Specific Weed Management Practices

Reducing Herbicide Application

Reducing herbicide rates will decrease costs and reduce risks of herbicide carryover into the next crop, environmental contamination, and adverse side effects With proper timing and application methods, rates can be significantly reduced without loss of weed control. In some cases, guidelines may be available for application rates of herbicides that are below those recommended on the label. The EPA allows reduced rates if supported by research data. Consult your state cooperative extension office to see whether such guidelines exist for a particular herbicide. When the amount of herbicide applied was decreased in a California study, more weeds survived but it was found practical to remove the surviving weeds by hand hoeing.

Recent research has shown that glyphosate gives better control of many weeds as well as improved knockdown of cover crops when it is applied in lower volumes of water per acre. Lower spray volume with a higher herbicide rate gives a higher herbicide concentration so less total active ingredient is needed.

For example, quackgrass can be controlled as well with .73 pounds A.I. glyphosate per acre in 22 gallons of water as with .97 pounds in 80 gallons, reducing the amount applied and the costs. Recent research has also shown that ‘lower than labelled' rates of newer post-emergence herbicides are effective when weeds are small. Broadleaf herbicides can be used at one-quarter to one-half of the usual recommended rate if weeds are sprayed when they are smaller than 1 inch. Similar results were observed with postemergence grass herbicides.

Several practices will increase the efficiency of herbicide use so the total amount of herbicide applied can be reduced. Placing herbicides only in a band 4 to 8 inches wide along the crop row, rather than over the entire field greatly reduces the total amount of herbicide applied. Weeds are controlled in the row by the herbicide and those in the row middles can then be controlled by cultivation. Similarly, when mapping shows localized infestations, these problem areas can be spot treated rather than treating the whole field. In sandy soils, rates of soil-applied herbicides can be decreased while on soils with high organic matter, higher rates are needed.

Using the most effective herbicide for the predominant weed and for the soil type also increases herbicide efficiency. For example, it has recently been reported that clomazone will control velvetleaf at rates one quarter that required to kill many other weed species. Another strategy for reducing herbicide input is blending two or more herbicides at low rates to exploit the strengths of each product for controlling particular weed species. There are also selected herbicide combinations that provide better control in combination than when applied separately.

Newer herbicides are more active and are sometimes used at rates as low as one ounce per acre or less. Since many of these newer materials degrade rapidly, they should impose a lower chemical load on the environment and reduce the possibility of residues compared to older herbicides, some of which were applied at rates of several pounds per acre.

Cover Crops to Smother Weeds

Using cover crops to smother weeds is a long established and widely used cultural practice. Cover crops compete with weeds for resources and also may have chemical or allelopathic properties which reduce weed growth. Cover crops can either be planted ahead of the vegetable crop, as a winter crop for example, or they can be seeded at the same time the crop is planted to form a living mulch under the crop as it develops. The use of living mulches and cover crops is discussed in more detail in the Cover Crop section. Cover crops which are particularly effective in weed control are described below by season.

Summer Cover Crops.

Grasses, such as sorghum-sudan grass hybrids, buckwheat, and legumes such as soybeans grown in narrow rows quickly form a complete cover, outcompeting weeds. Part of the weed control effect of sorghum-sudangrass may be allelopathic, with its decomposition releasing a chemical that suppresses weed germination. Some vegetables may also be sensitive to these residues, however.

Winter Cover Crops.

Winter rye plantings will smother cool-season weeds and inhibit later seedling growth of broadleaf weeds because of the presence of allelopathic substances in the crop residues. Unless the rye or ryegrass is incorporated several weeks before planting, however, residues also inhibit germination of small seeded crops such as lettuce.

In Maryland, researchers found that the dense mat provided by hairy vetch residues left on the surface as a mulch inhibited the establishment of weed species such as lambsquarters which require light for germination. Larger seeded weed species without a light requirement for germination, such as velvetleaf and green foxtail, were inhibited only slightly, however. Corn planted into the vetch residue emerged normally, but poor stands of other crops could result if the combination of heavy residues and lack of tillage on poorly drained soils lead to saturated conditions.

Mulches

Mulches control weeds by excluding light and by forming a physical barrier to growth. Experiments in Ohio compared yields of tomatoes and sweet corn on plots with no mulch to those in plots with 4 to 6 inches of straw or 6 to 8 inches of newspaper mulch. Highest yields for both crops were found on plots covered in shredded newspaper. Both mulches suppressed annual weeds but, unfortunately, neither suppressed perennials such as Canada thistle and yellow nutsedge. Additional examples of mulch use are given in some of the crop chapters. Before laying down a mulch, especially a plant mulch, the area to be covered should be weed-free.

Plant Mulches

To eliminate light necessary for weed seed germination, a plant mulch must be 4 to 6 inches deep between the rows but only 2 to 3 inches deep under a crop canopy. Living mulches, cover crops which are seeded between the crop rows once the main crop is established, can also be used in some situations to control weeds. See the Cover Crops for a discussion of the use of living mulches in vegetable production.

Advantages: Plant mulches conserve water, prevent soil crusting and may improve water penetration. They also keep the soil cooler in summer. Maximum soil temperature is reduced more than minimum soil temperature and live hairy vetch was found to reduce maximum soil temperature more than desiccated hairy vetch. Mulches keep plants clean by reducing splashing. When turned under, they add organic matter to the soil. Effects of plant mulches in improving soil structure are discussed in Chapter 1.

Disadvantages: Plant mulches can cool the soil, which is usually beneficial for the fall crop, but bad in the spring, before the soil warms up and plants are established. None of the plant mulches controls perennial weeds such as bermudagrass. They all will tie up soil nitrogen if not at least partially decomposed when incorporated in the soil. Also, mulches intercept light rains, especially if they become completely dry, reducing water availability.

Plastic Mulches

Black plastic is most commonly used. Colored plastics, usually red, have been shown to increase yield in some studies and to produce a stockier plant, but these materials are usually more expensive. Clear plastic warms the soil faster in the spring but encourages weed growth to such an extent that even preplant incorporated herbicides are not considered adequate for control. Plastic mulches that transmit selective wavelengths are now being marketed. Like clear plastic these allow infrared radiation to penetrate and warm the soil, but they also block out the wavelengths that weed seeds need for photosynthesis. Although experimental, these specialty mulches may offer an advantage over black plastic for spring production of warm-season crops. White plastic is used for weed control in summer plantings to avoid excessively high soil temperatures.

Advantages: Black plastic provides good weed control in the row, conserves moisture, and warms the soil in the spring. When coupled with drip irrigation, black plastic improves yield, earliness, and fruit size in many crops.

Disadvantages: In addition to the installation cost of $200 to $300 per acre, conventional plastics and the remaining photodegradable plastic must be removed from the field and disposed of, costing an additional $60 to $75 per acre. For best results, drip irrigation tubing should be placed under the plastic, at an additional cost. Plastic disposal is increasingly restricted in agricultural areas. Hand weeding at the plant holes is needed if no herbicides are incorporated under the plastic. Yellow nutsedge and some other weeds can grow through plastics.

Cultivation

Tillage is still a widely used method of weed control. It is often combined with other field operations such as sidedressing fertilizers, breaking up surface crusts, and hilling up potatoes to prevent tuber greening. Cultivation is most effective against seedling weeds. Improper timing of cultivation can bring additional weed seeds to the soil surface. Light at the soil surface then releases seeds from dormancy, resulting in flushes of germination after each cultivation. Buried weed seeds become sensitized to light and even a split-second exposure to sunlight may trigger germination and growth. Because of this light sensitivity, night cultivation can drastically reduce the germination of certain weed seeds.

The 'stale seedbed' technique also takes advantage of the need of seeds for light to germinate. Seedbeds are formed about two to three weeks before the crop is planted. After 7 to 10 days of good growing conditions (moist soil, warm temperatures), there is a flush of weeds which can be killed with herbicides or flame weeders. Assuming all the weeds are killed and the soil on the bed is undisturbed except for the minimal soil disturbance from seeding or transplanting the crop, fewer weeds should germinate during the season to compete with the crop.

Many vegetable growers use rolling cultivators, because these allow shallow cultivation and can be adjusted for many row spacings and crop configurations. Moldboard plowing is the most effective means for reducing weed populations, however, because it buries seeds deeply enough that germination and establishment is reduced. However, moldboard plowing is considered more destructive of soil structure than chisel plowing. Any form of prolonged tillage will impair soil structure and increase soil compaction as well as subject the field to soil erosion and loss of fertilizer inputs. See Weed Management for weed control practices on erosion-prone soils.

General Guidelines for Mechanical Cultivation:

• Adjust equipment accurately for each cultivation.
• Establish straight rows and beds far enough apart to avoid injuring crop plants during between-row cultivations.
• Withhold irrigation after cultivation to prevent cultivated weeds from re-establishing.

Soil Solarization

In the hottest, sunniest months of the year, weed seeds as well as some of the pathogens and nematodes in the top layers of moist soil can be killed by covering the soil with clear plastic film for 4 to 6 weeks. During solarization, soil temperatures should reach 140 degrees F at a depth of 2 inches and 102 degrees F at a depth of 18 inches. Researchers have developed models to predict soil temperatures under clear mulch based on atmospheric conditions, but the main limitation to soil solarization is finding a time between crops when temperatures under the plastic are high long enough to be effective.

Weeds vary in their susceptibility to high soil temperatures. Winter annual grasses, barn-yardgrass, black nightshade, cocklebur, chickweed, field bindweed, hairy nightshade, prickly lettuce, red-root pigweed, shepherdspurse, velvetleaf and wood-sorrel have been reported to be controlled by solariza-tion. Bermudagrass, johnsongrass, nutsedge, purslane and established field bindweed are only slightly affected, however.

The effects of solarization on soil biota and on crops are not well understood. Although many pathogens and the Rhizobium symbionts which allow legumes to fix nitrogen are killed, there is some evidence that beneficial soil bacteria and fungi are less sensitive than pathogens to solarization. Some beneficial organisms actually increase following soil solarization, while others decrease but seem to recover rapidly. Crop plants often grow faster and more vigorously in solarized soil, even if no known major pathogens were present. Although not well understood, this growth stimulation is most often attributed to increased availability of soluble nutrients.

Examples of successful use of solarization for control of annual dicots in the South include experiments in Texas and Mississippi on strawberries. Experiments at Auburn University in Alabama found that applying chicken manure at a rate of 8 tons per acre and then covering the soil with plastic controlled morningglory and sicklepod. See Disease Management for reports from California that chicken compost under clear plastic also reduced pathogenic Pythium propagules.

General Solarization Guidelines

1. Solarization is most likely to be effective during long days of high temperatures and no wind.

2. Clear 1 mm thick plastic should be adequate and is not expensive, but in windy areas, thicker (1.5 to 2 mm) plastic may be necessary to prevent tearing. Any tears in the plastic should be repaired with clear tape. If the plastic will be used as a mulch with the following crop, it must have UV inhibitors. Otherwise, sunlight and high temperatures start to break the plastic down after 4 to 6 weeks This is particularly true for the cheaper grade builders plastics. Polyethylene (PE) and polyvinyl chloride (PVC) performsimilarly. PE is cheaper but does not heat the soil as much since it is more permeable to long wave radiation. This means it has less 'greenhouse' effect in warming the soil than the PVC film.

3. The soil-plastic contact should be as tight as possible in order to raise the soil temperature as high as possible to as great a depth as possible.

   Thus, the area to be solarized must be leveled and free of weeds and large clods of soil that could raise the plastic off the ground.

4. Moist heat is more lethal to pathogens than dry heat. The soil can be moistened just before the plastic is applied, but irrigating with drip lines placed under the plastic is usually more effective.

5. Plastic must completely cover the soil surface. There is an edge effect of up to 24 inches in which temperatures will be cooler and solarization less effective so solarization in narrow strips is not likely to be effective. If the area to be solarized is wider than the plastic available, 2-to-3-foot wide strips can be joined with heat-resistant glue or tape.

6. Plastic must be left in place for 4 to 6 weeks. Any tears or sections where the plastic breaks down should be patched and replaced.
7. Deep cultivation should be avoided before planting the next crop to avoid bringing non-solarized weed seeds to the surface.

Goats, pigs, sheep, and other animals will eat weeds, but will also root or graze on any crop plants present in the field. Pigs are sometimes useful to root out tubers of nutsedge and quackgrass before the next crop. Geese are one of the few animals that can be used once the vegetable crop emerges.

In 1960, 175,000 geese were used as weeders in the United States, mainly to remove grasses from broadleaved crops such as cotton. Young White Chinese weeder geese at least 6 weeks old are the best feeders and should be used at rates of 3 to 5 geese per acre, depending on the weed population to be removed. African weeder geese are considered by some to be even better than the Chinese breed.

Geese prefer tender young shoots of grasses (such as goosegrass) and sedges over broadleaved vegetables, but should be removed when 75 to 90 percent of their preferred food is gone or they may begin eating the crop. Moving shade and a water source to different locations in the field encourages geese to weed more thoroughly. Geese must be fenced in and protected from predators.