Agricultural pest management
Integrated Weed Management
(Reviewed 6/07 , updated 11/08 )
Weeds compete with cole crops for sunlight, nutrients, and water. Weeds in and around the field before planting sometimes harbor pathogens, nematodes, insects, or vertebrates that can invade or spread to the crop soon after planting. Weed control is especially important in precision-planted crops where loss of seedlings to competition can substantially reduce the vigor and uniformity of the overall stand. Also, any weeds that go to seed contribute to weed problems in succeeding crops.
Cole crops are either direct seeded or transplanted in the field. Cauliflower and Brussels sprouts are primarily transplanted, which simplifies weed control in these crops. The use of transplants allows for earlier crop maturity, a more uniform stand, and increased weed management options. An integrated weed control program relies on several management methods, both cultural and chemical, to keep weed populations at tolerable levels. Management of insects, diseases, and nematodes can affect a program's success because damage by pests may limit the crop's overall ability to outgrow competing weeds.
In direct-seeded crops, the first 30 days after seeding are the most important for weed control. As the crop grows older, most cole crops shade and compete well with weeds.
Careful water management is also important in weed control. Poorly maintained furrows may cause water to collect in parts of the furrows, favoring the growth of water-loving weeds, as well as soilborne disease organisms.
Regional differences in weed problems can be marked. The winter-spring weeds, which are favored by cool, moist conditions, predominate most of the year along coastal areas. In the southern desert area, however, early-planted crops must compete with weeds that germinate under the warm-to-hot conditions of early fall in the desert. Later fall plantings compete with winter annual weeds. In the San Joaquin Valley, grass weeds and the early fall-winter weeds are the most common.
Choice of an herbicide depends largely on the weed species to be controlled, but it is also influenced by soil type, irrigation method, and crop rotation. Very few herbicides are available for use in cole crops and several important weeds are not controlled by registered materials. Herbicides also vary in their selectivity or potential for damaging the crop at certain stages of development. When trying a new material or if you suspect an herbicide may be causing crop damage, leave part of a row untreated to check for effectiveness on weeds or crop sensitivity. No herbicide registered for cole crops provides satisfactory control of all the weeds likely to be found in the crop. Sometimes combinations of herbicides or sequential applications will be required. Check with your farm advisor or agricultural commissioner to make sure that you have the latest information on available materials, their compatibility, and recommended rates.
Proper application is just as important as the right choice of herbicide for controlling weeds without injuring the crop. Usually, lower rates are recommended for sandy soils. Timing of applications relative to rain or irrigation is important, as some herbicides may lose effectiveness when leached from the soil surface by excess water. In other cases, water can serve to move the herbicide into the soil after surface application, although care must be taken not to apply too much; 0.5 to 1 inch is often enough. With the exception of oxyfluorfen (GoalTender ) registered for use in broccoli and cauliflower, many of the herbicides used in cole crops do not kill emerged weeds, so cultivation is needed before application to remove emerged weeds. If there is a chance that residues will damage subsequent crops, use a band application to reduce the total amount of herbicide in the soil. Then dilute the herbicide residue after harvest with deep plowing and disking. Use application equipment suited to field conditions and calibrate sprayers before each use.
Herbicides may be applied before planting (preplant), after planting but before the crop emerges (postplant preemergence), or after planting with crop emergence (postplant postemergence). In some districts, fall-planted cole crops may receive a layby treatment before winter rains. The choice of timing depends on your schedule and the proper method for the materials you choose. For example, many materials are incorporated with a sprinkler irrigation, so the timing of the first irrigation may be the most important determinant of when materials are best applied.
To plan a weed management program, you must know which weeds are present and their relative abundance. Survey each field for weeds before the first cultivation and at harvest. If possible, conduct the first survey while the previous crop is still in the ground. Make a record of weeds that are mature and producing seed; these weeds will be the sources of weed problems in succeeding crops. Repeat the survey midway through the growing season to check on the effectiveness of weed control measures. Small weed seedlings may be difficult to identify but if you look around the area there may be a few plants that are a bit larger and easier to identify. Areas of the field with perennial weed infestations can be managed separately, especially if a GPS is used to mark their location. Most herbicides used in cole crops are effective only on germinating weeds, so it is essential to know what the target weeds are before they grow. Normally, this information comes from routine weed surveys carried out during the previous crop. In some cases, it may be necessary to take soil samples for seed germination tests to determine the weed spectrum before planting.
To conduct a weed survey, walk through the field in a regular grid pattern and rate the degree of infestation for each weed species. Use a numerical scale or rate infestations as light, medium, or heavy. Check the area surrounding the field as well as the field itself. Maintain a file on each field to track long-term trends in weed species and density.
Weed control is easier and cheaper in fields that are not infested with difficult-to-control weeds. Problem weeds include burning nettle, annual sowthistle, shepherd's-purse, London rocket, purslane, hairy nightshade, chickweed, and nutsedges. If problem weeds are present in significant numbers, the best strategy is to rotate to a crop in which they can be successfully controlled. Certain hard-to-control weeds such as yellow nutsedge may be best controlled in coastal California by rotating to a crop that receives preplant fumigation (e.g., strawberry). Planting date can have an impact on weed problems in a given region. For instance, fields planted between October 1 and October 15 in the southern desert are usually fairly weed-free.
Sanitation is critical in a weed management program. Some weeds can produce thousands of seeds in a single season. To reduce seed production, disc or mow harvested fields before weeds flower and produce seeds. If weedy species are present, cultivate areas around the field such as field edges, fence lines, roadsides, and irrigation ditches regularly to prevent weed seed production. Cultivation equipment and irrigation water must also be kept free of weed seeds and vegetative propagules to avoid spreading weed populations. High pressure washing with water is necessary.
Several foliar-active herbicides are available that can be used on fallow beds to control troublesome weeds before planting cole crops. Paraquat (Gramoxone) controls most emerged annual weeds and grasses and burns back perennials. Glyphosate (Roundup, etc.) will control most annuals and many perennial weeds. Neither, however, will completely control field bindweed, nutsedge, burning nettle, or cheeseweed. Make fallow bed treatments after weeds have germinated following rains or irrigation. A common practice in the desert is to sprinkler irrigate to germinate weeds and make an aerial herbicide application a few weeks later.
Cultivation and bed preparation
Preplant plowing, followed by irrigation and one or two discings before bed formation, will destroy many weeds. Deep plowing and inverting soil to a depth of 16 inches has significantly reduced sowthistle and groundsel infestations in the central coast region, and may also serve to reduce certain disease organisms. If done with moldboard plows, deep plowing can reduce the nutsedge population by 95 to 98%. Plowing must be followed by rain or irrigation and discing of emerging weeds before they flower. Using cultivation to bury seeds is not as effective against weeds, such as cheeseweed, that have hard-coated seeds. These seeds can survive for years at lower depths until they are brought to the surface by subsequent plowing where they will then germinate.
Proper bed preparation is important for successful weed cultivation after the crop is planted. Poorly leveled land will cause water to collect in low areas of the field, favoring growth of water-loving weeds. Effective cultivation of bed tops requires precise row spacing and careful alignment of cultivating tools. GPS-assisted, auto-guidance systems have been shown to produce precision aligned beds, permitting accurate, close cultivation.
A stale seedbed method can also be used for cole crops. The concept depends on controlling the final flush of weeds before crop emergence, followed by minimal soil disturbance to reduce subsequent weed flushes. To do this, prepare a seedbed and preirrigate it to germinate weed seeds. Once bed surfaces are dry enough to allow equipment on the field, the crop can be direct seeded. In the case of direct-seeded broccoli, an additional step can be taken to treat the field with an herbicide or with a propane flamer to kill all emerged weeds just before the crop emerges.
The preplant, preemergent herbicides commonly used in cole crops in California are bensulide (Prefar), trifluralin (Treflan), napropamide (Devrinol), and DCPA (Dacthal).
DCPA can be applied at planting to control many broadleaf and grass weeds. It is not very effective at controlling weeds in the mustard family, however.
Bensulide is relatively insoluble, readily absorbed into organic materials, leaches very little in soil, and consequently has a long residual period. It is not effective on volunteer grain crops and only controls a select group of broadleaf weed species.
Trifluralin must be mechanically incorporated 2 to 3 inches (5–7.5 cm) deep; once incorporated, it remains stable. Crop safety with this material is marginal under the arid conditions of the southern desert areas and in wet, cold coastal winters; this material is not widely used in these situations. Trifluralin has a somewhat limited spectrum of weeds that it will control and has a long residue period. Residues harmful to acutely sensitive crops, spinach, sugarbeets, milo, and corn, may persist up to 12 months.
Napropamide can be applied preplant and incorporated with power-driven rotary tillers. It provides excellent control of all annual grasses, including volunteer cereals and a large number of broadleaf weeds. It has long residual properties and a narrow range of crop tolerances; do not plant certain crops, especially lettuce, sugarbeet, and cereals, following its use in cole crops.
When cole crops are grown from transplants, preemergent herbicides such as trifluralin, oxyfluorfen (Goal XL, GoalTender), ammonium nitrate solution AN-20 (20-0-0), and napropamide (Devrinol), can be applied before transplanting. By treating the field before transplanting, the transplants can be irrigated immediately following the transplanting operation.
The GoalTender fomulation can be applied for postemergent control in direct-seeded or transplanted broccoli and cauliflower. GoalTender provides good control of a broad spectrum of broadleaf annual weeds and is safe on transplanted cauliflower or broccoli. It is less effective in controlling large lambquarters and grassy weeds and does not control yellow nutsedge.
Rates are dependent upon soil type. Transplanting should be completed with minimal soil disturbance and treated soil surfaces should be left undisturbed for as long as possible, however timely cultivation after weed emergence may also be necessary. Transplants may temporarily show leaf cupping or crinkling symptoms but rapidly outgrow symptoms, providing transplants are hardy and not severely stressed before planting.
Control of weeds after planting is most critical during the seedling stage. Once established (4–5 inches tall), most cole crops, with the exception of cabbage, can shade out weeds. Scout for flowering wind-dispersed weeds (such as annual sowthistle) and destroy them before they produce seed to prevent dispersal and establishment in fields.
Effective cultivation of bed tops requires precise row spacing and careful alignment of cultivating tools. When plants have two to three leaves, sweeps or knives can be set as close as 2 inches on each side of the seed rows as long as they cultivate shallowly; closer cultivation will cut feeder roots. When crop seedlings are tall enough that they will not be buried, usually when they have three to four leaves, arrange tools so they move a 1-inch layer of soil toward and into the seed row. This mulch of dry soil will prevent many weed seeds from germinating.
Fields may be cultivated up to four or more times between planting and harvesting. Cauliflower is nearly all transplanted, and because of its low planting density, which facilitates cultivation, it may or may not require hand hoeing. Broccoli is usually planted to a stand and not thinned, but it is typically hand hoed or cultivated at least once if economically feasible. Weed control in Brussels sprouts relies largely on cultivation. Fall-planted cole crops in the coastal valleys may require more cultivation than summer-planted crops because cooler temperatures slow their growth rate. It may take 45 to 60 days for them to grow large enough to shade out weeds and eliminate the need for further weed control activity.
Surface banding of fertilizer
Surface banding is an effective way to apply nitrogen and it gives the crop a competitive advantage over the weeds. The waxy cuticle that cole crops develop once they have at least three true leaves prevents damage to the crop, unless the plants are very wet. However, because weeds lack this cuticle, they will be burned by the fertilizer; this effect is most pronounced on warm days. Use a shielded spray to avoid spraying the growing point or emerging new leaves of the cole crop plant.
Table 1. Effect of Surface Banding of Ammonium Nitrate Fertilizer on Weed Species in Cole Crops.
After planting, DCPA (Dacthal), bensulide (Prefar), and napropamide (Devrinol) can be applied and sprinkler incorporated before the crop emerges.
Clethodim (Select Max) can be used for controlling small seedling annual grasses and some perennial grasses; it also controls annual bluegrass but not sedges or broadleaf weeds. Later growth stages of annual grasses are more difficult to control. Effectiveness is reduced when grasses are under moisture stress.
Oxyfluorfen (GoalTender formulation) is registered for postemergent use in broccoli and cauliflower to control broadleaf weeds at the two to three true leaf stage of the crop. It controls emerged weeds such as cheeseweed and burning nettle, as well as common purslane, sowthistle, and nettleleaf goosefoot, which are not burned back by fertilizer applications.
Sethoxydim is a selective, postemergent herbicide for control of annual and perennial grass weeds. However it does not control Poa annua (annual bluegrass) nor does it have any effect on sedges or broadleaf weeds. All grass crops (such as cereal grains and turf) are susceptible. Application made to control volunteer cereals (barley, corn, oats, rye, and wheat) should be made before tillering.
UC IPM Pest Management Guidelines: Cole Crops
R. F. Smith, UC Cooperative Extension, Monterey County