Chapter 4 Weed Management
Contents
4.1 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.
4.1.1
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, Tillam, and Treflan. Several herbicides are extremely toxic to Galinsoga: *AAtrex, Dual, *Lasso, Lorox, Lexone, 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, Bladex, 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 Banvel 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.
Caution. 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.
4.1.2
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:
WHITE SPROUTS = seeds 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 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.
4.2.2 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:
§ 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 seed 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
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.
4.2.4
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.
4.2.5
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 Section 7.1.
4.2.6
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 4.2.2.
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
Table 4.2.1 Commonly used herbicides on vegetables in New York. Not all registered products are listed in
|
|
Aim (carfentrazone) |
*AAtrex (atrazine) |
Alanap (naptalam) |
Assure II (quizalofop p-ethyl) |
Balan (benefin) |
Basagran (bentazon) |
Buctril (bromoxynil) |
Callisto (mesotrione) |
Chateau (flumioxazin) |
Clarity (dicamba) |
Curbit (ethalfluralin) |
Devrinol (napropamide) |
*†Dual II Magnum (metolachlor) |
*Eptam (EPTC) |
Eradicane (EPTC + safener) |
Fusilade DX (fluazifop-p-butyl) |
Goal (oxyfluorfen) |
*Gramoxone (paraquat) |
Karmex (diuron) |
*Kerb (pronamide) |
†*Lasso (alachlor) |
Lorox (linuron) |
|
Mode of Action1 |
3 |
7 |
9 |
2 |
6 |
4 |
4 |
11 |
|
9 |
6 |
6 |
10 |
10 |
10 |
2 |
3 |
3 |
7 |
6 |
10 |
7 |
|
Asparagus |
X |
|
|
|
|
|
|
|
|
X |
|
X |
X2 |
|
|
X |
|
X |
X |
|
|
X |
|
Bean, dry |
X |
|
|
X |
|
X |
|
|
|
|
|
|
X |
X |
|
|
|
X |
|
|
X |
|
|
Bean, snap |
X |
|
|
X |
|
X |
|
|
|
|
|
|
X |
X |
|
|
|
X |
|
|
|
|
|
Beet |
X |
|
|
|
|
|
|
|
|
|
|
|
X2 |
|
|
|
|
|
|
|
|
|
|
Broccoli |
X |
|
|
|
|
|
|
|
|
|
|
X |
|
|
|
|
X |
X |
|
|
|
|
|
Brussels sprouts |
X |
|
|
|
|
|
|
|
|
|
|
X |
|
|
|
|
|
X |
|
|
|
|
|
Cabbage |
X |
|
|
|
|
|
|
|
|
|
|
X |
X2 |
|
|
|
X |
X |
|
|
|
|
|
Carrot |
X |
|
|
|
|
|
|
|
|
|
|
|
X2 |
|
|
X |
|
X |
|
|
|
X |
|
Cauliflower |
|
|
|
|
|
|
|
|
|
|
|
X |
|
|
|
|
X |
X |
|
|
|
|
|
Cucumber |
X |
|
X |
|
|
|
|
|
|
|
X |
|
|
|
|
|
|
X |
|
|
|
|
|
Eggplant |
X |
|
|
|
|
|
|
|
|
|
|
X |
|
|
|
|
|
X |
|
|
|
|
|
Lettuce and Endive |
|
|
|
|
X |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
X |
|
|
|
Melon |
X |
|
X |
|
|
|
|
|
|
|
X |
|
|
|
|
|
|
X |
|
|
|
|
|
Onion |
X |
|
|
|
|
|
X |
|
X |
|
|
|
X2 |
|
|
X |
X |
X |
|
|
|
|
|
Peas |
|
|
|
X |
|
X |
|
|
|
|
|
|
|
|
|
|
|
X |
|
|
|
|
|
Pepper |
X |
|
|
|
|
|
|
|
|
|
|
X |
X2 |
|
|
|
|
X |
|
|
|
|
|
Potato |
X |
|
|
|
|
|
|
|
|
|
|
|
X |
X |
|
|
|
X |
|
|
|
|
|
Pumpkin |
X |
|
|
|
|
|
|
|
|
|
X |
|
X2 |
|
|
|
|
X |
|
|
|
|
|
Spinach |
|
|
|
|
|
|
|
|
|
|
|
|
X2 |
|
|
|
|
|
|
|
|
|
|
Squash |
X |
|
|
|
|
|
|
|
|
|
X |
|
|
|
|
|
|
X |
|
|
|
|
|
Sweet corn |
X |
X |
|
|
|
X |
|
X |
|
|
|
|
X |
|
X |
|
|
X |
|
|
X |
|
|
Tomato, field |
X |
|
|
|
|
|
|
|
|
|
|
X |
X |
|
|
|
|
X |
|
|
|
|
|
Watermelon |
X |
|
X |
|
|
|
|
|
|
|
X |
|
|
|
|
|
|
X |
|
|
|
|
|
*Restricted use only |
|
|
1Mode of action |
|
|
|||||||||||||||||
|
(†) Not for use in Nassau/ Suffolk |
1=amino acid inhibitor |
|
|
|
7=systemic photosynthesis inhibitor |
|||||||||||||||||
|
Counties |
|
|
|
2=lipid synthesis inhibitor |
|
|
8=amino acid derivative |
|
|
|
||||||||||||
|
|
|
|
|
3=cell membrane disrupter |
|
|
9=growth regulator |
|
|
|
|
|
||||||||||
|
|
|
|
|
4=contact photosynthesis inhibitor |
10=shoot growth inhibitor |
|
|
|
||||||||||||||
|
|
|
|
|
5=pigment inhibitor |
|
|
|
|
11=enzyme inhibitor |
|
|
|
|
|||||||||
|
|
|
|
|
6=root growth inhibitor |
|
|
|
|
|
|
|
|
|
|
|
|
|
|||||
this table or in crop sections.
|
Matrix (rimsulfuron) |
†Outlook (dimethenamid) |
Permit (halosulfuron) |
Poast (sethoxydim) |
Prefar (bensulide) |
Princep (simazine) |
Prowl (pendimethalin) |
Pyramin (pyrazon) |
Raptor (imazamox) |
Reflex (fomesafen) |
Reglone (diquat) |
Ro-Neet (cycloate) |
Roundup Ultra (glyphosate) |
Sandea (halosulfuron) |
*Select (clethodim) |
Sencor (metribuzin) |
Sonolan (ethalfluralin) |
Spin-Aid (phenmedipham) |
*†Stinger (clopyralid) |
Strategy (ethalfluralin+clomazone) |
Sutan + 6.7E (butylate+safener) |
Treflan (trifluralin) |
(2,4-D) |
|
|
1 |
10 |
1 |
2 |
6 |
7 |
6 |
4 |
1 |
|
3 |
10 |
8 |
1 |
2 |
7 |
6 |
4 |
9 |
5,6 |
10 |
6 |
9 |
Mode of Action1 |
|
|
|
|
X |
|
|
|
|
|
|
|
|
X |
X |
X |
X |
|
|
|
|
|
X |
X |
Asparagus |
|
|
X |
|
X |
|
|
X |
|
X |
X |
X |
|
X |
X |
X |
|
X |
|
|
|
|
X |
|
Bean, dry |
|
|
|
|
X |
|
|
X |
|
X |
X |
|
|
X |
X |
|
|
|
|
|
|
|
X |
|
Bean, snap |
|
|
|
|
|
|
|
|
X |
|
|
|
X |
X |
|
X |
|
|
X |
X |
|
|
|
|
Beet |
|
|
|
|
X |
X |
|
|
|
|
|
|
|
X |
|
X |
|
|
|
|
|
|
X |
|
Broccoli |
|
|
|
|
X |
X |
|
|
|
|
|
|
|
|
|
X |
|
|
|
X |
|
|
X |
|
Brussels sprouts |
|
|
|
|
X |
X |
|
|
|
|
|
|
|
X |
|
X |
|
|
|
X |
|
|
X |
|
Cabbage |
|
|
|
|
X |
|
|
|
|
|
|
|
|
X |
|
X |
X |
|
|
|
|
|
X |
|
Carrot |
|
|
|
|
X |
X |
|
|
|
|
|
|
|
X |
|
X |
|
|
|
X |
|
|
X |
|
Cauliflower |
|
|
|
|
X |
X |
|
|
|
|
|
|
|
X |
X |
X |
|
|
|
|
X |
|
|
|
Cucumber |
|
|
|
|
X |
X |
|
|
|
|
|
|
|
|
X |
X |
|
|
|
|
|
|
|
|
Eggplant |
|
|
|
|
X |
X |
|
|
|
|
|
|
|
X |
|
X |
|
|
|
|
|
|
|
|
Lettuce and Endive |
|
|
|
|
X |
X |
|
|
|
|
|
|
|
X |
X |
X |
|
|
|
|
X |
|
|
|
Melon |
|
|
X |
|
X |
|
|
X |
|
|
|
|
|
X |
|
X |
|
|
|
|
|
|
|
|
Onion |
|
|
|
|
X |
|
|
X |
|
X |
|
|
|
X |
|
|
X |
|