Organic Weed ControlWeed Control: Tools and Strategies

Alex Stone

Bonnie Hoffman-Cox

Oregon State University

and the Peace Corps, Bolivia

What is a WEED?

A plant that grows where it is not wanted

Photos: William B. McCloskey (1, 2) and Pat A. Clay (3); ©2003 Regents, University of Arizona

"Weeds are truly thieves in the fields. Weeds draw moisture and plant food from the soil, robbing the growing crop... they steal profits when permitted to grow unhampered."

John Deere equipment handbook (1937)

Compete With

Crops For:Light

Nutrients

Space

Aggressive growth

Success in disturbed areas

Rapid and abundant reproduction(Seed, or Vegetative propagules)

Characterized By:

Impacts of Weeds

• Estimated to account for ~10% of all crop losses globally (http://www.hdra.org.uk/organicweeds)

• National survey of organic producers: weed management was top concern (OFRF, 1999, http://www.ofrf.org)

What does a grower want?

EconomicalEffectiveDependableNon-damaging

Guiding Principles

Monitoring– Where are the weeds?

• Weed maps, species present– How can we predict weed emergence?

• Glass plate check• Weedcast modeling

In crop row

In wheel tracks

On shoulders of beds

Between crop rows

Where are the weeds?

Guiding Principles

Monitoring– Where are the weeds?

• Weed maps, species present– How can we predict weed emergence?

• Glass plate check• Weedcast modeling

Emergence Potential, Emergence Timing, and Seedling Height for 1 Weed Species

Each Feature Shown for Multiple Weed Species

Guiding Principles

Prevention– Exhaust the seed bank– Prevent build-up of weed seed bank– Grow a healthy crop

• Encourage rapid emergence and growth• Provide adequate water, nutrients, temperature• Give the crop the advantage

Timing: Get weeds when they’re vulnerable

• Intervene early• Hot, dry days• Small weeds

– White-thread stage

Guiding Principles

Non-chemical Tools and Strategies

• Cultural• Mechanical• Thermal

• Crop rotation• Cover cropping• Concentrating resources• Mulching• Stale seedbed

Cultural Weed Control

Photo by Ron Morse

Conventional control 2-year rotation

MaizeSoybean

MaizeSoybean

Low-input control 4-year rotation

MaizeSoybeanOats/Forage legume

Forage legume

Crop Rotation

Schematic key:

(Kelner, Juras & Derksen, 1996, and Derksen, 1997)

Cover croppingWeeds Bare ground

(McLenaghen et al., 1996)

Cover crop

Irrigation: overhead vs. drip

Fertilizing: broadcast vs. banding

Cover crops: strip planting legumes and grasses

Concentrating Resources Near the Crop

Legume: Sunn Hemp Grass: Sudangrass

Pho

to b

y R

on M

orse

Mulching

• Organic• Plastic• “Grown-in-place” Organic

Plastic

Grown-in-Place

Straw

Grown-in-Place Mulch

• High biomass cover crop

Grown-in-Place Mulch

• High biomass cover crop• Before planting, mechanically kill

– Disengaged flail mower– Brillion seeder (cultipacker) – Roller crimper

Roller-Crimper

Pearl Millet Cover Crop

Rolled with roller-crimper at

flowering

Advantages to rolling:

• residue in one direction

• stalks left intact

Grown-in-Place Mulch

• High biomass cover crop• Before planting, mechanically kill

– Roller crimper– Disengaged flail mower– Brillion seeder (cultipacker)

• Plant– Sub-surface tiller transplanter– No-till seeder

Photo by Ron Morse

Photo by Nancy Creamer

Photo by Nancy Creamer

Stale SeedbedGive the crop the advantage

• Small weed seeds germinate from top 1-2” of soil

• Prep field or bed• Encourage 1st flush of weeds • Weed ‘em

– Minimize soil disturbance• Plant quickly into clean bed• Less effective in early-season

(Grubinger, 1999)

http://www.extension.umn.edu/distribution/cropsystems/DC7376.html

Cultural Weed Control• Modifications to

farming practices

• Techniques can be used in combination

• Requires “long-view”planning

– Off-season– During cropping

season

Photo by Ron Morse

Mechanical Weed Control

• Physical interventions– Uproot– Cut– Bury

• Creates soil disturbance– Soil quality– Draws up new weed seeds

• Best with straight crop rows, level beds

Thermal Weed Control

• Use of heat to kill emerged weed seedlings

• Rupture plant cells, not singe or char

• Kills weeds without disturbing the soil surface

• Propane burners– Hand-held– Tractor-mounted

Timing of use • Stale seedbed• Pre-emergence for slow germ crops• Post-emergence for tolerant crops

Photo by Hoaf Co.

Unshielded multi-burner tractor-mounted

Open-flame burners

Open single-burner backpack

Shielded tractor-mounted

Reflective or insulated shields

Radiant heat (ceramic or steel)

Less fuel efficient Less preciseVery noisy

More fuel efficientMore precise

Less noisy

Hot water, steam, insulating foam

1930’s to1960’s

1980’s to 1990’s

1990’s to present

Evolution of Thermal Weed Control

Rob Heater, Stahlbush Island Farms

Considerations in flaming• Heat delivered depends on

– flame intensity– ground speed– distance between burner and

weed

• Field evenness affects heat distribution

• Less effective on some weeds– grasses– purslane

…and

Safety

Operator safety

Motor fuel tank

Check fire danger

Conclusion

• Many organic weed management options available

• Improved old ideas

• New developments

Conclusions

• Good timing and an integrated plan• Combinations of cultural, mechanical, and

thermal strategies• Creativity and resourcefulness

Keys to Successful Organic Weed Management

ReferencesBowman, G. (ed.) 1997. Steel in the Field: A farmer’s guide to weed

management tools. Beltsville, MD: Sustainable Agriculture Network.Diver, S. 2002. Flame Weeding for Vegetable Crops. Fayetteville, AR:

Appropriate Technology Transfer for Rural Areas.Grubinger, V. P. 1999. Sustainable Vegetable Production from Start-up

to Market. NRAES-104. Ithaca, NY: Northeast Regional Agricultural Engineering Service.

Liebman, M. et al. 2001. Ecological Management of Agricultural Weeds. Cambridge, UK: Cambridge University Press.

Smith, R. et al. 2000. Weed Management for Organic Crops. Pub. 7250. University of California, Division of Agriculture and Natural Resources.

Sullivan, P. 2003. Principles of Sustainable Weed Management for Croplands. Fayetteville, AR: Appropriate Technology Transfer for Rural Areas.