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Organic Weed Management. Alisha Rupple , University of Arkansas Heather Friedrich, University of Arkansas . Weeds: Top Issue for Organic Farmers. Successful Management Requires Multiple approaches Continual effort Knowledge of the biology of weeds species

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organic weed management
Organic Weed Management

Alisha Rupple, University of Arkansas

Heather Friedrich, University of Arkansas

weeds top issue for organic farmers
Weeds: Top Issue for Organic Farmers
  • Successful Management Requires
    • Multiple approaches
    • Continual effort
    • Knowledge of the biology of weeds species
      • Reproduction, lifecycle, establishment annual, perennial, wandering perennial, broadleaf, grass
      • Cornell Organic Weed Database,
basic weed ecology
Basic Weed Ecology
  • Weeds are nature’s way of keeping bare ground covered and increasing biodiversity
  • Dynamic system involving the interaction of weeds, crops, humans and environment
  • Factors affecting weed ecology are identical to those affecting crop ecology:
    • Light, temperature, water, pH, nutrients, organic matter, insects and diseases, etc
impact of high weed pressure
Impact of High Weed Pressure

Crop yield

Weed numbers or size

Compete with crops for nutrients, water, and light

Reduced yields

Lower crop quality

Harbor pest insects and diseases

Increase irrigation costs

But, complete elimination of weeds is unnecessary

multiple prevention and elimination strategies
Multiple Prevention and Elimination Strategies
  • Cultural
  • Mechanical
  • Biological
  • Chemical (organically


“Many hammers approach.”

Liebman and Gallandt, 1997

organic weed management practices crop rotations on organic farms mohler johnson 2009
“Organic weed management practices” Crop Rotations on Organic Farms, Mohler & Johnson, 2009
cultural strategies
Cultural Strategies
  • Buy quality crop seed with low/no weed seeds present
  • Do not allow weeds to form seed
  • Thoroughly compost (>130°F for ≥15 days) all manure and plant residues to ensure destruction of weed seed
  • Stale seedbed technique
    • Prepare soil for planting and bring weed seeds to the surface; allow weeds to germinate, kill weeds with light tillage/minimal soil disruption. May be repeated. Plant main crop.
cultural practices
Cultural Practices

Improve crop competitiveness

  • Improve soil tilth, aeration, and fertility to optimize crop growth
  • Increase crop density through narrow row spacing and increased seeding rate
  • Use transplants, rather than seed, when possible
  • Plant at optimal soil temperatures to prevent slow germination of crop
  • Choose competitive crop cultivars
  • Manage fertility according to crop needs; avoid excess application
cultural practices9
Cultural Practices

Reduce weed numbers

  • Mulch (wood chips, mow and blow, paper, living, plastic, etc)
  • Use weed-suppressive cover crops
    • Quick germinating, high biomass
    • Field with high weed pressure may warrant full year of cover cropping and fallow to reduce weeds
  • Crop rotations
    • altering narrowly spaced crops with closely spaced crops, shallow rooted/deep rooted crops, cold/warm season crops
  • Intercrop
    • Clover underseeded in sweetcorn
  • Prevent seeds from germinating by blocking light, can smother out some weeds
  • Conserve water, minimal soil disruption
  • Use local resources: straw, fabric, wood, newspaper, plastic
  • Be careful of weed seeds in straw or hay
    • Avoid hay, unless you know its free of weeds
  • Especially good for perennial systems: blueberries, blackberries, flowers, trees
  • Living mulches – ie constant cover of clover on orchard floor

Wood chips


Shredded paper


cover crops
Cover Crops
  • Smother weeds by out-competing for light, water, nutrients
  • Release allelopathic chemicals that suppress weed germination
  • May reduce weed emergence by 75-90%
  • e.g. sudan grass, buckwheat, annual rye grass, sesbania, many more

Field pea-oat-mustard cover crop

Managing Cover Crops Profitably, 3rd ed. SARE

crops profitably. 3rd ed. SARE

Red Clover

crop rotations
Crop Rotations
    • Weeds tend to infest crops with similar life cycles
    • Change crop ecology: shallow/deep roots, cold/warm season, row/drilled crops, foliage density, and heavy/light feeders
  • Change cultural practices: cultivation, mowing, fertilization, herbicide application, and planting/harvest dates
physical and mechanical practices
Physical and Mechanical Practices
  • Mowing
    • Prevents seeding
    • Depletes storage reserves
    • Better control for broadleaves
  • Soil solarization
    • Effective control of winter annuals
    • Limited control of perennials
    • Cost prohibitive on large acreages
    • Avoid tillage deeper than 3” after solarization
  • Hand weeding
  • Cultivation
  • Flaming
  • Should be shallow to lessen disturbance to weed seed bank
  • Better for perennial and biennial control than annual weed control
    • Exhaust root system by depleting storage reserves
    • Requires 6-8 timely treatments in yr 1, then 3-5 the following year

Wheel hoe

Thoroughly clean equipment before moving it between fields to prevent weed transport

Various hoes

potential downsides of cultivation
Potential Downsides of Cultivation
  • Exposes bare ground:
    • increased erosion,
    • decreased biodiversity,
    • speeds decomposition of OM,
    • increases water run-off
  • Major cause of soil compaction
  • Cost: expensive equipment, fuel
  • Should not be done in wet conditions
use cultivation wisely
Use Cultivation Wisely
  • USDA-ARS research showed organic methods can increase OM more than conventional no-till
    • Teasdale et al., 2007. Potential Long-Term Benefits of No-Tillage and Organic Cropping Systems for Grain Production and Soil Improvement. Agron J. 99:1297-1305
  • Negative effects of tillage may be offset by the use of cover crops and additions of organic matter (compost, manures, mulch, etc)
  • Must still use caution to avoid negative effects of cultivation
type of horticultural cultivators
Type of Horticultural Cultivators
  • Spyders™
  • Torsion weeders
  • Spring hoe weeders
  • Finger weeder
  • Basket weeder
  • Multiple-component weeder frame
  • Brush weeder
  • Rotary tilling cultivators
  • Rear- or front-tine tillers
  • Hand implements: push-pull hoes, hand scrapers, etc

Sustainable Vegetable Production from Start-Up to Market, Grubinger, 1999

other cultivators
Other Cultivators

Sustainable Vegetable Production from Start-Up to Market, Grubinger 1999, NRAES 104

  • Intense heat sears the leaf, causing the cell sap to expand and disrupt cell walls
  • seedlings are most susceptible
  • Broadleaf weeds are more susceptible than grasses
  • May be used in wet soil conditions


Sustainable Vegetable Production from Start-Up to Market, Grubinger, 1999

uses for flaming
Uses for Flaming
  • Stale seedbed technique:
    • Planting delayed after seed bed preparation (tillage, irrigation, etc)
    • Flaming knockdowns flush of weed seedlings prior to planting
  • Peak emergence technique:
    • Crop seeded promptly after seedbed preparation
    • Just before crop germinates, flaming used to kill weed seedlings
    • Good method for direct-seeded, slow-germinating crops
    • Glass or plastic can be used as a crop-germination indicator: crop grown under cover germinates 2-3 days before uncovered crop; flaming should occur when crop germinates under cover
  • Post-emergent flaming:
    • Emerged crops protected by: directing flame away from crops, shielding the crop, or flaming at a time when crop stems are resistant to heat
    • Older plants able to recover from heat damage, while young seedlings are killed
flaming tips
Flaming Tips
  • Bigger weeds and wet weeds are harder to kill
    • Target weeds while seedlings up to 3 to 4 leaves
    • Avoid flaming with morning dew
      • Light drying winds and hot days increase effectiveness
  • Avoid flame deflection by soil clods or excessive dust = protect weeds
  • Match equipment for your needs
    • Save on fuel, time
  • Make time for adjustments
    • Burner placement, fuel pressure, tractor speed
  • How much is enough?
    • When you squeeze plant leaf between finger and thumb, want to see your thumb imprint = cells have burst and weed will die/setback
biological control practices
Biological Control Practices
  • Insects: may consume large numbers of weed seeds or feeding injury to plant or vector virus
    • Thistle & adult thistle-head weevil, Rhinocyllusconicus
    • Multiflora rose & rose rosette disease transmitted by fungi or a (mite)
  • Selective grazing
    • Sheep: clean fields after harvest
    • Weeder geese: useful against grass weeds and in perennial systems
  • Biofumigation
    • Use of Brassica species (canola, Indian mustard) as cover crop or in rotation
    • May be incorporated or left as residue
    • Brassicas produce glucosinolates, which may by converted to cyanate compounds during decomposition
    • Cyantes toxic to many bacteria, fungi, nematodes, insects, and germinating seeds
chemical control
Chemical Control

Organic options

  • Corn gluten meal (pre-emergent herbicide)
    • Suppresses many common grasses and herbaceous weeds
    • WeedBan™ and Corn Weed Blocker™
    • Look for non-gmo sources
  • Commonly based on vinegar or lemon juice or clove oil ingredients (post-emergent burndown herbicide)
    • Perennials may require multiple applications
    • Corrodes metal sprayer parts
    • Burnout™, Bioganic™, AllDown ™, MATRAN™, and Weed Bye Bye™
    • Post-emergent chemicals are phytotoxic (burn plant tissue); use caution when applying in crops

Organic herbicide in an orchard

Cost can be decreased by knowing pattern of weed distribution (spot treatment v. overall application

12 steps to sustainable weed management mark schonbeck
12 Steps to Sustainable Weed Management – Mark Schonbeck

Pre-season Planning

Step 1. Know the weeds on your farm.

Step 2. Plan cropping systems to minimize open niches for weeds.

Step 3. Keep the weeds guessing.

Step 4. Design the cropping system and select tools for effective weed control.

preventive cultural practices
Preventive (Cultural) Practices

Step 5. Grow vigorous, competitive crops.

Step 6. Put the weeds out of work – grow cover crops.

Step 7. Manage the weed seedbank: minimize deposits and maximize withdrawls.

control tactics
Control Tactics

Step 9. Utilize biological processes to enhance weed control.

Step 10. Bring existing weeds under control before planting weed-sensitive crops and long-term perennial crops.

Step 8. Knock the weeds out at critical times.

No-till organic farm; Weed-free bed of weed-sensitive onion crop.

enhancing and fine tuning the weed management strategy
Enhancing and Fine-tuning the Weed Management Strategy

Step 11. Keep observing the weeds and adapt practices accordingly.

  • Keep notes
  • What is suitable for one crop may not be for another

Step 12. Experiment and stay educated. Keep up on new developments and practices.

  • Night time cultivation
  • Soil solarization
  • Others…

Weed free bed of carrots

  • Using multiple approaches (“many hammers”) to manage weeds will yield greater impact than relying on a few practices.
  • Develop a weed management strategy that is designed for the needs of your farm.
  • Big Hammers
  • Competitive crops
  • Rotation
  • Cover Crops
  • Mulches
  • Weed predators
  • Livestock/grazers
  • Cultivation tools
  • Rollers/roller-crimper
  • Flamers
  • Growers Observation
  • Little Hammers
  • Solarization
  • Organic herbicides
  • Bioherbicides
  • Soil microorganisms
  • Crop-weed interactions
  • The Sustainable Weed Control Rag, Mark Schonbeck, e-Organic,,
  • Sustainable Vegetable Production from Start-up to Market, Grubinger, 1999. NRAES-104
  • Crop rotation on Organic Farms: A Planning Manual, Mohler and Johnson, 2009. NRAES-177
  • Managing Cover Crops Profitably, 3rd ed. SARE
  • Michigan Field Crop Pest Ecology Management. Cavigelli et al. 2000, MSU Extension Bulletin E-2704
  • Cornell Organic Weed Management Website, l

This presentation address general organic production practices. It is to be to use in planning and conducting organic horticulture trainings. The presentation is part of project funded by a Southern SARE PDP titled “Building Organic Agriculture Extension Training Capacity in the Southeast”

Project Collaborators

  • Elena Garcia, University of Arkansas CESHeather Friedrich, University of ArkansasObadiah Njue, University of Arkansas at Pine BluffJeanine Davis, North Carolina State UniversityGeoff Zehnder, Clemson UniversityCharles Mitchell, Auburn UniversityRufina Ward, Alabama A&M UniversityKen Ward, Alabama A&M UniversityKaren Wynne, Alabama Sustainable Agriculture Network