realize thatnutrients from fertilizers are required byplants for proper growth and development,they may not realize that fertilizers are notreally plant food. Plants make their ownfood through photosynthesis, a chemicalreaction in leaves involving water, carbon

dioxide (CO2) and light energy. The endproducts, carbohydrates, are used by plantsfor energy and growth and are the trueplant food.

Carbohydrates can be stored in stem andcrown tissues when they are made fasterthan they are used. Storage is greatest infall and is beneficial since the plant needs


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carbohydrates for recovery from injurywhen turf is damaged by pests, drought, heat, and mower injury the following year.Depletion of carbohydrates is fastest in spring, especially under low mowing heightsand high nitrogen fertility. If depleted too quickly, the turf may go into the summermonths in a weakened state. This is one reason why turfgrass professionals do notapply excess amounts of nitrogen and mow below optimum heights of cut in spring.

structure, growth, and development of cool-season grasses

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