Integrating Non-Chemical Weed Controls Into Your System

This article by Brian Caldwell, Cornell Cooperative Extension,
appeared in the July 2001 issue of
“The Vegetable Growers News,” Vol. 33(2).

Weed control is a real challenge for vegetable growers. For many reasons, there is increasing interest in non-chemical controls. At the same time, many growers are working harder to improve their soil quality. Some non-chemical weed controls are good for the soil; some are not. Herein I consider six non-chemical weed-control practices, ranked roughly from the best to the worst in terms of soil impact.

MULCHING WITH ORGANIC MATERIAL
Growers commonly do this for berry crops; why not more for the vegetables? One example I know of that works very well is to put down clean straw or hay in the aisles between black plastic. It also helps with disease control. Mulch also works well with summer squash and some brassica crops. Galinsoga and many other annual weeds will be well controlled by a good mulch. Perennial weeds will not, and will thrive under mulch. For them, try cover crops and fallow treatments.

CROP ROTATION
Preferably, this includes a legume-grass sod crop, but at least with crops that are tilled in different seasons. Weeds will build up more if they are consistently favored by tillage at their favorite time. Warm-season weeds such as pigweed, purslane, and crabgrass are favored by tillage in June and July - and will be smothered out by mulch or cover crops at the time. Galinsoga seeds have only a short life in the soil. A field rotated into three years of hay would have a dramatic reduction in galinsoga pressure, plus an improvement in soil tilth. Many Midwestern studies have shown yield increases of 10 percent or more in corn and soybeans in their first year of rotation, compared to continuous cropping. This is called the rotation effect. No doubt it works for most vegetables are well -- take advantage of it.

ROTATION AND HERBICIDES
Use crops in rotation to get rid of pesky weeds via wider herbicide options. This should be a cornerstone of weed IPM. For instance, growing sweet corn allows one to use Dual, which can help clean up a field with heavy annual grasses, galinsoga, nightshade, and nutsedge. If you cultivate the corn as well, so much the better for your weed control. If horse nettles are a problem in a field, plant an early-maturing crop such as peas, snap beans, cucumbers, summer squash, etc. Then either till repeatedly after the crop is harvested (i.e., fallowing the field), or spray with Roundup and Banvel or 2,4-D. Velvetleaf is quite susceptible to cultivation, so rotating into a crop that you can cultivate thoroughly can help. Field crops may present more herbicide options. As much as possible, plan so that herbicide carry-over does not preclude your rotation choices.

COVER CROPS
Use them in rotation, as above. Fill in windows before or after a crop of five weeks or more during the growing season with a cover crop that will grow rankly in that season. Plant it thickly. For instance:

Warm season - sudangrass, Japanese miller, cow peas, soybeans, buckwheat;

Cool season - oats, rape, field peas;

For overwintering - rye, hairy vetch, red clover.

If the window is less than five weeks, consider fallowing during that period by harrowing the field every 10 days. As cover crops grow, they smother weeds that germinate. When you till the cover crop under, the weeds are also destroyed before they go to seed. Even if the cover crop has not reached its full growth, till the stand under if you see weeds beginning to flower.

CULTIVATION
Learn about the best tools for your crops and soils and how to use them. Talk to other growers who cultivate. Two excellent resources are the video “Vegetable Farmers and Their Weed Control Machines’ and the book “Steel in the Field."

STALE SEEDBED
This is a technique used to clean up a field before planting the crop. Prepare your seedbed, but delay planting until a flush of weeds emerges. Then shallowly harrow them, flame them or spray with a slow rate of Roundup before seeding the crop. You will be depleting the weed seed-pool in the top inch of soil. This process may be repeated several times if your planting schedule permits. Stale seedbed does not work well against perennial weeds unless herbicides or repeated flaming are used.

FALLOW
This involves repeated discing or harrowing of the field during a substantial midsummer time period (often six to eight weeks) to till weeds. It can be done after some very early crops, such as spinach or lettuce, or incorporated into a year off with cover crops. Fallowing is an effective way of cleaning up fields infested with most weeds. It is a last resort for most growers because of having to take the field out of production, and because it is relatively hard on the soil.