Ecologically Based Weed Management for Organic Farms

The Weed Paradox

Weeds are the most costly category of pests in crop production.

Organic farmers consider weeds a topresearch priority.

Yet …

The Weed ParadoxIf it weren’t for weeds, the world might have already run out of fertile topsoil.

Why?

Weed Ecology 101

Weeds are pioneer plants that perform several functions after fire, flood, clearcutting, tillage, or other disturbance leaves soil exposed:

•Protect the soil from erosion.•Absorb and conserve nutrients.•Replenish organic matter, feed soil life.•Provide habitat for other organisms.•Begin the process of succession.

Cropland Weeds:

• Are adapted to frequently disturbed, fertile soils.

• Germinate in response to light and other tillage cues.

• Grow and develop rapidly.• Accumulate and respond to

abundant nutrients.• Reproduce prolifically by

seed, or by rhizomes, tubers, or other vegetative means. Common ragweed and

jimsonweed in tomato

How Humans “Make” Weeds

• Decide what plants are “unwanted.”

• Hold succession back at early stages to produce desired crops (tillage).

• Create open niches (bare soil).

• Introduce exotic plants into the region.

Replacing Herbicides with Steel?

The Organic Farmer’s Dilemma:

How can I manage weeds without tilling the soil to death?

Ecologically based weed management.

Step 1: Know the Weeds

• Correct ID for major weeds• Life cycle, growth habit, season

– Summer annual, winter annual, simple perennial, invasive perennial

• Reproduction, seed germination cues• Impacts on crop production• Response to control tactics (cultivation, etc.)• Weak points = opportunities for management

Summer Annual Weeds

• Emerge, grow, set seed, and die within one season

• Frost-tender• Rapid, aggressive growth in

hot weather• Reproduce prolifically by

seed• Worst in warm season crops• Usually controlled by timely

cultivation and mulching

Pigweed (left) and lambsquarters (right) are summer annuals

Example: the Pigweeds

Smooth pigweed (left), spiny amaranth (center), and Palmer amaranth (right) emerge in late spring, triggered by high, fluctuating soil temperatures. They respond to high soil N, set viable seed ~10 days after pollination, and die with frost.

Palmer Amaranth

• Grows 1.5 times as fast as other pigweeds

• 2 tons/ac in 30 days• Heat-loving; 95 – 110°F• Can complete life cycle on

one rainstorm• 500,000 seeds per plant• Severe crop losses• Herbicide resistant strains

Young Palmer amaranth, showing pointsettia-like habit of growth

Pigweed – some Weak Points• Seedlings tiny and vulnerable to:

Shallow cultivationFlame weedingMulchingCrop competition (shading)

• Susceptible to cereal grain allelopathy Winter cover crops grown to maturity & rolled

• Very small seed – little nutrient reserveEmerging seedling dependent on soil nutrientsSlow release N favors crop over pigweeds.

Invasive Perennial Weeds• Emerge from perennial

underground rhizomes, tubers, bulbs, roots, etc.

• Propagate and spread by rhizomes, tubers, etc.

• Indefinite life span• May also reproduce by seed• Very difficult to control by

cultivation or mulching• The most damaging weeds

Yellow nutsedge, an invasive perennial of moist soils, reproducesthrough small tubers.

Example: Purple Nutsedge

• Heat and drought tolerant• Spreads by rhizomes & tubers• Intense competition for water

and nutrients• Allelopathic to many crops• Tremendous underground

reserves (5 – 8 tons/ac)• Rapid regrowth after tillage• Grows through black plastic

Can you find the peppers in thisnutsedge?

Nutsedge – some Weak Points• Draws down reserves to regrow after tillage

Repeat cultivation at 3 – 4 leaf stageUndercut basal bulb in cultivation when practical

• Shade-intolerant and short grow taller, dense-canopy crops (e.g., snap bean)

• Susceptible to allelopathySorghum, sweet potato in crop rotation

• Dislikes mycorrhizae build soil life, grow mycorrhizal host crops • Solarization breaks tuber dormancy (esp. purple)

Broadleaf versus Grass

Broadleaf seedling growing point above ground – easy to kill by shallow cultivationor flaming when weeds notmore than 1 inch tall.

Grass seedling growing point below ground – harder to kill by cultivation;flaming ineffective; cultivateas soon as they emerge.

Step 2: Minimize Niches for Weed Growth

Open niche in time: soil left exposed between harvest of one crop and emergence of the next crop is an open invitation to the weeds.

Open niche in space: bare soil between crop rows requires cultivation for weed control until the crop canopy closes.

Cover crops fill post-harvest niche

Timely planting of winter rye + hairy vetch after vegetableharvest (left) fully occupied the bed, while common chickweed did the job when no cover was sown (right).

Relay planting closes post-harvest niche more rapidly

Clover cover crops were interseeded into brassicas (left) and tomato (right) when vegetables were at mid-growth. After vegetable harvest, clover rapidly covers the ground.

Mulch partially closes weed niches

An organic mulch effectively closes the bare soil niche for weed seedlings. Established grasses or perennial weeds will break through.

Step 3: Select Best Weed Control Tools

• Match cultivation equipment with bed and row spacings.

• Choose tools and tactics to address expected major weeds in the crops to be grown.

• Consider soil and climate conditions in selection of cultivation tools.

Step 4: Keep the Weeds Guessing through Crop Rotation

• Include high diversity of production and cover crops in the rotation.

• Alternate cool and warm season vegetables.• Follow several years of annual vegetables with 2–

5 years in perennial sod.• Vary timing of tillage, planting, and harvest.• Vary method and depth of tillage.• Vary cultivation and other control tactics.

Perennial Sod Break in Annual Crop Rotation can Reduce Weed Problems

• Rotate to grass-clover for 2 – 3 years.

• Disrupts annual weed life cycles.

• Ground beetle and other weed seed consumers thrive without tillage.

After ten years in vegetables, this field wasrotated to oats overseeded with red clover(shown here after oat harvest), which reduced annual weed populations.

Step 5: Grow Vigorous, Competitive Crops

• Maintain healthy, living soil.• Choose vigorous, locally-adapted varieties.• Use high quality seed.• Transplant vigorous “starts.”• Optimize planting dates, growing conditions.• Select row and plant spacings to close canopy.• Feed and water the crop, not the weeds.

Carrots are weed-prone, but the ‘Danvers’ type varieties have more vigorous tops that close canopy within 60 days, aiding weed control.

Season Extension

Summer squash (left) and snap bean (right) were grown for an early market under low tunnels (covers now removed). Good season extension technique maintained near-optimal conditions for the crop, and thereby facilitated weed control.

Feed and Water the Crop, Not the Weeds

In-row drip irrigation waters lettuce (left) and tomato (right) while leaving between-row weeds dry.

Avoid Over-fertilizingAn all-legume green manure has released enough soluble N to stimulate the growth of pigweed in this broccoli.

Slower-release N sources (e.g., compost, or roll-down rye + vetch), or banding N (e.g., feather meal) in the row would have reduced pigweed growth relative to the crop.

Step 6: Put the Weeds Out of Work - Grow Cover Crops!

Cover crops:•Occupy open niches•Protect, feed, and restore the soil•Provide habitat and restore biodiversity•Outcompete weeds for light, water, nutrients•Suppress weed seedlings by allelopathy•Alter the light stimulus for weed germination

Winter Rye (Secale cereale) with Hairy Vetch (Vicia villosa)

Advantages of grass + legume:• Higher biomass and better

weed suppression than either alone (3 – 4 t/ac)

• Rye supports vetch vines to provide maximum shade.

• Balanced C:N “feeds” soil better, no N leaching or tieup, less apt to promote weeds with excessive soil N.

Buckwheat (Fagopyrum esculentum)

• Frost tender summer annual• Short life cycle, ideal for 30-50 day

fallow period• Planted anytime from spring frost

date to August• Rapid canopy closure (15-20 days)• Smothers weeds, reduces weed

seed germination• Beneficial insect habitat

Cowpea (Vigna unguiculata)

• Heat loving summer annual• Rapid canopy closure, dense

shade• Tolerant to drought and low

soil fertility• Strong weed competitor• Good N fixer• Varieties with long vines and

heavy canopy.

Forage Radish (Raphanus sativus)

• Aggressive weed competitor

• Rapidly covers ground, inhibits weed germination

• Plant in August, canopy closes in 3 weeks, winterkills at 15 – 20°F

• Weed suppression persists into spring after winterkill

Fodder radish (shown)and daikon radish fightweeds and make excellent cover crops.

Step 7: Manage the Weed Seed Bank

Minimize “deposits”:• Remove weeds before they propagate• Exclude new invaders through sanitationMaximize “withdrawals”:• Stale seedbed• Encourage weed seed predation

Draw Down the Weed Seed Bank

Stale seedbed or false seedbed:• Till soil, prepare seedbed• Wait until weeds emerge• Cultivate or flame-weed• Repeat if needed• Make final cultivation very

shallow – better yet, flame• Plant immediately after final

weeding

Weeds have emerged in this stale seedbed, and it is time to cultivate again.

Weed Seed Consumers

Organic mulches (left) and low-growing vegetation provide habitat for ground beetles (right) and otherweed seed consumers.

Step 8: Knock the Weeds Out at Critical Times

• Start with a clean seedbed.

• Get the weeds when they are small.

• Keep crop clean through its Minimum Weed-Free Period.

• Hit invasive perennials at 3–4 leaf stage.

• Prevent weed propagation.

Check that Seedbed!

• Lightly stir the soil surface and look for “white threads”

• Waiting just five days from seedbed preparation to planting can mean a weedy crop.

• Cultivate shallowly in sunny weather to knock out germinating weeds.

Timely Cultivation – When Weeds are Small

• Hoeing or cultivating at this stage saves fuel, soil, labor.

• Within-row weeds this small can be killed by burying with soil (if crop is large enough).

• Some weeds become very hard to kill when larger.

• Timely cultivation is especially important early in crop establishment.

Minimum Weed-Free Period

• 4-6 weeks for vigorous starts and large seeded crops.

• 8-12 weeks for some small seeded or slow growing crops.

• Later-emerging weeds do not hurt yields by competition.

• Late weeds can vector or promote disease, contaminate harvest, or set seed.

This broccoli has passed through its minimum weed-free period.