Organic Weed Control Methods in Sustainable Agriculture
A Two-Pronged Approach to Season-Long Weed Control
Based on research by Eric Gallandt, PhD, weed ecologist at the University of Maine
Weed ecology research summary
Source: University of Maine weed ecology research
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In sustainable farming, a complete weed plan deliberately works on two fronts:
- Cut down the weeds already growing and the seeds in the soil.
- Keep new weed seeds from getting established.
The first usually takes far more time and labor than the second, but doing both together gives the best results.
Here are the basics of the two-part approach.
1 • Reduce Weed Seedling Density & Weed Seed Bank
Make Several Passes — With The Right Tools
Different Strokes
Photos: University of Maine
To thin out weeds competing with the crop, many growers rely only on "weeding," by which they mean cultivation or hoeing.
The working rate of a weeding method is how long it takes to do the job properly. From fastest to slowest:
- tractor cultivation;
- push tools such as wheel hoes;
- long-handled tools;
- short-handled tools; and
- finally hand pulling, the slowest of all.
Tractor cultivation and wheel hoes take about the same time whether weeds are few or many, but some weeds always slip through. Hand pulling and hoeing can get nearly every weed, but the time and cost rise with the number of weeds.
The lesson? several passes with tractor or push tools can make weeding much more efficient.
To learn more, see Evaluation of Scale-appropriate Weed Control Tools for the Small Farm, a Northeast Sustainable Agriculture Research & Education (SARE) project.
Target Small Weeds
Go after weeds while they're small.
Cultivation, including with hand tools, kills only some of the weed seedlings. This kill rate is highest on tiny seedlings, especially at the white thread stage, and drops sharply as weeds grow.
Some large-seeded weeds already have a lot of root at the cotyledon stage and are harder to kill by cultivation than small-seeded ones, as the photo shows: hempnettle and wild mustard (large seeds) next to white-thread lambsquarters and redroot pigweed (small seeds).
More Weedlings = More Cultivations
The bigger the weed, the smaller the share killed.
Some weeds always survive, and the number of survivors rises with the number of seedlings you started with. More seedlings means more escapes.
So as weed pressure grows, you have to cultivate more often to keep up.
Breaking the cycle of more weeds and higher weeding costs takes a broader plan that includes late-season work to:
- prevent annual weeds from setting seed; and
- prevent perennial weeds from storing up energy in their roots.
Weed When It's Dry
Physical weed control usually works by cutting, uprooting or burying small seedlings.
In moist soil, seedlings are more likely to re-root and keep growing; university field research has found that cultivation kills noticeably fewer weeds as soil moisture rises.
The lesson? Cultivate when the soil is dry.
Plant Into a Stale Seedbed
Annual weed seeds germinate in response to many environmental signals, including light, moisture, oxygen and nitrogen.
If you prepare beds ahead of time, cultivate them shallowly (about 2", or 5 cm) just before planting to kill seedlings that have emerged or are emerging.
Silage tarps can also make a stale seedbed by blocking light, a method called occultation. Prepare the bed as usual and cover it with the tarp. Weed seeds germinate in the warm, moist soil underneath, then die without light.
Flame weeding is another way to make a stale seedbed and is especially useful for slow-emerging crops such as carrots and beets. Flaming should target annual broadleaf weeds; grasses are less affected because their growing points sit at the base. The best time to flame is shortly, about a day, before the crop comes up; once the soil is cracking and the crop is emerging, the flame can kill it too. Flaming in fall can also help manage the seed bank.
2 • Prevent Weed Establishment
Mulch Crops To Spread Weeding Workload
Weed control for the whole season
Students lay about 1" of hay mulch between beds, overlapping the red plastic mulch.
Photo: University of Maine
Plastic and organic mulches do several jobs. They can:
- change soil temperature;
- hold moisture; and
- suppress weeds all season, freeing up labor for crops that need cultivating.
Plastic mulch is common for warm-season crops (such as tomatoes, peppers, eggplants, melons and basil), but it works for many other crops too (brassicas, other cucurbits, onions, garlic, head lettuce and cut flowers).
Keeping weeds down in the wheel tracks and paths between plastic-mulched beds is still hard, though. The research team's small student vegetable farm puts down a thick layer of hay between beds, at least 1" (about 2.5 cm) once compressed, for weed control all season. The mulch has to stay that thick and overlap the plastic, or weeds come up along the edge. It costs money and labor, but it improves the soil and spreads out the students' weeding, and a large share of the area needs no more weeding.
Living mulches are another choice between plastic-covered beds, but unlike straw, hay or leaves, they compete with the crop for water and nutrients and can encourage perennial weeds. If you use them, plan to limit competition, for example by feeding the crop with drip irrigation and banded fertilizer and mowing the living mulch often.
Incorporating Cover Crops to Suppress Weeds
Cover crops can also suppress weeds and shrink the seed bank. Smother crops can cut labor and herbicide costs while building soil health. Learn more about using cover crops against weeds here:- Cover crop chart • Comparison chart
- Cover crops for home gardens and raised beds • Article
- Ending cover crops in organic systems • Article
- Putting no-till practices to work • Article
