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. 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%). To improve efficacy, there are several principles to keep in mind.

1. Establish and then maintain the crops’ size advantage. Selectivity (which refers to killing weeds but not the crop) of cultivation tools are generally based on a size differential between the crop and weed.
2. Cultivation is most critical early in the growing season. Weeds are most effectively cultivated shortly after they germinate, and crops are most sensitive to weed pressure during their early stages of growth.
3. Carefully prepare the seedbed. 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.
4. 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. Seeding with a walk-behind seeder is generally not going to be suitable for later tractor-mounted cultivation tools.
5. Tools should be adjusted to work as shallowly as possible to minimize movement of seeds in the seedbank and break seed dormancy. 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.

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, and beets, 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 and available 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 in-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 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 until 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 (i.e., a slower tractor or walking speed) will be necessary.

Flaming is ideal for stale seedbeds because it kills weeds without soil disturbance, which would bring up new weed seeds to germinate. For longer-lasting weed control, apply the final flaming as late as possible prior to crop emergence after seeding or just prior to transplanting. However, broadleaf weeds should be flamed at the three-leaf stage, before they grow too large for effective flame control.

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.

Plant competition is a 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 through drip irrigation, instead of broadcasting.

Other cultural strategies that help to manage weeds include cover crops, mulches, fallowing, stale beds, crop rotation, and soil preparation.

Cover crops

Cover crops reduce overall weed populations in several ways.

• Shallow tillage between short cycles of cover crop growth kills weed seedlings and encourages germination of a new “flush” of weeds that can be killed with the next disturbance. Encouraging weed seed germination, but not allowing further weed seed production reduces weed seeds in the soil seedbank over time.
• While it is growing, a dense cover crop stand will suppress weed growth by shading the soil. A season-long cover crop can help suppress weeds that may have otherwise matured and set seed.
• Cover crops can slow soil warming in the spring, helping to slow weed seed germination and reduce the soil seedbank over time.

Despite these benefits, perennial weeds can increase when perennial cover crops are in place for a longer period of time, e.g., one or more years of red clover.

Mulches

Mulches are often used to control weeds. Mulches can be organic (straw, hay, grass clippings, dead cover crops), inorganic (plastic), or degradable (bio- or photo-degradable plastics).

• 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 reduce soil temperatures and slow soil warming, they can reduce nitrogen availability, 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 enough to allow 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 Herbicides and Plastic Mulches for more information. A downside of black plastic mulch is that they generate a lot of plastic waste material, which can be difficult and expensive to dispose of.
• Degradable plastic mulches are more expensive than plastic mulches and rip more easily, which exposes soil where weeds can germinate. They are also not approved for organic production. However, degradable plastics can be plowed into the soil after the growing season, reducing the need for plastic waste disposal.

Fallowing

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. Over time this strategy will reduce the weed seedbank of a field and can help manage established perennial weeds.

Crop rotation

Crop rotation can be a tool for managing weeds. Weed species present tend to be most like the 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.

Proper soil preparation

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

Stale seedbed is the practice of preparing a field for planting and then lightly disturbing the soil 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. Tools that can be used for this practice include chain-drag, spring-tooth harrow, light-weight disc harrows, tine weeders, or flame weeders. Stale seedbeds can be used in the spring before a crop is sown, or in the summer after a spring crop but before a fall crop. A summer fallow works better on summer annual broadleaves that emerge later in the season. Perennial weeds may be weakened but not killed. See Stale Seedbed Technique for more information.

Stale Seedbed Technique

The stale seedbed approach involves preparing the soil as if for planting, without actually planting the crop. After soils are prepared, weed seeds in the upper 1-2" of the soil are encouraged to germinate through adequate soil moisture and temperature (with irrigation or row covers). Generally, this happens within 2 weeks. 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 be used after a stale seedbed to further reduce germination. 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 that you would normally use, to the soil surface.
• If the crop will be direct-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 that 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 3 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.

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 weed species or families. Therefore, knowledge of the weed species present in the field is essential for good weed control. Once the weeds are identified, select the proper herbicide.

Herbicides can be classified several different ways and understanding what the classifications mean can help you better select the most appropriate herbicide to use for different situations.

Herbicides can be pre-emergent or post-emergent.

• Preemergence herbicides (PRE) are applied prior to the emergence of weeds. Some need to be incorporated into the soil with tillage (pre-plant incorporated) and others are applied to the soil surface and are sometimes incorporated with irrigation or rainfall (pre-soil-applied). In general, they work by preventing germinating weeds from growing, but some can also act on perennial plants that spread by rhizomes, tubers, and stolons. These herbicides prevent new weeds from emerging, but most will not impact weeds that are already emerged. 
• Postemergence herbicides (POST) work on actively growing weeds. They can be absorbed by the leaves or roots of the plants, depending on the chemical.

Herbicides can be systemic or contact.

• Systemic herbicides are absorbed by the plant and move through the plant to the site of action, where the herbicide kills the plant.
• Contact herbicides only affect the plant tissue they come into contact with. It is especially important to achieve good coverage of weeds when using these herbicides.

Herbicides can be selective or non-selective.

• Selective herbicides only work to kill certain species or types of plants. Selectivity is usually due to the physiology of the plant and the rate at which an herbicide is applied.
• Non-selective herbicides kill or injure any plant they come into contact with (as long as the weed has not developed resistance to the herbicide). These types of herbicides are used as burn-down products in situations where it is desirable for all living plant material to be killed.

These different classifications are usually well-defined, but not necessarily always exclusive (e.g. a pre-emergent herbicide can also have some post-emergent activity). The different types of classifications interact together. For example, 2,4-D is a postemergent, systemic, selective herbicide meaning that you apply it to actively growing weeds. It is absorbed by the plant and travels to meristems, causing death by uncontrolled growth, and only affects broadleaf plants (not grasses). Paraquat is another postemergent herbicide, but it is a contact and non-selective herbicide meaning that it is applied to actively growing weeds and kills all plant tissue the spray touches but does not harm tissue that is not sprayed.

Adjuvants are added to a pesticide mixture to improve its effectiveness. They include surfactants, stickers, penetrants, and compatibility agents. 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 (most dangerous): 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 (least dangerous): 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. See Pesticide Equipment Calibration.
• 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 an overview of herbicide use restrictions, safety concerns, and other information see Tables 28 and 29.
• For an overview of crops labeled for each herbicide, see Table 30.
• For an overview of weeds listed as controlled on each label, see Table 31.
• 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 psi 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 80-73º 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. See Pesticide Equipment Calibration.

Resistance Management

Each herbicide has a mode of action—how and where the chemicals in an herbicide work to kill a target weed. However, through natural random genetic mutations, weeds can develop resistance to a mode of action. Resistance in a weed makes it less sensitive to an herbicide. Since mutations that lead to resistance are genetic, they are inheritable. When an herbicide is used, weeds that are susceptible to an herbicide will be killed, but any with the genetic mutation that makes them less sensitive will survive. These then reproduce and pass on the genes that let them survive the herbicide exposure to their offspring. If the same herbicide is used repeatedly in the same field, over time the predominant genetic form of that weed in that field will be resistant to that herbicide. A higher rate of the same or a similar chemical from the same group usually will not control the weed and these weeds can become extremely difficult to manage.

There are many techniques that can help delay the onset of resistance.

• Integrate cultural, mechanical, and physical weed control techniques to reduce reliance on chemical tools.
• Scout fields so that you are aware of what is not controlled.
• Take steps to control escaped weeds, especially preventing seed set.
• 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, 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.

One of the most important ways to slow resistance development and extend the useful life of an effective product is to rotate the use of products with different modes of action. This can be done in several ways:

• Rotate the type of herbicide used on fields where the same crop is grown year after year.
• Rotate the crops grown on a field so that different herbicides are used on that field.

Even if herbicides have different active ingredients, they can share the same mode of action. It is very difficult to know which herbicides share a mode of action. To help growers effectively rotate herbicides, international groups have assigned group numbers to herbicides based on the mode of action. 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 herbicide categories. Herbicides with the same number have the same mode of action. Most labels now come with this group number assigned to them (some have not been updated to the HRAC/WSSA number, but will in the future).

When selecting herbicides to rotate between for resistance management, use the group number as your guide and NOT the product name or active ingredient. These resistance groupings are listed in Table 28.

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

The most common weed resistance issue that we have in New England is common lambsquarters that is resistant to atrazine.

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 and Plastic Mulches

With the increased use of plasticulture 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 soil is fumigated under plastic strips (e.g. Vapam), there is no need for an herbicide under the plastic. Herbicides can be used under the plastic but are often unnecessary under black plastic unless there is nutsedge or excessive weed pressure. If you use clear or white plastic without soil fumigation, then you need an herbicide under the plastic mulch. To get the best results, take the following steps for herbicide application:

1. Prepare a pressed bed.
2. Apply the herbicide to the bed surface.
3. 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.
4. Apply the plastic mulch.
5. 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.
6. For weed control between the plastic beds after planting, use a registered preemergence herbicide for that crop. Do not use nonregistered herbicides between beds 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).

Notes:

• Herbicide use under row covers can be dangerous, as some may volatilize and cause crop injury when used under plastic mulches or row covers. Some herbicides are prohibited from use under row covers. Be sure to observe any cautions on the product label.
• 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.
• Wherever possible, try it on a small scale before you apply it on a large scale.
• 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 plasticulture, these materials are best used if the plastic is applied the fall before planting. With all fumigants, be sure to space the injection shanks correctly. The label recommends a 6" spacing with Vapam.
• Backpack sprayers or modified boom sprayers are often used for herbicide applications in plasticulture 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').

About Herbicides

TRADE NAME1	ACTIVE INGREDIENT	HRAC2 GROUP	REI (hours)	ROTATION RESTRICTIONS	PRE-SA3	PPI4	POST5	WEEDS & CROPS LABELED
2,4-D Amine 4	2, 4-D	4	48	no	no	no	yes	Annual and perennial broadleaf weeds in asparagus and corn
Aatrex 4L*	atrazine	5	12	yes	yes	yes	yes	Some annual grass and broadleaf weeds in corn
Accent Q	nicosulfuron	2	4	yes	no	no	yes	Some grasses (including rhizotomous Johnsongrass) and some broadleaf weeds in corn
Aim EC	carfentrazone-ethyl	14	12	yes	no	no	yes	Broadleaf weeds in corn and cucurbits
Aquesta 4F	sulfentrazone	14	12	yes	yes	yes	yes	Mostly annual broadleaf weeds and yellow nutsedge in asparagus, beans, brassicas, cucurbits, eggplant, okra, peas, peppers, rhubarb, tomatoes, and turnip
Assure II	quizalofop	1	12	yes	no	no	yes	Annual and perennial grasses in legumes
Basagran 5L	bentazon	6	48	no	no	no	yes	Yellow nutsedge (high rate) and select broadleaf weeds in legumes and sweet corn
Broclean	bromoxynil	24	24 or 48	no	no	no	yes	Broadleaf weeds in bulb crops
Cadet	fluthiacet-methyl	14	12	no	no	no	yes	Broadleaf weeds in corn (processing sweet corn only)
Callisto	mesotrione	27	12	yes	yes	no	yes	Yellow nutsedge suppression and broadleaf weeds in corn and asparagus
Caparol 4L	prometryn	5	24 or 48	yes	yes	no	yes	Some annual grasses and annual broadleaf weeds in carrots, celery, and parsley
Casoron 4G	dichlobenil	29	24	yes	yes	yes	no	Some annual and perennial grass and broadleaf weeds and yellow nutsedge in rhubarb
Chateau EZ	flumioxazin	14	12	yes	yes	no	yes	Grasses and many annual and perennial broadleaf weeds in asparagus, legumes, garlic, artichoke, and peppers
Clarity	dicamba	4	24	no	no	no	yes	Annual, perennial, and biennial broadleaf weeds in asparagus
Clean Slate	clopyralid	4	12	yes	no	no	yes	Many broadleaf weeds in asparagus, beets, corn, spinach, and turnips
Command 3ME	clomazone	13	12	yes	yes	no	no	Annual grasses and some broadleaf weeds in pumpkins and peppers
Curbit	ethalfluralin	3	24	yes	yes	no	no	Many annual grasses and broadleaf weeds in cucurbits
Dacthal Flowable herbicide	DCPA	EPA issued an emergency order on August 6, 2024 prohibiting all use, distribution, and sale of this product
Devrionol 2-XT	napropamide	0	24	yes	yes	yes	no	Many grasses and some broadleaf weeds in asparagus, brassicas, and solanaceous crops
Dual Magnum**	s-metolachlor	15	24	yes	yes	yes	no	Annual grasses, yellow nutsedge, and some broadleaf weeds in corn, potato, pumpkin, and tomato. **Dual Magnum has an indemnified label for use in some crop due to potential for crop injury. Users must agree to a waiver of liability through the Syngenta website to access the labels for these uses.
Eptam	EPTC	15	12	no	yes	yes	yes	Annual and some perennial grasses, yellow nutsedge, and some broadleaf weeds in beans and potatoes
Evik DF	ametryn	5	12	yes	no	no	yes	Many annual grass and broadleaf weeds in corn
Fusilade DX	fluazifop-p-butyl	1	12	yes	no	no	yes	Many grasses in carrots, onions, spinach, and asparagus
Goal 2XL	oxyfluorfen	14	24 or 48	yes	yes	no	yes	Some grasses and some broadleaf weeds in onions and brassicas
Gramoxone SL 3.0*	paraquat	22	12 or 24	no	no	no	yes	Non-selective control of annual weeds and suppression of perennial weeds. May be fatal if swallowed or inhaled. Must complete an EPA-approved training every three years (list this at bottom of table).
Impact	topramezone	27	12	yes	no	no	yes	Some grasses and many broadleaf weeds in corn
Kerb 50-W*	pronamide	3	24	yes	no	yes	no	Some grass and broadleaf weeds in lettuce
Laudis	tembotrione	27	12	yes	no	no	yes	Some grasses and many broadleaf weeds in corn
Lorox DF	linuron	5	24 or 192	yes	yes	no	yes	Grasses and broadleaf weeds in carrots and parsely (has different pre- and post- emergence efficacy)
Matrix SG	rimsulfuron	2	4	yes	yes	yes	yes	Grasses, yellow nutsedge, and broadleaf weeds in potato
Metribuzin	metribuzin	5	12	yes	no	yes	yes	Grasses and broadleaf weeds in potatoes and tomatoes
Outlook	dimethenamid	15	12	yes	yes	yes	no	Many annual grasses, yellow nutsedge, and annual broadleaf weeds in beans, corn, and root vegetables
Poast	sethoxydim	1	12	no	no	no	yes	Annual and perennial grasses in most vegetables
Prefar 4-E	bensulide	0	12	yes	yes	yes	no	Annual grasses in many vegetable crops
Princep 4L	simazine	5	12 or 48	no	yes	no	no	Annual grass and broadleaf weeds in corn
Prowl H2O	pendimethalin	3	24	yes	yes	yes	no	Annual grasses and some broadleaf weeds in asparagus, corn, peppers, potatoes, garlic, and onions
Quinstar 4L	quinclorac	4	12	yes	no	no	yes	Some annual and difficult perennial grasses and broadleaf weeds including bindweed and Canada thistle in asparagus and rhubarb
Raptor	imazamox	2	4	no	no	no	yes	Some grasses, yellow nutsedge suppression, and many broadleaf weeds in legumes
Reflex	fomesafen	14	24	yes	yes	no	yes	Some grasses and many broadleaf weeds in beans and potatoes. Can only be used every other year.
Reglone	diquat	22	24	no	no	no	yes	Non-selective control of small annual weeds and suppression of perennial weeds
Ro-Neet	cycloate	15	48	no	no	yes	no	Some annual and perennial grasses, yellow nutsedge, and broadleaf weeds in beets and spinach
Roundup PowerMax 3	glyphosate	9	4	no	no	no	yes	Non-selective. Absorbed by the foliage of all weeds and translocated into the plant. Many formulations are available.
Sandea	halosulfuron-methyl	2	12	yes	yes	no	yes	Yellow nutsedge and broadleaf weeds in asparagus, beans, corn, cucurbits, and fruiting vegetables
Scythe	pelargonic acid	0	12	no	no	no	yes	Non-selective control of small annual weeds and suppression of perennial weeds
Select Max	clethodim	1	24	yes	no	no	yes	Grasses in many vegetable crops
Sharpen	saflufenacil	14	12	yes	yes	yes	yes	Broadleaf weeds in corn (not fresh market sweet corn) and legume harvest aid
Shield EX	tolpyralate	27	12	yes	no	no	yes	Some grass and broadleaf weeds in corn
Simazine 90 WDG	simazine	5	12	no	yes	no	no	Many annual grass and some broadleaf weeds in corn
Sinbar WDG	terbacil	5	12	yes	yes	no	yes	Some grass, yellow nutsedge (high rate), and broadleaf weeds in asparagus
Solicam DF	norflurazon	12	12	yes	no	yes	no	Grasses, yellow nutsedge suppression, and many broadleaf weeds in asparagus
Sonalan HFP	ethalfluralin	3	24	yes	yes	no	no	Many annual grasses and broadleaf weeds in beans and potatoes
Stinger	clopyralid	4	12	yes	no	no	yes	Many broadleaf weeds in beets, corn, spinach, and turnips
Surpass EC	acetochlor	15	12	yes	yes	yes	no	Some annual broadleaf and grass weeds in corn
Treflan HFP	trifluralin	3	12	yes	no	yes	no	Many annual grasses and some broadleaf weeds in many vegetables
Valor EZ	flumioxazin	14	12	yes	yes	no	yes	Grasses and many annual and perennial broadleaf weeds in sweet potato
Zidua	pyroxasulfone	15	12	yes	yes	yes	no	Annual grass, yellow nutsedge suppression, and some broadleaf weeds in corn
1Trade Name - Example herbicide trade name for the active ingredient. There may be several formulations of the same active ingredient with different trade names. Inclusion in this guide is not an endorsement of the product. 2HRAC - Herbicide Resistance Action Committee resistance management numbers. 3Pre-SA - This herbicide can be applied to the soil before weeds have emerged. 4PPI - This herbicide can be applied before weeds have emerged and incorporated into the soil. 5Post - This herbicide can be applied after weeds have emerged.

ACTIVE INGREDIENT	EXAMPLE TRADE NAME	ADDITIONAL TRADE NAME(S)	SIGNAL WORD1	RESISTANCE GROUP (HRAC2 CODE)	DERMAL LD503 (mg/kg)	ORAL LD504 (mg/kg)	RESPIRATOR REQUIRED?
2, 4-D	2,4-D Amine 4		D	4	>5050	1340	no
acetochlor	Surpass EC	Breakfree NXT, Cadence, Cadence NXT, Confidence, Fearless, GCS Acetochlor 7EC, Harness, Surpass NXT, TopNotch, Volley NXT	W	15	>2240 (similar material)	1415 (similar material)	no
ametryn	Evik DF		C	5	>2020	4494	yes
atrazine	Aatrex 4L*		C	5	>5050	>5000	no
bensulide	Prefar 4-E		C	0	>2000	960	no
bentazon	Basagran 5L	Bastante	D	6	>2000	1260	no
bromoxynil	Broclean	Moxy 2E, Palouse	C	24	>2	238	no
carfentrazone-ethyl	Aim EC	Antik EC, Longbow EC	C	14	>4000	4077	no
clethodim	Select Max	Arrow 2 EC, Ceridian 2 EC, Cleanse 2EC, Clethodim 2E, Dakota, Intensity Post-Emergence Grass, Intensity One Post-Emergence Grass, Shadow, Shadow 3EC, Tide USA Clethodim 2EC, Trizenta 3EC, Trizenta, Volunteer, Willowood Clethodim 2EC	C	1	>5000	>5000	no
clomazone	Command 3ME	Clomate 3ME	C	13	>5000	>5000	no
clopyralid	Stinger, Clean Slate	GCS Clopy 360SL, Spur, Stigmata	C	4	>5000	>5000	no
cycloate	Ro-Neet		C	15	>5000	3129	yes
DCPA	Dacthal Flowable	EPA issued an emergency order on August 6, 2024 prohibiting all use, distribution, and sale of this product
dicamba	Clarity	BL1, Clash Selective, Dicamba 4, Rifle, Sterling Blue, Strut, Topeka, WC-DCB	C	4	>2000	>2000	no
dichlobenil	Casoron 4G		C	29	slightly irritating	>5000	yes
dimethenamid	Outlook		W	15	>5000	695	yes
diquat	Reglone	Aceto Diquat 2L AG, Capone Desiccant, Nufarm Diquat 2 L, Nufarm Diquat SPC 2 L, Verdure-X	C	22	>5050	886	no
EPTC	Eptam		W	15	2750	1325	yes
ethalfluralin	Curbit, Sonalan HFP		D	3	>5 mL/kg	3267	no
fluazifop-p-butyl	Fusilade DX	Stockade	C	1	>2000	>5000	yes
flumioxazin	Chateau EZ, Valor SX	Chateau SW, Flumi 51 WDG, Flumi SC , Flumi SX, Flumioxazin 51% WDG, Maxunitech Flumi SC Ag, Outflank, Panther SC, Tuscany, Tuscany SC	C	14	>5000	>5000	yes
fluthiacet-methyl	Cadet		W	14	>2020	2537	no
fomesafen	Reflex	Forsyte 1.88 SL, Ringside, Top Gun Flex, Top Gun, Willowood Fomesafen 1.88 SL	D	14	>2000	>982	no
glyphosate	Roundup PowerMax 3	Abundit Edge, Buccaneer 5 Extra, Buccaneer Plus, Bullzeye HL-K, Cornerstone K, Cornerstone 5 Plus, Cornerstone Plus, Credit 41 Extra, Credit K6, Credit Xtreme, CropSmart Glyphosate 41% Extra, Duplicator 6, Duramax, Durango DMA, Four Power Plus, Gly Star Original, Gly Star Plus, Glyphogan, Glyphogan Plus, Glyphosate 4 Plus, Honcho K6, Imitator DA, Mad Dog, Mad Dog 5.4, Mad Dog K6, Mad Dog Plus, Makaze, Roundup PowerMAX, Roundup WeatherMAX, Shar-Max Glyphosate 41% SL, Tomahawk 4, Tomahawk 5, Willowood Glypho 5, Willowood Glypho 6	C	9	5000	5000	no
halosulfuron-methyl	Sandea	Halomax 75, Permit, Profine 75, Stadia	C	2	>5000	1287	no
imazamox	Raptor	Beyond Xtra	C	2	>4000	>5000	no
linuron	Lorox DF	Linex 4L Agricultural	C	5	>2000	3489	no
mesotrione	Callisto	Argos, Atticus Cavallo 4 SC, BL4, Explorer, Mesotrione 4SC, MesoTryOne 4L, Motif , Seeker 4SC, Seeker, Sotrion, Willowood Mesotrione 4SC	C	27	>5000	>5000	no
metribuzin	Metribuzin	Glory, Glory 4L, Glory FDF, Metricor 4F, Metricor DF, Me-Try-Buzin 4L, Me-Try-Buzin 75DF, Rancor 4 F, Rancor 75 DF, TriCor 4F, TriCor DF	C	5	>2000	2365	no
napropamide	Devrionol 2-XT	Devrinol DF-XT	C	0	>2000	>2000	no
nicosulfuron	Accent Q	Primero	C	2	>5000	>5000	no
norflurazon	Solicam DF		C	12	>2000	1140	no
oxyfluorfen	Goal 2XL	Collide, Galigan 2E, GoalTender, ScrollOVR, Scroll 2 XL, Willowood OxyFlo 2EC	W	14	>5000	3129	no
paraquat	Gramoxone SL 3.0*	Axill Solutions Paraquat 3SL*, Gramoxone SL 2.0*, Helmquat 3SL*, Paraquat 43.2% SL*, Paraquat Concentrate*, Parazone 3SL*, Quik-Quat*, Willowood Paraquat 3SL*	D (Poison)	22	1990	172.81	yes
pelargonic acid	Scythe		W	0	>2000	>5000	no
pendimethalin	Prowl H2O	AquaPen 3.8, Framework 3.3 EC Hebicide, Pin-Dee 3.3 EC, Prowl 3.3 EC, Satellite Flex, Satellite HydroCap, Stealth	C	3	>5000	>2000	no
prometryn	Caparol 4L		C	5	>5000	>5000	yes
pronamide	Kerb 50-W*	Kerb SC	C	3	>2000 (similar material)	>5000 (similar material)	no
pyroxasulfone	Zidua	Zidua SC	C	15	>2000	>2000	yes
quinclorac	Quinstar 4L		C	4	>5000	5000	no
quizalofop	Assure II	FirstAct, Targa	D	1	>2000 (similar material)	5900 (similar material)	no
rimsulfuron	Matrix SG	DuPont Matrix SG, Grapple, Hinge, Tetris SG, Tide Rimsulfuron 25WG	C	2	>2000	>5000	no
saflufenacil	Sharpen		C	14	>5000	>2000	no
sethoxydim	Poast		W	1	>5000	5000	no
simazine	Simazine 90 WDG, Princep 4L	Simazine 4L, Simazine 4L Flowable, Simazine 90DF, Sim-Trol 4l Simazine Flowable, Sim-Trol 90DF Simazine Dry Flowable	C	5	>2000	>5000	yes
s-metolachlor	Dual Magnum**	Brawl II, Charger MAX, Cinch	C	15	>2000	3425	no
sulfentrazone	Aquesta 4F	Maxunitech Sulfentrazone 4 SC, Passage, Shutdown, Spartan 4F, Sulfentrazone 4L, Sulfin 4SC, Zeus XC, Zone 4F	C	14	>2000	300 - 2000	no
tembotrione	Laudis		C	27	>5000	1750	no
terbacil	Sinbar WDG		C	5	>5000	>5000	no
tolpyralate	Shield EX		C	27	>2000	>2000	no
topramezone	Impact	Armezon	C	27	>2000	>2000	no
trifluralin	TreflanHFP	Treflan TR-10 Granular, Trifluralin 10G, Trifluralin 4EC, Trifluralin HF, Triflurex HFP, Trust	C	3	>2000	>4013	no
1Signal Word - (C) Caution is least dangerous, (W) Warning is intermediate, (D) Danger is most dangerous 2HRAC - Herbicide Resistance Action Committee number 3Dermal LD50 - The lethal dermal dose of the herbicide for 50% of a test population, usually of rats 4Oral LD50 - The lethal oral dose of the herbicide for 50% of a test population, usually of rats

*Federally restricted herbicide

**Indicates an indemnified label for use in this crop due to potential for crop injury. Users must agree to a waiver of liability through the Syngenta website to access the label for this use.

C = Control

H = Control at high rate

PC = Partial control/suppression

* Federally restricted herbicide