Vegetable growers should combine cultural practices to minimize the number of weeds in the crop, suppress competition by surviving weeds, and reduce weed seed production. Cultural controls may include any practice that makes the crop more competitive with weeds. Optimizing cultural controls will give the crop a competitive edge over weeds.
Weeds sometimes invade the farm from elsewhere. Slow down the onset of weed problems by using only plants, seeds, greenhouse media, and soil amendments that are certified to be free of weeds or weed seeds. Do not allow weeds to grow on compost or manure piles. Install screens on irrigation intake lines to ensure that weed seeds do not enter the system. Wash all soil from new machinery, or machinery that has been used on another farm before you use it in your fields. Be on the lookout for, and then control invasive weeds around buildings, along roadsides (Figure 1), ditches, and field edges. Maintain a dense grass cover around all field edges to provide a barrier to invading weeds. This will require regular mowing of the grass edge, maintaining a height of at least 3 inches. Mow frequently enough to keep the grass barrier from going to seed. Always direct the mower chute away from the crop. Perennial broadleaf weeds in the grass barrier can be further suppressed by spraying with 2,4-D, but only after vegetable crops have been harvested. 2,4-D can severely injure many vegetables, and should only be used when sensitive crops are not growing in nearby fields.
Is one weed too many? Maybe so, especially if it is allowed to go to seed. Be on the lookout for weeds that are new to your farm and destroy them, making sure they do not go to seed (Figure 2). Weeds that are close to flowering may have already set seed, so simply pulling them out and dropping them in the field may not prevent seed formation. A more reliable method would be collecting flowering weeds as they are pulled, and either burying them in a landfill or incinerating them. Watch for emergence of more plants in the area of the original infestation and destroy any that occur. Emergence may continue over several years. Try to identify how the new weed got on your farm, and take measures to prevent reinfestation. Remember, early eradicationeven if it means hand pulling and hoeing, will pay for itself many times over in lower weed control costs and less frustration.
Crop rotation affects weeds because different weed control measures are used in the rotation. Growers who use no herbicides consider crop rotation to be the basis for good weed management. Whether herbicides are used or not, a diverse crop rotation including spring and summer germinating crops, annual and perennial crops, and closed- and open-canopied crops permits a variety of tillage, cultivation, cutting, and other weed control practices at different times of the growing season. The different operations contributing to weed management in these crops ensure that no single weed species or complex of weeds dominates the field, a problem that usually occurs when a single approach to weed control is used repeatedly, year after year, in the same crop.
Vegetable growers should consider planting fields that are heavily infested with weeds to a field crop (e.g., wheat, oats, corn, or soybeans) or to a vegetable crop such as potatoes or sweet corn in which good, season-long weed control is attainable. To control creeping perennials, plant a winter cereal crop and attack the weeds with herbicides either just before harvest or soon after harvest. If planning an after-harvest herbicide, harvest high so that as much weed foliage is left as possible. Aggressive plowing and discing right after harvest may also reduce perennial weeds, even without herbicides, because the weeds may have been weakened by competition from the crop.
A stale seedbed involves preparing the seedbed in a normal manner but earlier. Planting is delayed from several days to several weeks, during which time herbicides, flaming, or light cultivation is used to kill emerged weeds, and then planting is completed without reworking the seedbed. In soils that tend to bake or crust severely, the stale seedbed method may not be feasible. However, on soils of good tilth, seeding should be successful even if rain occurs after the seedbed is prepared. The stale seedbed method is most commonly used with cole crops and vine crops. Applying Roundup™ herbicide prior to seeding to control weed seedlings is safe for virtually any crop. Gromoxone™ may also be used for direct-seeded asparagus, lettuce, and melons.
If shallow cultivation is to substitute for herbicides in stale seedbed, start much earlier because cultivation will probably stimulate more weed seeds to germinate. Strive to deplete the weed seeds in the upper layer, and to end up with a shallow layer of dry, fluffy soil at the surface by planting time. Then without additional tillage, plant crop seed into moisture below the cultivated zone.
Competitive vegetables that can be cultivated early, and that suppress weeds by rapidly shading the soil surface will reduce weed growth. Many factors contribute to a competitive crop including the following:
Managing the soil in a way that provides optimum conditions for rapid growth of crop roots and shoots will enable the crop to capture resources (sunlight, water, and nutrients) that would otherwise be available for weed growth. Planting into warm, moist soil of good tilth, with the proper pH and fertility gets the crop off to a quick start. Limit weed growth simply, by targeting fertilizer application to the crop row. Banding dry fertilizer, or injecting fertilizer into drip irrigation lines reduces the amount of fertilizer needed and restricts availability of nutrients to weeds growing between the crop row.
Potatoes, beans, squash, pumpkins, cucumbers, melons, and sweet corn are vegetable crops that compete well with weeds. Using high quality seed of a variety that is adapted to local conditions is one way to obtain rapid crop growth. Certain varieties may be more competitive than others due to more rapid seedling growth, and because they maintain a dense canopy that reduces weed growth. Transplants will always be more competitive with weeds than a seeded crop. Remember that crops and weeds "race" to obtain factors needed for growth. Select varieties that may win this "race" due to their excellent vigor.
Narrow rows and precise placement of seed will help to optimize the cropıs competitiveness by leading to a uniform, closed canopy of crop leaves in the shortest time possible. Gaps in the field, perhaps due to improper seeding, provide weeds with an opportunity to establish and spread. Therefore, take steps to ensure that the crop population is optimal.
Weeds are easier to kill when they are small. Cultivating and hoeing when weeds are small requires less energy, is more effective, and causes less crop damage than when weeds are large (Figure 3). Maximum control is achieved when cultivation (or tillage during summer fallow) takes place after weeds have germinated in the soil but before they emerge. This stage of weed growth is often called the white thread stage and can be targeted by cultivating about 5 days after planting. The cultivator should be capable of shallow stirring of the soil to uproot weed seedlings between the rows, while being able to throw soil onto the row (when the crop is tall enough) to cover emerging weed seedlings. Shallow cultivation will minimize subsequent weed emergence by creating a layer of dry soil at the surface, which is the zone in which most weeds germinate. Shallow cultivation is also less likely to bring buried weed seeds to the surface where they are likely to germinate. More details on mechanical weed control tools are found in Steel in the Field: A Farmerıs Guide to Weed Management Tools, available from the Sustainable Agriculture Network.
Weed control by flaming seedling weeds is practiced on many organic vegetable farms in Europe, and is gaining popularity in the United States. Flame weeders (Figure 4) can be used to kill weeds after crop seeding but before crop emergence (pre-emergence), and after the emergence of certain crops (post-emergence), such as corn. Flaming works by heating plant tissues for a short time, which ruptures cell membranes and coagulates the cell sap. For post-emergence trea™ents, shields are used to protect the crop from heat. Also, some crops (especially grasses) are less sensitive to heat shock than are small weeds. Remember that flaming is selective. In our experience, common chickweed was killed by post-emergence flaming of corn; however, wild radish survived. The main cost of this method is for propane (10 to 30 gallons per acre). Extreme caution should be exercised to prevent fires, burns, and explosions.
Synthetic and organic mulches are used to control weeds in vegetable production. Black, white, and other colors of plastic are commonly used for weed control and for improving crop quality, but they do not promote earliness (Figure 5). For earliness, try infrared transmitting mulches, which allow the soil to warm up but do not promote weed growth. Clear plastic mulch promotes crop earliness and weed growth.
Straw, compost, or other organic materials will prevent emergence of weed seedlings, provided a sufficiently thick layer is applied. Apply organic mulches around the base of vegetable transplants, or after a seeded crop has emerged but be careful not to cover young crop plants. Using residues from the preceding crop to suppress weeds is a relatively new concept, but has been successfully adopted by both conventional and organic growers. Fall-seeded cover crops such as winter rye are most commonly used to accomplish this. The cover crop is sown in the fall and spring growth is mowed or chemically killed prior to planting vegetables. The vegetable crop is either transplanted, or seeded with a no-till seeder.
Keep in mind several factors when using a residue mulch from a fall-seeded cover crop. First, try this approach initially on just a small area. Second, to effectively reduce weeds, the crop residue must heavily shade the soil. Care must be taken to establish and grow a healthy cover crop that will be used as the mulch. Approximately 3000 to 4000 pounds of residue per acre are required for weed suppression. Third, both the fall cover crop and the following vegetable crop require nutrients and water. A legume cover crop such as hairy vetch may satisfy the vegetable cropıs nitrogen requirement but additional phosphorus, potassium, and micronutrients may be needed. Therefore, soil nutrients (including limestone) required for both crops should be added to the soil prior to seeding the fall cover, unless additional nutrients needed by the vegetable crop can be supplied through an irrigation system. Injecting nutrients into irrigation water is good insurance against nutrient or water deficiencies in the vegetable crop. Fourth, the fall cover must be killed in the spring so that it will not compete with the vegetable crop. Finally, vegetables must be planted into the crop residue mulch with minimal soil disturbance (i.e., no-till).