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.