Cultural Weed Management

Plant competition is a foundation of weed management. Weeds compete with crops for water, light, and nutrients thereby reducing crop yield and quality. Competitive effects of weeds are density dependent, with increasing weed density causing increasing crop yield loss. The magnitude of loss varies based on the crop and weed species present and the timing of competitive interactions. Carrots, beets, and alliums, for example, are small-seeded, slow to emerge, and are poorly competitive early in the growing season. Larger-seeded crops (e.g. pea, bean, and corn), tubers (potato), or transplanted crops start with a significant initial size advantage over germinating weeds. A first principle of weed management is to establish this size advantage, and then maintain it with early weeding or mulching to improve crop competition. To this end, successful weed management requires good foundational agronomic and horticultural practices. Overall, the more rapidly a crop can cover the soil ahead of weed emergence, the more competitive that crop will be. Choose high quality seed, calibrate equipment to ensure accurate seeding rates and depth, consider decreasing the space between crops to increase soil shading, and where possible, supply resources selectively to the crop (e.g., apply fertilizers banded below/near the crop row, or through drip irrigation, instead of broadcasting). Remember, in plant competition, “the big get bigger!”

Other cultural strategies that help to manage weeds include cover crops, mulches, fallowing, stale beds, crop rotation, and soil preparation.

Cover crops

Cover crops reduce overall weed populations in several ways.

• While it is growing, a dense cover crop stand will suppress weed growth by shading the soil. A season-long cover crop can help suppress weeds that may have otherwise matured and set seed.
• Cover crops can slow soil warming in the spring, helping to slow weed seed germination.
• Some methods of cover crop termination can also kill emerged weeds within the field. If this is done before those weeds flower and set seed, it can help remove weeds from the seedbank.
• If a terminated cover crop is dense enough and left in place, it can become an effective mulch.

Despite these benefits, perennial weeds can increase when perennial cover crops are in place for a longer period of time, e.g., one or more years of red clover.

Mulches

Mulches are often used to control weeds. Mulches can be organic (straw, hay, grass clippings, dead cover crops), inorganic (plastic), or degradable (bio- or photo-degradable plastics).

• Organic mulches are effective if they are thick enough to keep weeds from emerging through them (usually at least 2-3"). Downsides of organic mulches are that they can be expensive, they reduce soil temperatures and slow soil warming, they can reduce nitrogen availability, and they can harbor animal pests. Cooler soil temperatures can be a problem in warm season crops. It is recommended that the mulch application be delayed enough to allow the soil to warm up sufficiently for the crop.
• Black plastic mulches will warm soil and eliminate weed pressure. However, weeds emerging through the planting holes and between strips of plastic mulch can still reduce yields if not controlled. Infra-Red Transmitting (IRT) mulches are less effective than black plastic for controlling weeds, and clear mulches can enhance weed growth. Some growers plant cover crops between plastic mulch strips as "living mulch", but these cover crops can also compete with the crop. Killing the living mulch before the crop is planted, mowing the mulch on a regular basis, or using raised beds will help to reduce but not eliminate competition. See the section on Herbicides and Plastic Mulches for more information. A downside of black plastic mulch is that they generate a lot of plastic waste material, which can be difficult and expensive to dispose of.
• Degradable plastic mulches are more expensive than plastic mulches and rip more easily, which exposes soil where weeds can germinate. They are also not approved for organic production. However, degradable plastics can be plowed into the soil after the growing season, reducing the need for plastic waste disposal.

Fallowing

Fallowing is not planting a field with the intention to reduce weed seed populations. Repeated shallow soil disturbance will encourage weeds to germinate while subsequent events kill seedlings before they go to seed. Over time this strategy will reduce the weed seedbank of a field and can help manage established perennial weeds.

Crop rotation

Crop rotation can be a tool for managing weeds. Weed species present tend to be most like the crop planted. Examples include grasses in corn, winter annuals with early-planted crops, and perennial weeds with perennial crops. Rotating crops among these groups will tend to disrupt this trend.

Proper soil preparation

Proper soil preparation can influence weed emergence. Soils which are rough and less firmly packed will yield fewer weeds than those that are more finely worked, more compacted, and more uniformly moist. However, as noted above, a well-prepared seedbed will help weeding tools function to their potential during later cultivation events.

Stale seedbed

Stale seedbed is the practice of preparing a field for planting and then lightly disturbing the soil on a regular basis to kill small weeds as they emerge, without bringing up new weed seeds from below the top few inches of soil. Tools that can be used for this practice include chain-drag, spring-tooth harrow, light-weight disc harrows, tine weeders, or flame weeders. Stale seedbeds can be used in the spring before a crop is sown, or in the summer after a spring crop but before a fall crop, but a summer fallow may work better on summer annual broadleaves that emerge later in the season and perennial weeds may only be weakened but not killed.

Stale Seedbed Technique

The stale seedbed approach involves preparing the soil as if for planting, without actually planting the crop. After soils are prepared, weed seeds in the upper 1-2" of the soil are encouraged to germinate through adequate soil moisture and temperature (with irrigation or row covers). Generally, this happens within 2 weeks. Weeds are then killed with Gramoxone, Roundup, Scythe, or flaming.

Killing emerged weeds with herbicides or flaming does not disturb the soil, and no new weed seeds will be brought close to the soil surface. After using the stale seedbed technique, care should be taken not to disturb the soil any more than is absolutely necessary during the seeding or transplanting process to minimize the amount of weed seeds that are brought up to the surface to germinate. Preemergence herbicides can be used after a stale seedbed to further reduce germination. Any cultivation performed after should be kept extremely shallow (3/4-1" maximum) so as not to reposition any additional weed seeds.

On sandy, loamy, or high organic matter soils, the soil should not crust and modern seeders should still work satisfactorily. On heavy clay soils, crusting could make this technique unusable.

Steps:

• Prepare the soil as if you are about to seed or transplant. The soil should have good moisture (irrigate with 1/4" of water if necessary).
• Wait as long as possible to allow weeds to germinate and emerge. Allow weed seedlings to grow to the third-leaf stage, or at least to the first true leaf.
• If you're using transplants: flame the soil or make an application of Gramoxone, Scythe, Aim (for broadleaves only), or Roundup (if registered for the crop) to the soil surface before transplanting. Transplant the crop, without dragging any additional soil off the bed, and then apply any preemergence herbicide that you would normally use, to the soil surface and use water to incorporate if appropriate.
• If the crop will be direct-seeded: Gramoxone, Scythe, Aim (for broadleaves only), or Roundup (if registered for the crop) or flaming may be applied just before or just after seeding (see the label). After seeding, apply any preemergence herbicide that you would normally use to the soil surface and use water to incorporate if appropriate. CAUTION: If the crop has already been seeded, be careful that the flaming process does not injure the crop seed or the emerging crop seedling.

Check the current herbicide label and recommendations by crop to determine if Gramoxone, Scythe, Aim, or Roundup is registered for use in that crop. Gramoxone, Scythe, Aim, and flaming will have minimal long-term effect on established perennial weeds. For cucumbers, melons, squash, pumpkin, peppers, and eggplant, Roundup must be applied at least 3 days prior to seeding or transplanting.

In cases where Roundup is registered, it can also be used for control of perennial weeds, such as quackgrass and dock, prior to soil preparation. After application, delay tillage for 3-5 days. There is no residual weed control. See the label for directions.