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General Principles Weeds reduce yield and quality of vegetables by competing directly for light, nutrients, and water and by interfering with harvest operations. Early-season competition is most critical and a major emphasis on control should be made in this period. Weed control requires good management practices in all phases of production. Because there are so many different kinds of weeds, they cannot be managed by a single method. The first step in managing weeds is identification. Some of the common problem weeds in New York State are highlighted below. Problem Weeds in Vegetable Production Galinsoga. Because of its biology and its tolerance to vegetable herbicides, galinsoga may quickly become a major weed once it is introduced into a vegetable field. Galinsoga is not sensitive to day length and, as a consequence, begins to flower and produce seed when it has about five or six pairs of leaves. It continues to form seed until it is killed by frost. Fresh seed drops onto the soil surface and soon sprouts because there is little or no dormancy. The new seedlings begin to flower at the fifth or sixth node, and the cycle is repeated. Three to five generations per season are often observed in Ithaca, New York. Cultivation is only partially helpful because Galinsoga roots quickly and easily reestablish themselves from cut stems and uprooted plants unless conditions are very dry for several days after cultivation. Many herbicides used on vegetables are only slightly helpful for control of Galinsoga. Ineffective chemicals include * †Dacthal, *Eptam, Prefar, Ro-Neet, and Treflan. Several herbicides are extremely toxic to Galinsoga: *AAtrex, * †Dual, * † Lasso, Lorox, and Sencor. Velvetleaf. This robust, annual weed is increasing rapidly in upstate New York areas. It often escapes in fields where preemergence herbicides are used without mechanical cultivation. It has fairly large seeds that last many years in the soil and are not destroyed when fed to cattle. Because of their size, seeds can germinate anywhere in the top several inches of soil. Seedlings emerge from a range of depths, appearing over a period of many weeks, and most surface-applied herbicides used at planting are relatively ineffective on seedlings that emerge late. Unfortunately, even late seedlings can produce mature seeds before frost. Although several herbicides are active against velvetleaf, they usually do not provide season-long control. Fields should also be cultivated. Alanap, *AAtrex, Basagran, *Eptam, Lorox, Prowl, Sutan, and 2,4-D have activity against velvetleaf. Herbicides such as * †Dacthal, Prefar, * †Dual, * †Lasso, and Treflan, however, have little activity regardless of timing. Nightshade. Nightshade, a warm-season summer-annual weed, is becoming a problem in New York. Eastern black nightshade (Solanum ptycanthum) is the most common and widespread species, but hairy nightshade (Solanum sarracoides) is predominant in some areas. These weeds are particularly problematic in tomato, potato, snap bean, and dry bean fields. Few herbicides currently registered for vegetable crops effectively control nightshade. Therefore, to stop an infestation, it is important to identify the weed and eradicate it before the plants produce seeds. *AAtrex, used in sweet corn, will control nightshade, and thus corn can be used as a rotational crop to reduce control problems in other vegetables. The acetamides (* †Lasso, * †Dual, Frontier) suppress nightshades. Additional research will be required, however, to determine the effectiveness of these herbicides in controlling the different nightshade species. Quackgrass. This common perennial problem grass is controlled most efficiently by a combination of chemicals and tillage. Check specific crop recommendations for control. Nutsedge (nutgrass). Tubers do not sprout until the soil is warm. In most fields, weeds such as lambsquarters, mustard, ragweed, and quackgrass emerge two or three weeks earlier. Nutsedge grows vegetatively until midsummer, forming the first tubers as the days shorten slightly in July. As days shorten more sharply in August and September, tuber formation is greatly accelerated. In the fall, even small plants form tubers. In many vegetable crops, even temporary control is difficult. Furthermore, reducing the severity of a heavy infestation or eliminating nutsedge from a field is difficult because tubers may last four or more years after they have formed. Fortunately, nutsedge is sensitive to dense shade, and successful control measures need to capitalize on this characteristic. For example, when planted early at close spacing, most pumpkins and squash provide the shade needed. For dry and snap beans, potatoes, and sweet corn, nutsedge can be controlled by selective herbicides. See specific crop information. Some general suggestions follow: use heavy crop shading if possible; plant crops at close spacings; cultivate between rows until crop foliage takes over; plant and harvest early on fields for which selective chemicals are not available. Fall tillage and nonselective chemicals can then be used. When selective chemicals are available, delay planting and treatment until tubers have sprouted. Herbicides do not damage dormant tubers. Both cultural practices and herbicides are needed; neither can do the job alone. Perennial broadleaf weeds. Perennial broadleaf weeds such as bindweed, Canada thistle, horse-nettle, and milkweed are not easily controlled while vegetable crops are growing. Cultivation alone is only partially adequate, and effective herbicides are very damaging to crops. Rotation and time of planting should be scheduled, so infested areas are free of crops during late summer and early fall. Suitable herbicides are available, and at this time, perennials are fairly susceptible because, as days shorten, they move food from their leaves downward into overwintering parts in preparation for spring growth. Herbicides applied at this time also are moved downward, causing severe damage. Vigorous green leaves must be present at the time of herbicide spraying. If time permits, the fields should be worked a week or so after spraying, and any areas of regrowth should be given a second application. Severe infestations often require two or more seasons for adequate control. Hormone-type herbicides such as 2,4-D and Clarity are particularly effective on bindweed and Canada thistle, whereas horsenettle and milkweed require Roundup. See labels for dosages. Better control of perennial broadleaf weeds is achieved with combinations of herbicides applied after harvest. Using combinations allows growers to reduce the rates of both herbicides while increasing the effectiveness of weed control when compared to either herbicide alone. Weeds in and around greenhouses. Weeds in and around greenhouses and other plant-growing structures are a nuisance as well as a source of insects, diseases, and rodents. Perennial weeds can spread to an entire farm if transplants are infested. Although it is tempting to try to control weeds with chemicals while the crop still occupies the greenhouse, this is extremely dangerous. It is preferable to control weeds with chemicals while the structure is empty, generally in the summer and fall; next, as a preventive measure, lay heavy (six mils or thicker) black plastic over the soil before starting the next crop. Frequent mowing of surrounding areas favors sod and reduces perennials that might harbor pests. Treating a narrow strip next to the walls with Roundup or *Paraquat makes mowing easier. See Virginia Tech Weed Identification GuideCaution. Do not permit herbicide sprays to contaminate the upper surface of the plastic. They remain there indefinitely and can damage plant roots or foliage. Do not use hormone sprays such as 2,4-D or Banvel anywhere near a greenhouse. They have a long residual period, toxic vapors, and can contaminate equipment. Weed Maps A weed map illustrates problem areas and provides information for future control decisions. When weed maps are kept over a period of years for a given field, changes in weed locations and populations can be noted and decisions about control can be adjusted accordingly. Growers can target areas of severe infestations for specific control practices, rather than treat a larger area needlessly or fail to control problems at all. Weeds or weed species may be unevenly distributed over a field. Localized areas of severe weed infestations or atypical conditions, such as poorly drained areas, high spots, and field edges, may be recorded on a weed map. Two weed maps should be prepared during the season. The first should be completed soon after planting to evaluate the success of the current season program. The second should be made at or near harvest to help predict weed control practices that will be necessary for next year. First, make a rough sketch of the field, including landmarks, boundaries, crop row directions, compass directions, roads, planting date, map preparation date, and any other important details. The following information should be indicated on the map. Weed species. If the species is unknown, at the very least distinguish annuals from perennials, and broadleaves from grasses and yellow nutsedge. Weed size. The following ratings can be used:
Density of each species. Use the following code:
Distribution of weeds. Rate as follows:
Cornell Weed Ecology and Management Laboratory website Prevention The higher the weed seed populations in the soil, the more difficult it is to achieve good weed control. Therefore, weeds should be prevented from seeding in and around fields whenever possible. Weeds should be destroyed immediately after harvest. Keeping weeds out of fence rows and off ditch banks can also help. See report on estimating weed seed banks Crop Rotation A crop rotation is a planned sequence of crops grown in succession in the same field over an extended time. Crop rotations prevent continuous and uniform management practices from being employed that select for weed species best adapted to a particular system. Crop sequence in a rotation dictates: herbicide use, tillage type, timing of tillage relative to weed and crop emergence, and harvest date relative to weed and crop maturity. The more dissimilar that the crops and their management practices are in rotation, the less opportunity a weed species has to become dominant over several years. Crop variables which will effect yield include: planting/maturation dates, growth habit, competitive ability, crop-specific cultural practices (e.g. number and timing of cultivations), and fertility management. One can say that the most dominant factor influencing weed species composition over time is crop rotation, and that rotation is the most effective means available for suppressing weeds. Traditional rotations include:
Rotations with these components are superior to monoculture systems in maintenance of organic matter, topsoil conservation, and higher yields. Cornell cover crop decision tool Cornell buckwheat cover crop handbook See report on using buckwheat as a weed control cover crop Cultivation The high cost of skilled and hand labor has caused growers to rely increasingly on chemicals rather than on cultivation and hand hoeing or pulling. Unfortunately, chemicals alone cannot do an efficient, satisfactory job. A few weeds always escape, and new, chemically-resistant weed species may begin to increase. One early, shallow cultivation often solves both of these problems and benefits crop growth and development.
Stale Seedbed Technique In several vegetables, such as the field-seeded vine crops, weed control is difficult because there are no fully satisfactory selective herbicides available. A stale-seedbed technique often helps in these situations. This procedure involves preparing the seedbed in a normal manner, but perhaps a little earlier than usual; delaying planting for two to three weeks; then spraying with a herbicide to kill emerged weeds; and planting without reworking the seedbed. In soils that tend to bake or crust severely, the stale seedbed method is often not feasible. In good tilth, however, modern seeders work satisfactorily even if rain occurs after seedbed preparation and before seeding. *Gramoxone Extra and Roundup are safe for vine crops with a stale seedbed. Mulches Black plastic is the most widely used mulch and can be a good alternative to herbicides in some cases. Black plastic stops most weeds from germinating but some, like nutsedge, can poke through the mulch and become a problem. Clear plastic results in warmer soil temperatures than black which can speed harvest but it does create an ideal situation for weeds and herbicides must be used under clear plastic to avoid competition to the crop. IRT or infrared-transmitting plastic is the third type of plastic mulch used. It does a better job than even black plastic in eliminating weeds and warms the soil better than black but not as well as clear. For more information about mulches, see Mulches in Chapter 7- General Culture. Chemical Herbicides used in combination or alone. Because growers are often too busy to accomplish all necessary jobs, they may wish to combine herbicides with other pesticides or fertilizers. Most such combinations are not recommended, however, because they frequently lead to inferior control or injury to crops. It is particularly dangerous to combine liquid fertilizers and postemergent weed killers. Nevertheless, certain combinations of herbicides are highly desirable. In these combinations, each chemical is used at a relatively low dosage, and a larger variety of weeds can be controlled than with single chemicals. A guide to the relative effectiveness of some common herbicides against some of the more important weeds in vegetables is provided in Table 4.2. The vegetable being grown dictates the choice of herbicide for a particular field. In many situations, the target weeds cannot be controlled chemically. If a different crop is grown, however, another herbicide may control the weeds readily. For example, ragweed is not harmed by most herbicides that can be used on cabbage. Conversely, several herbicides used in corn and beans are harmless to the crop but toxic to ragweed. This principle applies to many crops and weeds. Thus, crop rotation is a simple, effective means of obtaining good, long-term weed control. Herbicide incorporation. Herbicides require incorporation for the following reasons: to reduce loss of efficacy through photodegradation, volatilization, or evaporation; to activate the herbicide through contact with soil moisture; to place the herbicide in the area where it will have the greatest effect; and to prevent drift. Herbicides have different physical properties, eg. solubility and volatility, that effect their movement and persistence in the soil. Depth of incorporation is a function of these physical properties. Those herbicides that are volatile tend to require deeper incorporation while those that need moisture for activation tend to require shallow incorporation. Combining herbicides that have different requirements will result in loss of efficacy of one of the herbicides and should be considered when deciding to combine applications. Two important herbicides that should be incorporated for best results are Eptam and Treflan. Eptam is incorporated (two to three inches deep) to reduce losses from volatility or evaporation. Treflan, on the other hand, is incorporated to ensure its presence in moist soil in the vicinity of sprouting weed seeds. Treflan will fail in dry soil because its solubility is exceptionally low. In contrast, Eptam's volatility is reduced in dry soil, and its performance is improved in moist soil. Granular formulations of Eptam are somewhat less volatile and sometimes more effective than liquid sprays. Treflan is much more stable than Eptam and need not be incorporated for several hours or may be incorporated with irrigation. The acetamide herbicides (* †Lasso, * †Dual, * †Outlook) do not require incorporation if precipitation is adequate or if irrigation is possible. However, surface applications during drought periods will perform poorly and in this situation, incorporation is recommended. These herbicides require shallow incorporation (called 'surface blending' on some product labels) in the top one to two inches of soil. If these herbicides are incorporated too deeply, efficacy is lost due to dilution in the soil profile. Chemicals to be incorporated should not be applied to rough, cloddy soil because the herbicide will be distributed unevenly as the clods break up. Several tools are used for incorporation, but all are not equally satisfactory. The double disk, a popular tool for incorporation, is highly effective in soil preparation, but large, modern disks may give poor results. Many have a preset angle, and the operator can control only depth and tractor speed. Too often, the preset angle results in an average which in practice is inadequate in some fields and excessive in others. An excessive angle throws too much treated soil to the outer edges of the disked area and leaves too little in the center. An inadequate angle causes poor mixing. Although the latter problem can be overcome by cross disking and the soil may be thoroughly mixed with excessive angle and cross disking, the uneven lateral distribution of the treated soil will cause irregular or streaked patterns of poor weed control and occasional crop injury. These problems can be avoided by the use of a spring-tooth harrow. In fields relatively loose and free of clods, the old-fashioned corn or potato weeder (sometimes called a finger weeder) also gives excellent results. These tools produce shallow incorporation and require a smaller amount of herbicide and much less power. The spike-tooth harrow, rotary hoe, and cultimulcher have been unsatisfactory for incorporating herbicides. Water volume and nozzle pressure. For most herbicides, the water used during spraying serves no purpose other than to properly distribute the chemical. If appropriate equipment were available, herbicide concentrates could be applied without water. From a practical standpoint, a rate of 20 to 40 gallons per acre handles almost all herbicides well. Less water may cause excessive nozzle plugging or result in spray droplets so small that they are easily carried away by wind. Higher rates of water require too much time to refill the sprayer. About 40 psi pressure at the nozzle is preferred for
most herbicides. Lower pressure may give an uneven spray pattern, and
higher pressure in regular flat-fan nozzles will cause many fine, fog-like
droplets, which often result in excessive volatility and serious drift.
Furthermore, small droplets of postemergence sprays cause heavier than
normal herbicide deposit on plant leaves, which often gives excellent
weed control, but can cause crop damage. Weeds should be sprayed when
they are less than one inch tall. At this stage, high pressure is unnecessary.
A few growers have tried to reduce chemical dosage by increasing sprayer
pressure, sometimes successfully. However, until research on this can
be evaluated, normal rates and pressures at the appropriate time are the
best practice.
Managing Herbicide Resistance and Persistance Triazine-resistant weeds. Since the early 1970's many weeds have developed resistance, first to the triazine herbicides and more recently to paraquat and the sulfonyl urea and imidazolinone herbicides. Triazine-resistant common lambsquarters was discovered in New York in 1978 and now infests 100,000 acres; triazine-resistant redroot pigweed now infests approximately 10,000 acres. Initially, triazine resistance developed in fields where the triazine herbicides were used exclusively and where corn was grown as a monoculture without rotation. Weed populations that were originally mixtures of both susceptible and resistant biotypes gradually became the predominant, resistant types because the susceptible ones were controlled continuously. The infestation has spread as a result of feeding cows silage corn contaminated with the seeds of these resistant species and then spreading the manure back on the fields. Most fields associated with dairy production in New York State are probably infested with triazine-resistant common lambsquarters. Controlling the resistant species can be difficult, but the following suggestions may help:
Herbicide carryover and soil residues. Several vegetable herbicides can be excessively persistent even when used at rates suggested on the label. *AAtrex, Sencor, Devrinol, and Treflan are examples of herbicides that often interfere with fall cover crops and sometimes injure susceptible vegetables the following spring. To help avoid persistence, Cornell occasionally suggests dosages well below those stated on the label. Other ways of reducing danger from herbicide carryover are to apply two different herbicides at their lowest labeled rates; use low initial dosages and repeat if needed; use only extremely shallow incorporation when necessary; avoid applying persistent herbicides after late June or early July because degradation occurs mostly in the warmer months; and moldboard plow deeply before planting a succeeding crop. World Wide Web Sites:
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