Weed Management

Weeds compete with crops for water, light, and nutrients thereby reducing crop yield and quality.  Competitive effects of weeds are density dependent, with increasing weed density causing increasing crop yield loss. The magnitude of loss varies based on the crop and weed species present and the timing of competitive interactions. Carrot, beet and Alliums, for example, are small-seeded, slow to emerge, and are poorly competitive early in the growing season. Larger-seeded crops (e.g. pea, bean, and corn), tubers (potato), or transplanted crops start with a significant initial size advantage over germinating weeds. A first principle of weed management is to establish this size advantage, and then maintain it with early weeding or mulching to reduce the density of competition weeds. To this end, successful weed management requires good foundational agronomic and horticultural practices regarding seedbed preparation, fertility and moisture management, timing and density of planting, choice of variety, as well as necessary early management of insect pests and plant pathogens. Remember, in plant competition, “the big get bigger!”  

Weed Biology and Ecology

Weed identification is now easier than ever, with apps like “Picture This” getting better every year. While apps still perform poorly in identifying grasses and weed seedlings, recognizing your mature weedy flora is a great place to start. Weeds of the Northeast by Uva, Neal and DiTomaso is another useful reference for weed identification.

Weedy plant species often share certain traits or characteristics that contribute to their success as the early colonists of disturbed sites, which is after all, the ecological job or “niche” of a weed. Fundamental weedy traits include rapid growth, high amount of seed production, and most notably, seed/bud dormancy to ensure offspring germinate and attempt to grow over several future years-an impressive bet-hedging strategy. Weedy species are also known for their “plasticity,” in other words, genotypic flexibility whereby an individual may exhibit different morphology in response to its environment (e.g., bushy growth in full sun but erect growth in a dense crop). 

Annual plants complete their life cycle in one year: seeds germinate, seedlings grow to maturity, flower, and reproduce all within a single growing season. Winter annuals germinate in late summer or fall, overwinter, and set seed the following spring. Summer annuals germinate in spring or early summer and set seed before fall, often in response to shorter days. Timing of germination and thus emergence is affected by species specific dormancy characteristics and environmental conditions including light quality (e.g., affecting phytochrome), temperature, moisture, gas exchange, nitrate, among many other factors. These in turn are dramatically affected by soil disturbance (tillage or cultivation). Thus, it should be no surprise that annual weeds dominate the weedy flora of vegetable farms that rely on soil disturbance for residue management, seedbed preparation, amendment incorporation and weed control. 

Weed seedbanks vary widely across farms. The seedbank refers to weed seeds on the soil surface or buried in the soil. Generally, seedbanks are larger on organic farms compared to farms using herbicides. Larger seedbanks result in a higher density of weed seedlings, or greater “weed pressure” as described by some farmers. The “bank” metaphor is useful in thinking about management, specifically the aim to reduce “credits” to the bank, i.e., avoid or reduce weed seed rain, while encouraging “debits” or losses. A common misperception is that weed seeds last “forever” in the soil, so “why bother to manage the seedbank?” While it is true that a seed may occasionally last many years, perhaps decades, by far, most seeds germinate or die in the first year. In fact, many annual weed seeds have a half-life of less than one year. In other words, 50% of the seeds produced this season will be gone in less than a year. This is true for several important weeds in our region, including, redroot pigweed, common lambsquarters, hairy galinsoga, yellow foxtail, and crabgrasses.

Vegetable farmers are fortunate in having many options related to weed seedbank management. Consider a field that will be used for a sequence of short season crops like radish or leafy greens. Shallow tillage for seedbed preparation in this case serves to encourage weed germination, the most important of seedbank debiting mechanisms. This way, crop harvest can be completed and residues incorporated before weeds mature, thus preempting seed rain. 

Emergence periodicity is a useful aspect of weed biology that can be used to optimize fallowing or stale seedbed events that aim to reduce the germinable weed seedbank. Shallow tillage breaks seasonal dormancy for species ready to germinate. Thus, if targeting summer annuals, shallow tillage in June or July will encourage germination, while winter annual species will remain in the seedbank. 

Perennial weeds can live for more than one year and while most produce seed, vegetative propagation by stolons, rhizomes, or roots are generally more important.  Tillage operations often drag perennial weed fragments from sod headlands or farm roads into vegetable fields. Shallow tillage around field margins throughout the growing season can establish a fallow zone to avoid this dispersal from tillage.

Monitoring weeds is an important but often neglected part of a weed management plan.  Weed maps of field areas are extremely helpful in planning weed control strategies. A weed map can illustrate problem areas so that growers can target specific problems in specific areas and help plan for future crop rotations. Over time, weed maps can show shifts in weed pressure and indicate the possible need for a strategy change. Maps can also highlight the importance of managing dispersal, e.g., avoiding tillage that drags quackgrass rhizomes from grass alleys into fields, or working around a weed patch to avoid tillage dispersal. 

Dispersal is critical to the success of weeds, but rarely a priority for management. It is widely thought that weed seeds are blowing in from neighbors, hitching rides from birds or mammals, or washing in with irrigation or surface waters. These are important dispersal mechanisms, but they are rare events, generally moving only a very small fraction of a batch of seeds. In fact, well over half of most seeds are dispersed right around the base of the mother plant, and most remaining seeds not much further. In a natural setting, these seeds would be very crowded, with intense competition among the weeds when a cohort germinates the following year. In a farmed system, however, tillage events serve to disperse the seeds, spreading them out to the benefit of the weed. These local dispersal factors drive the patch dynamics that will be evident in your weed maps, with dense areas where reproduction was high, lower density radiating from the patch where tillage moved seeds, and then many other locations where weeds may be absent. 
 

Physical Weed Control

Physical weed control refers to actions that remove or kill weed seedlings, aiming to reduce weed density and thus maximize crop yield and quality. In vegetable crops, this often includes hand weeding, which is effective but expensive. Hand weeding time/cost is dependent on weed density, so efforts to reduce the weed seedbank and increase the efficacy of weeding tools will often improve net returns. The availability and cost of labor are key considerations, although it is common to perform some hand weeding in almost all vegetable crops.  When relying on hand labor, start weeding operations with wheel hoes that can cover a lot of area quickly no matter the weed density. Next move to long-handled hoes designed for weeding: notable favorites among veteran organic vegetable farmers include the Glaser stirrup hoe and the colinear hoe. Using tools to get as close to the crop row as possible will reduce the final hand weeding labor. When tools are used for weed removal, physical weed control is sometimes referred to as “mechanical weed control,” or simply “cultivation.”

Cultivation is an important component of weed control in vegetables, particularly when the use of herbicides and/or mulches is to be minimized or avoided. Efficacy and selectivity are important indicators of cultivation performance. Efficacy or “effectiveness,” refers to the proportion or percentage of weeds that are killed. A central problem with cultivation is that efficacy is low (60% is common) and highly variable (some places in the field may have 95% efficacy, while another may have only 5%). Herbicides, in contrast, have very high efficacy and very low variability. Selectivity refers to killing targeted weeds, but not the crop. Cultivation tools are not particularly sophisticated in their operation, rather, selectivity is generally based on a size differential between the crop and weed. A first principle of physical weed control is to establish and then maintain the crops’ size advantage.

Cultivation strategies should start with careful seedbed preparation. Flat, firm, residue-free soil surface conditions will allow weeding tools to consistently function throughout the field. A light-weight field cultivator or soil conditioner with a rolling basket is often a good choice. While large-seeded or transplanted crops do not require a perfect seedbed for planting and stand establishment, later cultivation operations will benefit from these early season efforts to prepare a nice seedbed.

Weeds are most effectively cultivated shortly after they germinate, and crops are most sensitive to weed pressure during their early stages of growth. Thus, cultivation is most critical early in the growing season. 

Tools should be carefully adjusted, first in the shop, and then in the field after some testing. Row-crop tools should target the same number of rows that were planted, or a simple fraction of this. For example, if using a one-row seeder, plan to cultivate one row at a time. If planting two or four rows, use a two or four row cultivator. Hand planting with a push seeder is generally not going to be suitable for later tractor-mounted cultivation tools.

Tools should be adjusted to work as shallowly as possible to minimize movement of seeds in the seedbank and dormancy breaking. After carefully adjusting spacing, place a 3/4" thick board under the gauge wheel of parallel linkage units (depth-controlling units), and then drop tools to the floor and tighten. Mark lines on the shop floor indicating crop row spacing to allow precise adjustment relative to the row. Magnetic levels are handy to adjust top-linkage of three-point hitch tools, and a protractor can be used to check the angles of tools to either avoid soil movement or hill as desired.

Cultivation tools can be broadly grouped by the area of soil they disturb, with so-called “blind cultivation” working the entire tool width, and “row-crop cultivators” aiming to control weeds as close to the crop as possible.

Blind cultivation is performed after crops have been planted, over the top of them, so that both in-row and between-row areas are cultivated. To minimize crop damage, this should be done before the crop has emerged and/or once it is well rooted. Slightly deeper planting depths and slightly higher plant populations are recommended to compensate for some crop loss that may occur. Very tender crops such as leafy greens are not amenable to this technique, but a surprising array of crops are suitable for blind cultivation, including corn, cucurbits, beets, etc. depending on stage of growth and equipment used.

Tine harrows have multiple rows of flexible metal tines that cover the entire soil surface, wiggling slightly as they are pulled along, uprooting or dislodging very small weeds. They work best at relatively high speeds, and the wide span of the tool makes cultivation quick. They are rear-mounted and available in many widths. The tension on the tines can be adjusted on some units or by the pressure on the 3-point hitch on other units. Gauge wheels can be used to maintain uniform depth. Tine harrows work best in friable soils free of rocks, and when weather allows weeds to dry out on soil surface after uprooting.

Rotary Hoes have many narrowly spaced metal wheels each with about 16 curved teeth that work the surface of the soil. Used within the first few weeks of planting vigorous crops like corn or beans, they destroy weeds that have just germinated. Ground-driven, spring-loaded wheels do little damage to the crop and work well at high speeds on dry, rock-free soils with little residue. They are rear mounted, in many widths. The teeth, or spoons, lose their effectiveness if not sharp.

Between-row cultivation can be performed with varying degrees of aggressiveness, so that soil may or may not be pushed into the row to obtain some between-row weed control. Commonly used cultivation setups consist of a shank (either straight, C-, or S-shaped) attached to a toolbar, with a cultivating tool (shovel, sweep, knife, hilling disc, etc.) attached to the bottom. The more curve to the shank, the more it will vibrate and flex in the soil. Trip-shanks have a release mechanism that allows the shank to pop up when it hits a rock. Shanks can be arranged on multiple toolbars to offer complete coverage between multiple rows in beds. There are many kinds of tools that can be attached to the shanks, and these should be selected to disturb only as much soil as is needed to kill the weeds present. In general, one starts the season with smaller, shallower tools to kill small weeds when the crop is small; as the season goes on, if larger weeds are present, then more aggressive tools are needed. As the crop grows it is possible to more aggressively push soil into the row without causing damage. For example, shovels might be followed by sweeps and then hilling discs. Another strategy is to adjust the angle of cultivating tools such as hilling discs or rolling cultivators as the season goes on. Start by pulling soil away from the row when cultivating a young crop; that creates a small hill that can be pushed back into the row when the crop is a bit older to bury weeds.
In small and/or tender crops like leafy greens, shallow tools are needed that will not move soil into the row and cause damage. A side-knife (or beet-hoe) may be used to cultivate horizontally next to the crop, just below the surface.

Basket weeders are relatively high-speed, between-row cultivators good for control of small weeds in narrow rows when crop is small. The wire baskets work the soil surface and do not move soil into the row, but they don't work well in crusted or rocky soil. The front set of baskets are ground driven, and they turn the rear set of baskets a bit faster via a chain and gearing, causing scuffing of the soil. These are available in 2-6-row units that can be rear- or belly-mounted.

Finger weeders consist of steel cone wheels that are ground-driven by spike tines on the bottom, with rubber fingers on the perimeter.  The rubber fingers work the soil just below the surface, uprooting small weeds located very close to the crop. Finger weeders work best for control of small weeds in dry, friable soil with few rocks or residues. Clay soils may stick to fingers.

Spring-hoe, torsion weeders, and spyder weeders are flexible blades and square metal stock that disturb soil around the base of plants, and ground-driven spyder wheels with staggered teeth in an uneven pattern that break clods and throws soil into row, or pull it away, depending on the angle. These can be toolbar-mounted, either under the belly or in the rear. They can be used together, separately, or in conjunction with other cultivators.

Rolling cultivators have gangs of soil-driven 'spider wheels' that mount independently on a toolbar. The angle that they work the soil, and thus their aggressiveness, is usually adjustable. The number of gangs grouped together determines cultivator width, and these are usually rear-mounted, but pairs of gangs may be belly mounted to work a row or two. Soil can be thrown into row to bury small weeds or to form hills, depending on angle of the gangs. This is a relatively heavy, aggressive tool.

Reigi weeders require a rear operator who steers a pair of rotating horizontal wheels in and out of the crop row. The wheels are turned by a PTO-driven belt, and they have stiff tines on them that root out weeds. The wheels come in several sizes useful for various row spacings, and the units come in 1- or 2-row models. These are very effective for killing weeds in and next to the row in widely spaced crops like pumpkins, first-year strawberries, or sweet corn with a lot of skips.

Flame weeders can be used to kill weeds before planting without causing soil disturbance that brings up new weed seeds, after the crop has been planted (but before it emerges), inter-row weeding, and for stale seed bed preparation. Flame weeding, also known as thermal weeding or flame cultivation, exposes plants to brief periods of high temperature that causes the water in the plant tissue to expand rapidly, rupturing plant cells and leading to tissue damage.  Plants are not burned or incinerated, but "blanched". They will not show symptoms of injury for several hours after exposure.  Some weeds, such as purslane, can tolerate high temperature, and grasses with their growing points below ground are not controlled by flaming. When weeds are moist from rain or dew, more heat (a slower tractor or walking speed) will be necessary.

Like with contact herbicides, flaming kills weeds without soil disturbance, it is ideal for stale seedbeds. Once broadleaf weeds reach the three-leaf stage, they should be flamed to prevent them from growing too large. For longer lasting weed control, apply the final flaming as late as possible prior to crop emergence after seeding or just prior to transplanting.

Hand-held propane torches are commonly used to flame single rows at a time, but multi-row bed-flamers and tractor-mounted flamer kits are also available. Larger units require greater attention to safety during construction and operation. Safety is a big issue with flaming. Consult with a gas professional if constructing your own flaming unit. Do not mount propane tanks intended for stationary use onto tractors. Flame against the breeze and avoid areas with dry residues or dry hedgerows. Liability concerns may hinder the use of flaming.
 

Cultural Weed Management

Cultural options include cover crops, fallowing, plant competition, mulches, soil preparation, stale beds, and crop rotation.

Cover crops alone do little to reduce overall weed populations, but shallow tillage between short cycles of cover crop growth is effective.  The tillage kills weed seedlings and encourages germination of a new “flush” of weeds that can be killed with the next disturbance.  Through these cycles, the objective is to encourage weed seed germination but not to allow further weed seed production.  A dense stand will provide weed suppression while it is growing which is important for a season-long cover crop in which weeds may mature and set seed.  Cover crops can also slow the warm-up of soil and provide shade, both helping to slow weed seed germination and reduce the soil seed bank over time. Perennial weeds will increase in long-term sod crops, e.g., one or more years of red clover.  

Fallowing is not planting a field with the intention to reduce weed seed populations. Repeated shallow soil disturbance will encourage weeds to germinate while subsequent events kill seedlings before they go to seed. Even in the absence of a cover crop, this strategy will reduce the weed seed bank of a field.
Plant competition is the foundation of weed management.  Remember, “the big get bigger.” Large-seeded crops and transplants have an initial size advantage over weeds.  Decreasing the space between crops will also increase soil shading.  Overall, the more rapidly a crop can cover the soil ahead of weed emergence, the more competitive that crop will be. Choose high quality seed, calibrate equipment to ensure accurate seeding rates and depth, and where possible, supply resources selectively to the crop, e.g., apply fertilizers banded below/near the crop row, or drip irrigation, instead of broadcasting.

Mulches are often used to control weeds.  Mulches can be organic (straw, hay, grass clippings, dead cover crops) or inorganic (plastic).  Organic mulches are effective if they are thick enough to keep weeds from emerging through them (usually at least 2-3").  Downsides of organic mulches are that they can be expensive, they slow soil warm up or reduce soil temperatures, and they can harbor animal pests.  Cooler soil temperatures can be a problem in warm season crops.  It is recommended that the mulch application be delayed allowing the soil to warm up sufficiently for the crop.  Black plastic mulches will warm soil and eliminate weed pressure.  However, weeds emerging through the planting holes and between strips of plastic mulch can still reduce yields if not controlled.  Infra-Red Transmitting (IRT) mulches are less effective than black plastic for controlling weeds, and clear mulches can enhance weed growth.  Some growers plant cover crops between plastic mulch strips as "living mulch", but these cover crops can also compete with the crop.  Killing the living mulch before the crop is planted, mowing the mulch on a regular basis, or using raised beds will help to reduce but not eliminate competition.  See the section on using herbicides in combination with plastic mulches later in this section.

Proper soil preparation can influence weed emergence.  Soils which are rough and less firmly packed will yield fewer weeds than those that are more finely worked, more compacted, and more uniformly moist. However, as noted above, a well-prepared seedbed will help weeding tools function to their potential during later cultivation events.

Stale seedbed or summer fallowing is performed on fields that have been prepared for planting, either in the spring before a crop is sown, or in the summer after a spring crop but before a fall crop. The soil is then lightly disturbed on a regular basis to kill small weeds as they emerge, without bringing up new weed seeds from below the top few inches of soil. Early in the year, broadleaves will not be controlled if they have not yet emerged, so a summer fallow works better on them. Perennial weeds may be weakened but not killed. Tools that can be used for this practice include chain-drag, spring-tooth harrow, light-weight disc harrows, or tine weeders.  See additional information on the stale seedbed technique later in this section.

Crop rotation can be a tool for managing weeds. Weed species present tend to be most like crop planted. Examples include grasses in corn, winter annuals with early-planted crops, and perennial weeds with perennial crops. Rotating crops among these groups will tend to disrupt this trend.

About Herbicides

Herbicides are chemicals designed to control weeds. The use of these materials must be exact for satisfactory results. Proper rate selection, timing of application, activation, and observance of all precautions on the label must be followed to obtain optimum performance. Each herbicide controls certain weeds or families of weeds. Therefore, knowledge of the type of weed species present in the field is essential for good weed control. Once the weed problem is known, select the proper herbicide.

Herbicides can be systemic, meaning that they are absorbed and moved throughout the plant, while others are contact herbicides, meaning that they only affect the plant tissue they come into contact with. 

Preemergence Herbicides (PRE): These herbicides are applied prior to the emergence of weeds.  In general, they work by preventing weeds that are germinating from seeds from growing, but some can also act on perennial plants that spread by rhizomes, tubers, and stolons.  Application is timed to coincide with when the target weeds are about to begin germinating.  These herbicides prevent new weeds from establishing, but most will not impact weeds that are already emerged. 

Preplant: These herbicides are applied before the crop is planted.  They can be preemergence products that are incorporated into the soil or applied to control existing vegetation. 

Postemergence Herbicides (POST): These herbicides work on actively growing weeds.  They can be absorbed by the leaves or roots of the plants, depending on the chemical. 

Adjuvants: These products are added to a pesticide mixture to improve its effectiveness.  They include surfactants, stickers, penetrants, compatibility agents, etc.  Pesticide labels may list specific types of adjuvants that will maximize effectiveness of the pesticide.  Be sure to use the proper category of adjuvant if the manufacturer makes a specific recommendation.

When adjuvants are recommended, it is because research has shown that their addition increases efficacy of the herbicide.  If the label advocates the usage of an adjuvant, do not omit the adjuvant solely to save money.  The most common adjuvants used with herbicides are nonionic surfactants (NIS) and crop oil concentrates (COC) which can consist of petroleum, vegetable, or methylated vegetable or seed oils. They increase penetration of the herbicide through the leaf cuticle.

Herbicides and Crop Rotation Restrictions

Some herbicides can have long-lasting activity.  Many herbicides have crop rotation restrictions where they have been applied.  For example, Devrinol has a 60-day plantback interval for leafy greens. Check the label of each product for details.

Toxicity of Herbicides

The toxicity of pesticides varies by the active ingredient, concentration of active ingredient, and the formulation of the product (e.g. liquid, powder, etc.).  The toxicity of a pesticide is expressed in terms of oral (administered internally) and dermal (applied to the skin) LD50. LD50 is the dosage of poison that kills 50% of test animals (usually rats) with a single application of the pesticide product and is expressed as mg/kg of body weight. The lower the LD50 value, the more toxic the material.

The acute (short-term) toxicity of the formulated product is conveyed on label by a “signal word” stated on the front page. 

DANGER - pesticide product is highly toxic by at least one route of exposure. It may be corrosive and cause irreversible damage to the skin or eyes.  If the product is highly toxic if eaten, absorbed through the skin, or inhaled, then the word “POISON” must also be included in red letters.

WARNING - pesticide product is moderately toxic if eaten, absorbed through the skin, inhaled, or it causes moderate eye or skin irritation.

CAUTION - pesticide product is slightly toxic if eaten, absorbed through the skin, inhaled, or it causes slight eye or skin irritation.

General Principles for Safe Use

  • Know the herbicide. Read the label.
  • Check the output of sprayer frequently.
  • Replace worn nozzles. It may be necessary to replace them several times a season if the sprayer is used constantly.
  • Rinse spray equipment immediately after use. Use one sprayer for herbicides and another for insecticides and fungicides.
  • For restrictions on rates, timing, and crops for which the herbicide is approved, see Table 27.
  • For degree of susceptibility of each weed to a specific herbicide, see Table 28.
  • For limitations and special requirements of the herbicide, refer to the product label (or supplemental label).

Rate Selection

Always check the label to determine the proper rate to apply. For most soil-applied herbicides, knowledge of the type of soil and the percentage organic matter usually determines the rate. Generally, the more clay and/or organic matter present in the soil, the higher the herbicide rate necessary for good weed control. For postemergence herbicides, the type of weed, as well as its size, will usually determine the rate.

Incorporation of Herbicides

Some herbicides must be incorporated into the soil to be effective. Herbicides are incorporated because they are volatile and evaporate into the air if left on the soil surface or they will decompose when exposed to sunlight. Herbicides differ in their incorporation requirements; check the product label for the manufacturer's requirements.

Herbicide Sprayer Systems

  • Select a sprayer and pump that can deliver a volume of 20-50 gallons per acre. Most herbicides are applied at rates of 20-40 gallons of water per acre. Pressures of 20-40 p.s.i. at the nozzle are recommended for most herbicides. Higher pressures result in finer droplets and increase the chance for more drift. Lower pressures sometimes cause uneven spray patterns.
  • Use 50-mesh screened filters for nozzles and suction lines.
  • Select 80º-73º flat fan nozzles. Because of wear, brass tips used exclusively for applying wettable powders should not be used on more than 30 acres before being replaced. Use stainless steel or hardened stainless steel tips for longer wear. Stainless steel nozzle tips are more than twice the cost of brass tips but last about 20 times longer. Hardened stainless steel tips are only slightly more expensive than stainless steel tips but last three times longer. Ceramic nozzles are the most durable.
  • Calibrate sprayers frequently and check for wear, especially when wettable powders have been used.

Resistance Management

Pesticide resistance is an inheritable (genetic) characteristic of a pest that makes it less sensitive to a pesticide.  Repeated use of the same pesticide (or pesticides with the same mode of action) over time kills pests that are susceptible to the pesticide and leaves behind individuals that are less sensitive. These then reproduce and pass on the genes that let them survive pesticide exposure to their offspring. With herbicides, this usually takes several years to accomplish. This is because of the seed bank in the soil that preserves individuals that are not resistant for many years.

International groups have been founded for a cooperative approach to resistance management. They have assigned group numbers to pesticides to help growers make decisions on how to rotate pesticides. They are based on mode of action –how and where the chemicals in the pesticide work on the target. The Herbicide Resistance Action Committee (HRAC) and The Weed Science Society of America (WSSA) have developed a harmonized classification system of herbicides using numbers to designate the categories. A key step in resistance management is to minimize the continuous use of herbicides with the same mode of action through rotations and combinations of products. One of the purposes of these classification systems is to make it easier for farmers and farm advisors to understand which herbicides share the same mode of action without having to actually know the biochemical basis.

Most labels now come with a group number assigned to them. Some active ingredients are available under several different product names, and sometimes different active ingredients have the same mode of action.

Resistance management may include alternating or sequencing products with different modes of action or limiting the total number of applications per season. When selecting herbicides for resistance management, use the group number as your guide and NOT the product name or active ingredient.

The most effective way to extend the useful life of an effective product is to rotate herbicides within fields if the same crop is grown or to rotate crops so that different herbicides might be used in following years. The most common weed resistance issue that we have in New England is common lambsquarters that is resistant to atrazine. To help select pesticides with a different mode of action, see chemical resistance groupings in Table 27.

Once a weed develops resistance to a group of pesticides with a particular mode of action, a higher rate of the same or a similar chemical from the same group usually will not control the weed.

There are many other techniques that can help delay the onset of resistance. Using other options and recognizing weeds that may be resistant is critical.

  • Integrate chemical control with effective cultural, mechanical, and physical options.
  • Scout fields so that you are aware of what is not controlled and can take steps to control the escaped weeds.
  • Good rate selection, spray coverage, and herbicide activation helps do the job right the first time and avoids unnecessary repeat applications: use the proper size nozzles and the correct angle or orientation and the right amount of water per acre.
  • Time postemergence applications so that the weeds are at the right stage of growth.

NOTE: The group number is specific for each type of pesticide (insecticides, herbicides, and fungicides). For example, there is no problem when using material from the herbicide Group 1 and an insecticide or fungicide from Group 1.

Groundwater Concerns

The following herbicides have the potential to impact groundwater due to their chemical characteristics and toxicological profile and have been discussed in the crops sections. Check with your state for restrictions on their use in sensitive areas.  For example, Massachusetts has Zone II designations and regulations pertaining to the use of these herbicides in those areas.

  • Atrazine (Aatrex)
  • Bentazon (Basagran)
  • Chlorthal-Dimethyl (Dacthal, DCPA)
  • Dimethanamid (Outlook)
  • Diuron (Karmex)
  • Fluthiacet-methyl (Cadet)
  • Metolachlor (Dual Magnum)
  • Metribuzin
  • Pronamide (Kerb)
  • Simazine (Princep)

Herbicides Alphabetically Listed by Active Ingredient

* Federally restricted

bensulide (Prefar): preplant incorporated herbicide for control of various grass and certain broadleaf weeds in many vegetable crops.

bentazon (Basagran): postemergence herbicide in beans, peas and sweet corn for control of yellow nutsedge, smartweed and jimsonweed.

bromoxynil (Broclean, Maestro, and others): Note: some brands are registered in some states, and not others. For examples, Maestro is not registered in NH and VT while Broclean is registered in all New England states.  Check state pesticide registrations before selecting a product to make sure it is legal to apply in your state.  Used for postemergence control of broadleaf weeds when applied at the seedling stage. For use in garlic and onions.

carfentrazone (Aim): postemergence control of many broadleaf weeds in sweet corn, pumpkin, and winter squash.

clethodim (Select Max, Intensity, IntensityOne): postemergence grass herbicide for use in many vegetable crops.

clomazone (Command): preemergence control of most grasses and some broadleaf weeds in pumpkins and peppers.

clopyralid (Stinger): postemergence herbicide for control of many annual and perennial broadleaf weeds in beets, sweet corn, spinach and turnip.

cycloate (Ro-Neet): preplant incorporated herbicide to control annual grasses, nutsedge, certain perennial grasses and many broadleaf weeds in beets and spinach. MA, NH and ME only.

DCPA (Dacthal): preemergence herbicide for control of annual grasses and certain annual broadleaf weeds in many vegetables. Restricted use in MA.

dicamba (Banvel, Clarity): postemergence herbicide in asparagus.

EPTC (Eptam): for control of annual grassy weeds, nutgrass, perennial weeds such as Johnsongrass seedlings, quackgrass and several broadleaf weeds in beans (green) and potatoes. Restricted use in VT.

ethalfluralin (Curbit, Sonalan HFP): preemergence control of many annual grasses and broadleaf weeds in some vine crops (Cucurbit) and dry beans (Sonalan).

fluazifop (Fusilade DX): postemergence grass control herbicide for use on carrots, dry bulb onions, spinach and non-bearing asparagus.

flumioxazin (Chateau or Valor):  preemergence and limited postemergence control of many broadleaf weeds and some grasses.

fomesafen (Reflex): pre- and postemergence weed control in beans and potatoes.  Can only be used every-other year!

glyphosate (Roundup): many formulations available with varying % a.i. such as Roundup PowerMax, Roundup Pro, etc., in addition to many generic glyphosate products. A postemergence herbicide absorbed by the foliage of emerged annual and perennial weeds.

halosulfuron (Sandea): preemergence and postemergence herbicide for use in asparagus, beans, corn, cucurbits, and fruiting vegetables.

imazamox (Raptor): for early postemergence weed control in dry beans, lima beans, snap beans, and english peas.

linuron (Lorox): preemergence or postemergence herbicide for control of germinating and newly established broadleaf weeds and grasses in carrots and parsley. Restricted use in VT.

mesotrione (Callisto): pre- and postemergence control of many broadleaf weeds in sweet corn and asparagus. Generics are also now available.

metolachlor (Dual Magnum): preplant incorporated and preemergence weed control in corn, potato, pumpkin, and tomato.  Restricted use in MA and VT.

metribuzin (Metribuzin): preemergence and early postemergence herbicide for control of a large number of grass and broadleaf weeds in potatoes and tomatoes. Restricted use in MA and VT.

napropamide (Devrinol): preplant incorporated herbicide for use in peppers, eggplant, tomatoes, asparagus and cole crops. For good control of barnyard grass, crabgrass, fall panicum, goosegrass, lambsquarter, pigweed and purslane. Photodegradable, must be incorporated within 24 hours by discing or watering in.

norflurazon (Solicam): preemergence herbicide in asparagus.

oxyfluorfen (Goal): for control of certain annual grassy and broadleaf weeds in onions and crucifers.

*paraquat (Gramoxone): restricted-use pesticide. A contact "burndown" herbicide with no soil activity. May be fatal if swallowed or inhaled. Applicators must complete an EPA-approved paraquat training listed on the following website https://www.epa.gov/pesticide-worker-safety/paraquat-dichloride-training.... The training must be completed a minimum of every three years.

pelargonic acid (Scythe): postemergence contact herbicide for stale bed and directed/shielded applications in many vegetable crops.

pendimethalin (Prowl): preemergence herbicide for control of broadleaf weeds and grasses in asparagus, corn, peppers, potatoes, garlic, and onions. Restricted use in VT.

prometryn (Caparol): pre- and postemergence control of annual broadleaf weeds in carrots, celery, and parsley.

*pronamide (Kerb): preergence herbicide for weed control in lettuce.

quizalofop (Assure II): postemergence grass herbicide for use in beans and peas.

rimsulfuron (Matrix): preemergence and early postemergence herbicide for control of many grass and broadleaf weeds in potato. Restricted use in MA and VT.

sethoxydim (Poast): postemergence grass control herbicide for use in most vegetable crops. See label.

terbacil (Sinbar): preemergence and early postemergence herbicide for control of many broadleaf weeds in asparagus.

trifluralin (Treflan): preemergence herbicide that must be incorporated for control of many annual grasses and certain broadleaf weeds in many vegetable crops.

2,4-D (Amine 4): postemergence control of broadleaf weeds in asparagus and sweet corn. Restricted use in MA and VT.

Herbicides specific to corn

(not registered on other crops discussed in the New England Vegetable Managment Guide)

*acetochlor (Harness, Surpas): used for pre- and postemergence control of annual grasses and some broadleaf weeds in corn. Not registered for use in NH.

ametryn (Evik):  used for postemergence weed control in field corn and popcorn. Not for use in ME, NH, or VT.

*atrazine (AAtrex): provides season-long control of most broad-leaved weeds in sweet corn. Applied either preemergence, postemergence or in combination with other herbicides.

dimethenamid (Outlook): preemergence herbicide which provides grass and some broadleaf weed control in sweet corn.

fluthiacet-methyl (Cadet): Restricted use in some states. Not for use in fresh sweet corn. For use in field Corn, sweet corn grown for processing only, and popcorn.

nicosulfuron (Accent Q):  postemergence grass herbicide in corn.

pyroxasulfone (Zidua): used pre- or early postemergence for weed control in corn. Not registered in VT.

safluenacil (Sharpen) - used as a preplant surface, preplant incorporated, or preemergence to corn for broadleaf weed control. Not for use on sweet corn grown for fresh market use.  Some corn varieties are sensitive to injury form this herbicide. Also used as a harvest aid for dry beans.

simazine (Princep): Restricted use in some states. Used preemergence before weeds and corn have emerged.

topramezone (Impact):  postemergence control of many broadleaf weeds and some grasses in corn.

tembotrione (Laudis): postemergence herbicide for control of many broadleaf weeds in corn.

Note: Always check current registrations to be sure the herbicide is registered for the specific crop in question and state of use. Always read and follow all label instructions before using any pesticide!

Herbicides and Plastics

With the increased use of plastic culture for the production of various vegetable crops, it is very important to keep in mind a few points for proper weed management. Here are some tips:

  • If methyl bromide or Vapam fumigation is used under the plastic strips, there is no need for a herbicide under the plastic. If soil fumigation is not used and black plastic is, it may not be necessary to put a herbicide under the plastic (unless nutsedge or other weeds are excessive). If you use clear or white plastic, without soil fumigation, then you need a herbicide under the plastic mulch. To get the best results, take the following steps for herbicide application:
  • Prepare a pressed bed.
  • Apply the herbicide to the bed surface.
  • Activate the herbicide with rainfall or overhead irrigation (at least 1/2"); if the herbicide needs to be mechanically incorporated, do so and repress the bed.
  • Apply the plastic mulch. Note: Some herbicides may volatilize and cause crop injury when used under plastic mulches or row covers.  Some herbicides are prohibited from use under plastic. Be sure to observe any cautions on the product label. Wherever possible, try it on a small scale first.
  • If weeds are present between the plastic strips before planting, use a banded application of Gramoxone plus surfactant, or Roundup (if registered for stale bed use) to kill all existing vegetation. Depending on the level of weed vigor, a second application of Gramoxone plus surfactant may be needed before planting. Do not broadcast apply these herbicides to the surface of the mulch.
  • For weed control between the plastic strips, after planting, use a registered preemergence herbicide for that crop. Do not use nonregistered herbicides between strips as the crop roots will grow in that zone and, in many cases, the crop will be damaged or killed. Do not spray the surface of the plastic with any preemergence herbicides! During a rainfall the excess herbicide will wash into the holes where the crop was planted and will greatly concentrate the herbicide. Thus, the crop will be damaged or killed. When banded applications of herbicides are used, remember to adjust the rate of the herbicide downward to conform to the actual ground area being sprayed (with 30" beds and 30" areas between strips, only half the normal amount of herbicide would be required per acre since only half the amount of soil area per acre is actually being sprayed).
  • Backpack sprayers or modified boom sprayers are often used for herbicide applications in plastic culture systems. Be sure to calibrate them properly and maintain a constant pressure using a pressure gauge, especially on hand-pumped models. Maintaining a constant speed or walking pace is also necessary. Three miles per hour is a good speed (23 seconds per 100').
  • Use caution when spraying Gramoxone with a backpack or hand-held sprayer. Wear rubber boots, gloves and a mask or respirator. Avoid skin contact with the spray. If contact occurs, immediately rinse the area with water or rub soil on your skin. Soil will help soak up and bind the chemical, preventing its absorption into the skin. Spray when the wind is calm to avoid spray drift. Also, always use a surfactant and plenty of water with Gramoxone for good control.
  • Vapam is not recommended for use in the spring if the plastic is to remain in place. Under the cool soil conditions at this time of year, the soil will not aerate properly and the crop will be damaged. In plastic culture, these materials are best used if the plastic is applied during the fall before planting. Be sure with all fumigants to space the injection shanks correctly. The label recommends a 6" spacing with Vapam.
  • Herbicide use under row covers can be dangerous. Although several herbicides can be safely used under row covers, some can cause crop injury and even crop death. Generally, ventilated covers are safer than solid covers or hot caps, from an herbicide injury standpoint. This is especially true with an herbicide that is moderately or highly volatile. Test on a small scale before you apply it on a large scale.

Stale Seedbed Technique

Certain weeds may be present in crop areas which herbicides and other management techniques cannot control well in the crop system. The stale seedbed technique can result in improved weed management in problematic areas or with problematic weed species. After soils are prepared for planting, most of the weed seeds in the upper 1-2" of the soil will germinate within two weeks if soil moisture and temperature are adequate. The stale seedbed approach involves preparing the soil as if for planting, without actually planting the crop. Instead, weeds are allowed, even encouraged (with irrigation or row covers), to grow. Weeds are then killed with Gramoxone, Roundup, Scythe or flaming.

Killing emerged weeds with herbicides or flaming does not disturb the soil, and no new weed seeds will be brought close to the soil surface. After using the stale seedbed technique, care should be taken not to disturb the soil any more than is absolutely necessary during the seeding or transplanting process to minimize the amount of weed seeds that are brought up to the surface to germinate. Preemergence herbicides can also be used after to increase the efficacy of stale seedbeds. Any cultivation performed after should be kept extremely shallow (3/4"-1" maximum) so as not to reposition any additional weed seeds.

On sandy, loamy or high organic matter soils, the soil should not crust and modern seeders should still work satisfactorily. On heavy clay soils, crusting could make this technique unusable.

Stale Seedbed Steps:

  • Prepare the soil as if you are about to seed or transplant. If a soil-incorporated herbicide is used, it must be applied and incorporated at this time. The soil should have good moisture (irrigate with 1/4" of water if necessary).
  • Wait as long as possible to allow weeds to germinate and emerge. Allow weed seedlings to grow to the third leaf stage, or at least to the first true leaf.
  • If you're using transplants: flame the soil or make an application of Gramoxone, Scythe, Aim or Roundup (if registered for the crop) to the soil surface before transplanting. Transplant the crop (without dragging any additional soil off the bed) and then apply any preemergence herbicide, which you would normally use, to the soil surface.
  • If the crop will be seeded: Gramoxone, Scythe, Aim or Roundup (if registered for the crop) or flaming may be applied just before or just after seeding (see the label). After seeding, apply any preemergence herbicide which you would normally use to the soil surface.  CAUTION: If the crop has already been seeded, be careful that the flaming process does not injure the crop seed or the emerging crop seedling.

Check the current herbicide label and recommendations by crop to determine if Gramoxone, Scythe, Aim or Roundup is registered for use in that crop. Gramoxone, Scythe, Aim and flaming will have minimal long-term effect on established perennial weeds. For cucumbers, melons, squash, pumpkin, peppers and eggplant, Roundup must be applied at least three days prior to seeding or transplanting.

In cases where Roundup is registered, it can also be used for control of perennial weeds, such as quackgrass and dock, prior to soil preparation. After application, delay tillage for 3-5 days. There is no residual weed control. See the label for directions.

Table 27: Herbicides and Crops Registered

This is not a comprehensive list of all registered herbicides, but a reference of those more commonly used and registered in multiple crops.  Please refer to individual crop sections for more information on herbicides.

Herbicide
Active Ingredient
Resistance Group1
Oral LD50
Signal Word
Asparagus
Beans**
Brassicas**
Beets
Carrots
Celery
Cucurbits**
Corn, Sweet
Eggplant
Garlic
Lettuce
Onions**
Peas, green
Potatoes, Irish
Peppers
Pumpkins/Squash
Spinach
Tomatoes
Aim carfentrazone 14 5,000+ Caution R R R R R R R R R R R R R R R  R R R
Assure II quizalofop 1 5,900 Danger   R                     R          
Basagran bentazon 5 2,000 Caution   R           R         R          
Callisto mesotrione 27 5,000+ Caution  R             R                    
Caparol prometryn 5 5,000+ Caution         R R                        
Chateau/Valor flumioxazin 14 5,000+ Caution R                 R                
Command 3ME clomazone 13 5,000+ Caution R R R       R           R   R R    
Curbit/Sonalan ethalfluralin 3 3267 Danger   R         R             R   R    
Dacthal DCPA 4 10,000+ Caution     R       R   R     R           R
Devrinol napropamide 0 2,000+ Caution R   R           R           R     R
Dual Magnum s-metolachlor 15 3,425 Caution R^ R R^ R^ R^   R^ R   R^   R^ R R R^  R R^  R
Eptam EPTC 15 1,325 Warning   R                       R        
Fusilade fluazifop 1 5,000+ Caution R R     R         R R R            
Goal oxyfluorfen 14 5,000+ Warning                   R   R            
Gramoxone SL* paraquat 22 1098 Danger R R R   R   R R R R R R R R R R   R
Kerb* pronamide 3 5,000+ Caution                     R              
Lorox linuron 5 3,489 Caution R       R R   R         R R        
Matrix rimsulfuron 2 5,000+ Caution                           R       R
Metribuzin metribuzin 5 2,375 Caution R       R                 R       R
Outlook dimethenamid 15 695 Warning   R           R   R   R   R        
Poast sethoxydim 1 4,100 Warning R R R R R R R   R R R R R R R R R R
Prefar bensulide 0 960 Caution     R     R R   R R R R     R R    
Prowl pendimethalin 3 3,956 Caution R R     R   R R   R   R R R  R      
Ro-neet cycloate 15 3,129 Caution       R                         R  
Roundup glyphosate 9 5,000+ Caution R R R R R R R R R R R R R R R R R R
Sandea halosulfuron 2 1,287 Caution R R         R  R R           R R   R
Scythe pelargonic acid 17 5,000+ Warning R R R R R R R R R R R R R R R R R R
Select Max clethodim 1 5,000+ Caution R R R R R R R   R R R R R R R R R R
Sinbar terbacil 3 3,700 Caution R                                  
Stinger, Clean Slate clopyralid 4 5,000+ Caution R   R R                         R  
Treflan HFP trifluralin 3 5,000+ Caution R R R   R       R       R R R     R
2,4-D 2,4-D 4 1,161 Danger R             R                    

*Federally restricted      **not all herbicides for use in all vegetables in this group,  see crop section for details 
1Weed Science Society of America resistance code.  ^ indemnified label available in some states 

R = registered for crop; check label to confirm crop is listed before use. Note that some products are not registered in all states.

Table 28: Relative Effectiveness of Herbicides on Weed Species

Herbicide
Active Ingredient
Barnyard Grass
Crabgrass
Chickweed
Carpetweed
Fall Panicum
Fox Tail
Galinsoga
Johnsongrass (Rhizome)
Johnsongrass (Seedling)
Jimsonweed
Lambsquarter
Morning Glory
Mustard
Nightshade
Nutsedge yellow
Purslane
Pigweed
Quackgrass
Ragweed
Smartweed
Velvetleaf

Accent Q

nicosulfuron

 E

P  

   

E

 E

 

 P

 E

               

 P

     
Aim carfentrazone P P P G P P G P P G E E P G P G E P F G E
Assure II quizalofop E E N N E E N E E N N N N N N N N G N N N
Atrazine* atrazine G G E E P G E P P G E G E G P E E F E E P
Basagran bentazon P P G G P P G P P E F F G P G G P N G E G
Callisto - pre mesotrione F F E E P P E P P E G F P E P P E P E E E
Callisto - post mesotrione F F E E P P E P P E G F E E F P E P F E E
Command clomazone G G   N G G F N F F G P N   N G P N F F E
Curbit ethalfluralin E E G E E E P P E P F P   G P E E   F F F
Dacthal DCPA F E G G F E P P G P E P N P P G E P P P P
Devrinol napropamide G E E G G E G P P P F P P P P G G P G F P
Dual metolachlor G E G G E E E P F P F P P G G E G P F F P
Eptam EPTC G E F G G E F P G P G P P P G G G F P P E
Fusilade fluazifop E E N N E E N N E N N N N N N N N G N N N
Goal oxyflouren F F E E F F G P F F E P   E P E E   E E F
Gramoxone paraquat E E G E E E G P F G E G G G P G E F E G G
Impact topramezone F F E E F P E P P E G F P E P P E P E E E
Karmex diuron G G G G F G G P P G E F G G P E E P G F F
Kerb* pronamide G G   G G G P P G P G         G G   P   P
Laudis tembotrione F

P

P

F

Lorox Iinurin E E G E E E G P G E G G E G G G G P G G G
Matrix rimsulfuron E P     G E G     F F F G F P G E G P P P
Metribuzin-pre metribuzin G G G E E E E P F E E F E P P E E N G G G
Metribuzin-post metribuzin G G G E E E E P F E E F E P E E E P G G G
Outlook dimetheramid E E     E E E       P   P G G F F P P F P
Poast sethoxydim E E N N E E N E E N N N N N N N N G N N N
Prefar bensulide E E P P E E P P P N P P N P P P P N P P P
Princep simazine E G E E G G E P P E E G E G P E E F E E G
Prowl pendimethalin G E G G G E P P G P G P   P P G G   P P E
Ro-neet cycloate E E     E E P       E   N E G E E N N   F
Roundup glyphosate E E E E E E E E E E E E E E G E E G E E E

Sandea

halosulfuron

 P

   

 P

 

P

P

 

 

G

 

E

Scythe pelargonic acid E E G E E E G P F G E G G G P G E F E G G
Select clethodim E E N N E E N E E N N N N N N N N G N N N
Stinger clopyralid N N   N N N G N N P P N       N N N G P P
Treflan trifluralin G E E G G E P F G P G P P P P G G P F P P
2,4-D 2,4-D N N G G N N G N N E E G G E F G E N E F G
Zidua pyroxasulfone G E G G E E E P F P G P P G F E E P G F P

*Federally restricted herbicide

At recommended rates for your soil type or weed species: E=90% control or better, G=70%-90% control, F=50%-70% control, P=5%-50% control, N=less than 5% control