Southern SAWG - Weed Management

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 top research priority. Yet …

The Weed Paradox If 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 cultivation
Flame weeding
Mulching
Crop competition (shading)
Susceptible to cereal grain allelopathy
Winter cover crops grown to maturity & rolled
Very small seed – little nutrient reserve
Emerging seedling dependent on soil nutrients
Slow 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, reproduces through 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 this nutsedge?

Nutsedge – some Weak Points
Draws down reserves to regrow after tillage
Repeat cultivation at 3 – 4 leaf stage
Undercut basal bulb in cultivation when practical
Shade-intolerant and short
grow taller, dense-canopy crops (e.g., snap bean)
Susceptible to allelopathy
Sorghum, 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 cultivation or flaming when weeds not more than 1 inch tall. Grass seedling growing point below ground – harder to kill by cultivation; flaming ineffective; cultivate as 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 vegetable harvest (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 was rotated 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-fertilizing
An 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 fight weeds and make excellent cover crops.

Step 7: Manage the Weed Seed Bank
Minimize “deposits”:
Remove weeds before they propagate
Exclude new invaders through sanitation
Maximize “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 other weed 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 Smal l
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.

Southern SAWG - Weed Management

by MarkSchonbeck

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