General Practices to Control Weeds
Prior to the introduction of synthetic herbicides, weeds were controlled by a combination of crop rotation, smother crops, and mechanical cultivation.Crop rotation alone was not sufficient to control weeds, but it does introduce conditions and practices that are not favorable for a specific weed species, reducing growth and reproduction of that species.Even when synthetic herbicides are used, rotation has been shown to decrease weed seeds because different types of herbicides can be used on different crops.
After 7 to 8 years of standard chemical and mechanical weed control, from 139 to 278 weed seed were found per square foot in continuous corn compared to 19 to 65 weed seed per square foot in a corn/soybean rotation.Suspension of herbicide use for one year reduced yield in continuous corn by 10 to 27 percent, but did not reduce corn yeild in the two-year corn/soybean rotation. The soybean rotation may also be beneficial because of physical changes in the soil. On some soils, weed seed burial rates may be lower in a corn/soybean rotation because soybean fields apparently contain fewer deep soil cracks. Weed seeds falling into these cracks in the corn rotation build up the weed seed load because they are protected compared to seeds left exposed to the soil surface.
Crop Rotation
Most studies show crop rotation lowers weed densities. In one review of the crop rotation literature, weed densities were lower in 21 studies, higher in 1 study, and equivalent in 5 studies. Crop rotations should include some or all of the following types of crops and growing strategies in order to most effectively break the life cycles of weeds: cold and warm weather crops; legumes or grasses that suppress weeds (e.g. winter rye); and crops in which weeds can be mechanically or chemically controlled. Weed control may be difficult in some vegetables, not just because of plant characteristics, but because no effective herbicides are registered for them. Crops in which weed control is difficult should follow crops where effective weed control can generally be achieved (as in corn and sweetpotatoes). In addition, planting particular perennial crops or winter covers may disrupt the life cycle of the weed. Many California vegetable growers use an alfalfa rotation to control especially severe weed problemsand Idaho potato growers using spring grain-winter wheat rotations often reported fewer weed problems than those using a spring grain-spring grain-potato rotation. In Idaho, this was thought to be a result of winter wheat disrupting the life cycle of common summer annual weeds like wild oats and hairy nightshade.
Crop Competitiveness
Vegetable crops differ in their competitiveness with weeds. Carrots, lettuce, and onion seedlings are slow to establish in the field and do not produce leaves rapidly enough to shade weeds. Grower Example 1 describes one strategy for controlling weeds in carrots. Weed control is difficult in asparagus because row middles cannot be cultivated early to control weeds without destroying the rhizomes just below the surface. Other non-competitive vegetables include direct-seeded peppers, tomatoes, cole crops, and upright crops such as corn which allow light to penetrate in to the row middles, encouraging weed growth. Competitive vegetables that can be cultivated early and that rapidly shade the soil surface are: squash, beans, pumpkins, cucumbers, sweet corn, melons, potatoes and sweetpotatoes. For example, in a California study, cucumbers only required a 3-week critical weed-free period after emergence because vines grew rapidly, shading emerging weeds. Once the bare ground was covered, no more weeds germinated and those already present did not interfere with the crop. Some cultivars may be more competitive than others because of rapid early growth or their characteristic spreading habit. For example, on Long Island, NY 'Green Mountain' potatoes reduced nutsedge shoot dry mass by 89 percent and reduced the number of nuts produced by 70 percent in comparison to ‘Katahdin' because 'Green Mountain' plants quickly formed a dense canopy.
Transplanted crops are more competitive than direct seeded crops. For example, transplanted tomatoes and peppers required 4 to 5 weeks weed-free while 8 weeks were required for direct seeded peppers. Growers generally are advised to request 5-to-7-week old tomato transplants because these are vigorously growing and can be held several weeks before planting. For purposes of weed competition or planting into a no-till crop, larger transplants can be used as long as they are healthy and actively growing.
Healthy, vigorous crops are always more competitive than slow-growing crops or those with poor stands. Using high quality seed and following good cultural procedures as described in the following crop chapters will aid substantially in reducing weed problems.
| Grower Example 1: Carrots are planted at the stage in the rotation where the weed control on the previous crop was good. In order to know where to cultivate before the carrots emerged, David Patriquin, the grower, mixed in radish seeds so the final stand was approximately one radish seed every 3 to 4 inches. The radish seeds germinated in 3 days, allowing the field to be cultivated without damaging the carrot seedlings. Simultaneously, soil thrown on the crop row during cultivation killed the radish seedlings. |
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Converting Pasture Land
Depending on cropping history, weed populations can be very high in old pasturelands. As a general rule, thorough soil preparation, winter cover crops, and use of competitive crops or transplants are good precautions when initially bringing these areas into vegetable production.
General Weed Management Guidelines
bridgesj@unity.ncsu.edu