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Chapter 4: Weed Management

4.1 General Principles

Weeds reduce yield and quality of vegetables by competing directly for light, nutrients, and water. Weeds can serve as alternate hosts for insects and pathogens and uncontrolled vegetation can reduce air circulation around plants, creating more favorable conditions for plant disease development. Weeds that remain in-crop at the end of the season can significantly impede harvest operations. While a comprehensive weed control system integrates tools and practices throughout all phases of production, early-season competition can significantly impact future yield potential and control should be emphasized during this period. Weed species vary, considerably, with respect to their emergence patterns, life history traits, size and competitive ability, among other attributes, and cannot be controlled using a single method. Consequently, the first step in developing an effective management strategy is proper identification. Some online resources for weed identification are compiled at the New York State IPM Weed Identification Resources page. Common problem weeds in New York State are highlighted below.

4.1.1 Problem Weeds in Vegetable Production

Galinsoga. Galinsoga is an upright summer annual with opposite, egg- to triangular-shaped leaves with toothed margins. 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. The species 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 and continuing until the plants are killed by frost. Fresh seed that drops onto the soil surface can germinate almost immediately because there little or no dormancy. Three to five generations per season have been observed in Ithaca, New York. Cultivation is only partially helpful because Galinsoga can re-root, easily, and re-establish itself from cut stems unless conditions are very dry for several days following soil disturbance.

Velvetleaf. This erect, robust, summer annual weed is increasing rapidly in upstate New York areas. The species comes by its name, honestly, because its stems and heart-shaped leaves are hairy and soft to the touch. 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. Subsequently, seedlings can 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 late-emerging plants. Unfortunately, even late seedlings can reach reproductive maturity and produce mature seeds before frost.

Nightshades. Nightshades are warm-season, annual weeds.  Eastern black nightshade (Solanum ptycanthum) is the most common and widespread species in New York, although hairy nightshade (Solanum sarracoides) is predominant in some areas. It can be difficult to distinguish among the Solanum species, especially at the seedling stage. Eastern black nightshade is characterized by smooth egg- to triangular-shaped leaves and glossy, purple to black berries; hairy nightshade has hairy leaves and stems and green to yellow berries. These weeds are particularly problematic in tomato, potato, snap bean, and dry bean fields. Few herbicides currently registered for use in vegetable crops are effective for controlling nightshades. Therefore, to stop an infestation, it is important to correctly identify the weed and eradicate it before the plants produce seeds.

Quackgrass. Quackgrass is a common, cool-season, perennial grass that spreads by both rhizomes and seeds. The species can be identified by leaves that are rolled in the bud, a short membranous ligule and clasping auricles at the collar region. Quackgrass is most effectively managed by a combination of chemicals and tillage, although care must be taken to avoid spreading quackgrass rhizomes into clean fields via farm equipment. Check specific crop recommendations for more targeted control options.

Nutsedge (nutgrass). Nutsedge is a perennial weed with three-angled stems and long, grass-like leaves. The species spreads by both rhizomes and tubers. Dormant tubers can remain viable in the soil for years, making the species difficult to eliminate. Nutsedge does not emerge 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 when it begins to form daughter tubers as daylengths start to decrease in July. Tuber formation is greatly accelerated in August and September, when daylengths become even shorter. In the fall, even small plants can form tubers.

Both cultural practices and herbicides are needed to manage nutsedge infestations. The species is sensitive to dense shade and successful control programs need to capitalize on this characteristic. For example, when planted early and at a close spacing, most pumpkins and squash can provide the shade needed to suppress nutsedge growth. Cultivation can be used between rows to manage nutsedge until the crop canopy closes. 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 (dry and snap beans, potatoes, and sweet corn), delay planting and treatment until tubers have sprouted. Herbicides do not damage dormant tubers. See specific crop information for recommendations.

Perennial broadleaf weeds. Perennial broadleaf weeds such as field and hedge bindweed, Canada thistle, horse-nettle, and milkweed are not easily controlled while vegetable crops are growing. Crop rotations and harvesting and planting operations should be scheduled so infested areas are free of crops during late summer and early fall. At this time, perennials are more sensitive to systemic herbicides that move with the flow of carbohydrates from leaves, downward, into overwintering structures in preparation for spring growth. Vigorous green leaves must be present at the time of herbicide spraying as stressed plants are less susceptible to chemical treatments. 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. Cultivation, unless it is used frequently, can fragment and disperse root pieces, which can regenerate.

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. Weeds can spread to an entire farm if transplants become infested with propagules (seed, rhizomes, tubers, etc.). Although it is tempting to try to control weeds with chemicals while the crop still occupies the greenhouse, this can be extremely dangerous; accidental contact can cause significant crop injury. It is preferable to control weeds with chemicals while the structure is empty, generally in the summer and fall. As a preventative 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 greenhouse walls with a burndown herbicide makes mowing easier.

A word of caution. Herbicides that adhere to plastic surfaces can cause subsequent crop injury if plant roots and foliage touch them in turn. Do not use hormone herbicides anywhere near a greenhouse; they have long residual periods, can contaminate equipment and can produce vapors that are injurious to sensitive species.

4.1.2 Weed Maps

Weeds may be unevenly distributed throughout a field. Knowing the location of severe infestations or atypical conditions, such as poorly drained areas, high spots, and field edges that can facilitate the accumulation of particular species can help growers to implement targeted management practices. This information can be documented using a weed map. A weed map illustrates these problem areas within a field and provides information for future control decisions. When weed maps are kept over a period of years, changes in the location and size of weed populations and communities can be noted and decisions about control can be adjusted accordingly. Growers can target areas with specific control practices, rather than treat a larger area needlessly or fail to control problems at all.

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.

To make a weed map, 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:

  • White Sprouts = seeds are just germinating/emerging
  • Tiny = weeds show only cotyledons or first true leaf
  • Small = weeds are less than 1" tall (less than the diameter of a quarter)
  • Large = weeds are more than 1" tall (more than the diameter of a quarter)

Density of each species. Use the following code:

0 = None
1 = Scattered, few weeds
2 = Slight, 1 weed per 6 row feet
3 = Moderate, 1 weed per 3 row feet
4 = Severe, more than 1 weed per 3 row feet

Distribution of weeds. Rate as follows:

  • Spotty = found in a few places around the field
  • Local = found in a small portion of the field
  • General = found throughout the field

4.2 Management Options

4.2.1 Prevention

The weed seed bank is the ‘memory’ of a field. The size and composition of this community can be influenced by the types of weed and crop production practices employed, previously, in a system. The weed seed bank is also the reservoir of species that will impact future crops. 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; control efforts should not stop after the crop is out of the ground. Keeping weeds out of fence rows and off ditch banks can also help.

4.2.2 Crop Rotation

A crop rotation is a planned sequence of crops grown in succession in the same field over an extended period. Crop rotations can prevent the continuous employment of a uniform management practice over space and time; repeated use of one or a few weed control tools will select for weed species best adapted to that system. With respect to weed management, crop sequence in a rotation influences the type and timing of herbicide use, tillage type and the timing of tillage events relative to weed and crop emergence, and harvest date relative to weed and crop maturity. Other crop-related variables that can impact weed establishment include planting and maturation dates, crop growth habit and competitive ability, the number and timing of cultivations, and fertility management. The more dissimilar crops and their management practices are in a rotation, the less opportunity one or a few weed species should have to become dominant over time. Crop rotation is a significant factor influencing weed species composition; consequently, crop rotation is also an effective practice for suppressing unwanted vegetation.

Traditional rotations include:

  • a sod crop (legume or grass) which provides competition from densely-sown species, improves soil organic matter and does not disturb the soil (increasing seed decay time).
  • a "cereal" smother crop (provides competition for resources, allelopathy).
  • a legume seeding for forage crop (serves as a nitrogen resource).
  • a root or row crop -- also called "cleaning" crops because soils are disturbed and the crops are usually cultivated, disrupting weed growth, and deplete weed seed reserves in the soil by stimulating germination.

Rotations with these components are superior to monoculture systems in maintenance of organic matter, topsoil conservation, and higher yields.

4.2.3 Cultivation

Cultivation is an effective practice for managing annual species; rhizomatous perennial weeds, like field bindweed, are not likely to be controlled with soil disturbance, alone, and can even become more widely spread via root fragment dispersal. While many cultivation tools are designed to remove weed seedlings as close to the crop row as possible, some implements can remove weeds from in between crop plants. Although some weeds are likely to remain, within- and between-row soil disturbance may reduce the time needed to conduct expensive hand-weeding operations. Cover crops are a valuable tool for protecting soil health; they can also be effective at preventing weed seed germination and/or suppressing weed seedling emergence. Cover crops may also support populations of arthropods and vertebrate animals that can feed on weed seed. Cover crops can also serve as a source of weeds, so make sure that the seed you purchase is free of weedy contaminants.

4.2.4 Stale Seedbed Technique

In several vegetables, such as the field-seeded vine crops, weed control is difficult because limited availability of effective herbicides. 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, spraying with an herbicide to kill emerged weeds, and then 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 event occurs after seedbed preparation and before seeding. It is important to remember that not all weeds will be effectively managed using this strategy; species with deep seed dormancies may not respond consistently.

4.2.5 Mulches

Plastic mulches are used in several vegetable crops for a variety of reasons including soil warming, conserving soil moisture, protecting fruit from infection by soil pathogens, and minimizing or preventing weed competition with the crop. 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 or emerging but some, like nutsedge, can poke through the mulch and become a problem. Some herbicides may be compatible with mulched systems while others are not; review label recommendation before making chemical applications. Other types of plastic (e.g. clear and infrared transmitting) may affect weeds differently, requiring different control measures to be employed. For more information about mulches, see Chapter 7, General Culture, Section 7.1, Mulches.

4.2.6 Chemical

Herbicides used in combination or alone. Herbicide selection is dictated by many different factors including the identity, size and vigor of the target weeds, the type vegetable being grown currently, future crops, soil type and other site characteristics that can influence herbicide performance. To save time and make fewer passes across a field, growers may wish to combine the application of herbicides with other pesticides or fertilizers. This is not always advisable as the interactions between chemicals a can lead to inferior weed control or injury to crops. For example, crop damage has been reported for combinations of liquid fertilizers and herbicides; review labels, carefully, to avoid the potential for deleterious spray applications. On the other hand, tank-mixes of certain herbicides may be desirable in order to increase the spectrum of weed control as compared to a single chemical. Again, read labels to make certain that appropriate rates (which may be lower for tank mixes as compared to single-herbicide treatments) are being applied and to ensure that the products will not be antagonistic to each other.

Herbicide incorporation.

Soil-applied, residual 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. See herbicide labels for details on incorporation requirements.

 

Some herbicides can be incorporated using irrigation, alone, while others require physical mixing with the soil. Several tools can be 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 disk depth and tractor speed. In some situations, this angle may be excessive while in others it is inadequate. 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 using 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.

Chemicals to be incorporated should not be applied to rough, cloddy soil because the herbicide will be distributed unevenly as the clods break up, resulting in reduced weed control efficacy.

Water volume and nozzle pressure.

The water used in spray mixtures serves as a carrier for the herbicide. The amount of spray volume required is dependent on several factors including whether the herbicide is applied to the soil or to leaf tissue and whether a foliar herbicide is a contact or a systemic product. A rate of 20 to 40 gallons per acre may be appropriate for many herbicides, but applicators must read labels to ensure that specific use guidelines are being followed for each application situation. While herbicide performance is the primary reason for selecting a spray volume, other factors may also be of consideration. For example, depending on the herbicide, less water may cause excessive nozzle plugging or result in spray droplets so small that they are easily carried away by wind.

Pressure can significantly impact herbicide deposition, performance, and the potential for off-target injury. Lower pressures may give an uneven spray pattern, and higher pressures in regular flat-fan nozzles will generate fine, fog-like droplets, which can result in volatility and drift. 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. Like water volume, pressure selection is dependent on multiple factors including nozzle selection, desired droplet size, and herbicide active ingredient. 

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. 

4.3 Managing Herbicide Resistance and Persistence

Triazine-resistant weeds and other herbicide resistance.

Triazine-resistant common lambsquarters was discovered in New York in 1978 and now is ubiquitous in field corn production areas. Triazine-resistant redroot pigweed was confirmed in 1980 and may now be found on several hundreds of thousands of acres of crop land. Triazine resistance in common groundsel and common ragweed have also been reported in New York state. 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. 

While triazine-resistant lambsquarters, pigweed, groundsel, and ragweed are the only formally confirmed resistant weeds in the state, they are not likely to be the only ones. Horseweed and waterhemp are undergoing screening to evaluate their sensitivity to multiple herbicide modes of action. Glyphosate-resistant biotypes of Palmer amaranth 
have been documented in Pennsylvania and New Jersey and may occur in New York. Other herbicide-weed resistance combinations that occur in neighboring states include glyphosate-resistant ragweed, acetolactate synthase-resistant chickweed and foxtail, and common ragweed with resistances to three difference herbicide modes of action.

Controlling the resistant species can be difficult, but the following suggestions may help:

  1. Rotate crops. This alters the timing of cultural practices, changes the herbicides used, and facilitates the adoption of alternate control strategies to reduce selective pressures within a system.
  2. Rotate herbicides within crops. For example, when growing corn, select herbicide programs that do not rely entirely on triazines.
  3. Cultivate to prevent weed escapes from producing seeds. If herbicide resistance is suspected in a field, clean off equipment before moving to other sites to avoid transporting resistant weed seed. 
  4. Try to ensure that you are using seed, composts and manures that are free of herbicide-resistant weed seeds.
  5. Spread manure only on fields where an infestation of triazine resistant weeds already exist.

Herbicide carryover and soil residues. Several vegetable herbicides can be excessively persistent even when used at rates suggested on the label. *AAtrex, metribuzin, 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.

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Maintained by Abby Seaman, New York State IPM Program. Last modified 2018.