Weeds compete with the crop for water, light, and nutrients.  Weeds interfere with harvesting, reduce yields, and harbor insects and diseases.  Weed management option include physical, cultural, and chemical controls.  For most growers, integrating these 3 options provides the best overall control of weeds.

Physical/Mechanical Weed Management

Physical weed management includes hand weeding and cultivation. 

Hand weeding is expensive but effective.  The availability and cost of labor are key considerations, although it is common to perform some hand weeding in almost all vegetable crops.  When pulling weeds or using hand tools, minimize soil disturbance to avoid moving more weed seeds close to the soil surface where they can more easily germinate.

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. In general, 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. Weeds also will more easily die if the sun is out and the soil is not wet.  Also, it is imperative that cultivation tools be properly adjusted and that mechanical methods be complemented by cultural practices such as crop rotation, composting of manure, and mowing of headlands to keep nearby weeds from going to seed. Here is a summary of some commonly used weed control equipment and practices:

Flame weeders can also be used for stale seed bed preparation, killing broadleaf weeds before planting without causing soil disturbance that brings up new weed seeds. After the crop has been planted, but before it emerges, flaming can again be used. 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. Some weeds, such as purslane, and grasses with their growing points below ground are not controlled by flaming.

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 to blind cultivation, including corn, cucurbits, beets, etc, depending on stage of growth and equipment used.

Tine weeders (Einbock, Kovar, Lely) 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 on others, by the pressure on the 3-point hitch. Gauge wheels can be used to maintain uniform depth. Work best in friable soils free of rocks, and when weather allows weeds to dry out on soil surface after uprooting.

Rotary Hoes (Yetter, Deere) 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 less tender 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 various 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-shape) attached to a tool bar, 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 tool bars to offer complete coverage between multiple rows in beds. Planting crops using the same row spacing will make it easier to cultivate with a single set up by minimizing equipment adjustments for different crops.

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, and 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 (Buddingh) are relatively high-speed, between-row cultivators good for control of small weeds in narrow rows when crop is small. The wire baskests 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- to 6-row units that can be rear or belly-mounted.

Finger weeders (Buddingh) 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 single rows, to control small weeds in dry, friable soil with few rocks or residues. Clay soils may stick to fingers.

Spring-hoe, torsion weeders, and spyder weeders (Bezzerides) are flexible blades and square metal stock that disturb soil around 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.

Row crop cultivators (many brands are available) have gangs of S-shanks with sweeps that break up the soil while destroying small to moderate-sized weeds. The gangs work behind the tractor between the rows at uniform intervals. Shields may be added to protect the crop from moving soil, and a fertilizer hopper can be used for sidedressing during cultivation.

Rolling cultivators (Lilliston, BHC) 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.

Wiggle hoe (Friday) is an in-row cultivator that uses hand-steered side knives controlled by an orperator who moves them in and out of the row. Knives can reach very close to the row and eliminate the weeds, but if the operator isn't careful, the crop can be damaged.  Good for irregular plant spacing in the row or between rows.

Reigi weeders are a relatively new tool that also requires 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 one- or two-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.

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 weed populations.  They will, however, slow the warm up of soil and provide shade, both helping to slow weed seed germination.  Ultimately, some weeds germinate and will produce seeds adding to the soil seed bank.  Also, perennial weeds will be largely unaffected.  Soil disturbances between short cycles of cover crop growth are effective.  The tillage kills germinated weeds and also moves weed seeds near the soil surface where they can germinate prior to the next disturbance.  Through these cycles, the objective is to encourage weed seed germination but not to allow further weed seed production.

Fallowing is not planting a field with the intention to reduce weed seed populations.  For annual and biennial weeds, physical weed control is best.  Repeated soil disturbances (disking, rototilling) before weeds go to seed, even in the absence of a cover crop, will reduce the weed seed bank of a field.

Plant competition can also reduce weed pressure.  Use of transplants, rather than direct seeding where possible, will allow the crop to get a jump on the weeds and provide shading of the soil which will delay weed emergence and competition.  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.

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 inches).  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.  Therefore, it is recommended that the mulch application be delayed 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.  IRT are much less effective than black in controlling weeds and clear mulches enhance weed growth.  Some growers plant cover crops between plastic mulch strips but these cover crops will also compete with the crop.  Killing the mulch before the crop is planted, mowing the mulch on a regular basis, or using raised beds will help to reduce but not eliminate weed competition.  A separate section on using herbicides in combination with plastic mulches follows later in this section.

Proper soil preparation can also 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.

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 is also an option.  Weeds species present will tend to be most like crop usually 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.

Chemical/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. Certain considerations should be made in this process.

Toxicity of Herbicides

All pesticides are poisonous. However, some are more toxic than others. The toxicity of the pesticide is usually stated in the precaution on the label. For example, a skull and crossbones figure and the signal word "Danger" are always found on the label of highly toxic (Toxicity Class I) materials. Those of medium toxicity (Toxicity Class II) carry the signal word "Warning." The least toxic materials (Toxicity Class III) have the signal word "Caution." The toxicity of a pesticide is expressed in terms of oral and dermal LD50. LD50 is the dosage of poison that kills 50% of test animals (usually rats or rabbits) with a single application of the pure pesticide for a given weight of the animal (mg/kg of body weight).

The lower the LD50 value, the more toxic the material. Oral LD50 is the measure of the toxicity of pure pesticide when administered internally to test animals. Dermal LD50 is the measure of the toxicity of pure pesticide applied to the skin of test animals. Generally, an oral application is more toxic than a dermal one.

• For restrictions on rates, timing and crops for which the herbicide is approved, see Table 18.
• For degree of susceptibility of each weed to a specific herbicide, see Table 19.
• For limitations and special requirements of the herbicide, see reference no. 16 for more information.

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.
• Avoid skips and overlapping.
• Rinse spray equipment immediately after use. Use one sprayer for herbicides and another for insecticides and fungicides.

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.

Weed Sprayer Systems

• Select a sprayer and pump that can deliver a volume of 20 to 50 gallons per acre. Most herbicides are applied at rates of 20 to 40 gallons of water per acre. Pressures of 20 to 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 80° or 73° 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

Weeds that survive the application of a herbicide are likely to pass the trait that enabled them to survive on to their offspring. Repeat applications with the same type of herbicide will eventually remove almost all the susceptible individuals from a pest population and leave only those with the resistant gene. 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.

To preserve the useful life of a pesticide, it is necessary to practice resistance management. 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 17.

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: Resistance is not interchangeable between 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.