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University of Rhode Island GreenShare Factsheets

Cultural and Mechanical Weed Control in Vegetable Gardens

Weeds are unwanted plants in gardens that reduce available moisture, nutrients, sunlight and growing space needed by crop plants. Their presence can reduce crop growth, quality and yield. In addition, they can make harvest difficult. Weeds also provide cover for diseases, insects and animals (rodents, box turtles, snakes, etc.). Garden weeds are hard to control because they grow rapidly, produce vast numbers of seeds, and spread aggressively by vegetative structures and/or seeds. A combination of cultural and mechanical weed control methods can be quite effective in controlling weeds. There are several methods that can be used in a combined, coordinated effort to control weeds.

Cultural Methods:

• Fast growing vegetable crops can effectively suppress weeds by shading. One method of weed control is to select a crop that is capable of suppressing (shading) weeds. The following vegetable crops are listed according to their ability to suppress weeds.

Suppression of weeds by crops works best when the crop germinates quickly and gets a head-start on weeds. To achieve this, plant crops at the proper depth, with adequate moisture and fertilizer. It is equally important to purchase high quality vegetable seeds or transplants that are free of weed seeds or seedlings. Garden soils should be tested and soil test recommendations followed to stimulate rapid crop growth capable of weed suppression. See GreenShare Factsheet on soil testing for more information.

• Fast growing 'smother' crops can be used to reduce weed germination in succeeding crops. For this purpose, smother crops are usually planted in the fall and killed by tillage the following spring prior to planting vegetables. The straw residue from smother crops (i.e. rye, ryegrass, etc.) can inhibit early season germination of weeds such as common lambsquarters, common purslane and redroot pigweed by 75% or more.
NOTE: Avoid planting smother crops where small seeded crops such as lettuce will be seeded the following year or reduction in germination can occur. Smother crops are also effective in suppression of many winter annual weeds such as henbit and chickweed.

• Rotate crops to different areas of the garden so that a specific crop is never planted in one area for two years in a row.

• Mulches can be used to prevent weed germination and growth and ultimately reduce time and labor required to remove weeds. Mulches fall into two categories -- organic mulches, which are derived from plant material, decompose naturally in the soil, and inorganic mulches, which do not decompose, must be removed from the soil. Mulches help to increase soil moisture retention, enhance soil warming and reduce weeds.

Organic Mulches: Some of the most commonly used organic mulching materials are manures, bark chips, ground corncobs, sawdust, grass clippings, leaves, newspapers (shredded or in layers) and straw. Organic mulches allow some flexibility in fertilizing and watering, since they can be raked back from the plants. They should normally be applied in a uniform layer three to four inches deep around the base of the vegetable plant. Do not mulch with straw containing weed seeds. If straw does contain weed seeds, it should be moistened throughout, kept moist until the weed seeds germinate and then air dried several times to kill germinating seedlings.

Inorganic Mulches: Black plastic is the most frequently used inorganic mulch. Clear plastic is not recommended because it does not exclude the light that weed seeds need to germinate. Verify that there is adequate moisture in the soil before any mulch is applied, especially plastic films. It is also necessary to apply most of the fertilizer required by the plants before the plastic is installed. There are also several durable weed fabrics that are very effective in weed suppression. They are capable of lasting up to 20 years or more when covered with soil.

The decision of whether to use an organic or inorganic mulch really depends on the season of the year and what the gardener is trying to accomplish. Organic mulches should be applied after the soil temperature has warmed in the spring. If applied to cold soils, the soil will warm slowly and the growth rate of most vegetables will be reduced. Inorganic mulches can increase the soil temperature by at least 6 to 8 degrees F, making their greatest value early in the growing season when soils are naturally cool.

Mechanical Methods:

Since emerged weeds present at seeding or transplanting are capable of growing rapidly, it is important to kill all weeds prior to planting. Weeds that emerge after planting should be removed before they are more than 3 inches tall. Large weeds are difficult to remove without uprooting vegetable plants. Early season competition between vegetables and weeds may reduce crop growth, yield and quality. Removal by hand or cultivation can be used to manually rid the garden of weeds.

Weeds can be hand pulled or removed using hand tools. Several small hand tools are available that are very effective on small weeds and for working near garden plants. They include the springtooth hand cultivator and the trowel. These tools limit the gardener to working solely on their knees.

There are a variety of effective tools that allow the gardener to stand while removing weeds. They include the scuffle hoe, the Warren hoe, the onion hoe, and the Garden Weasel. Scuffle hoes come in several shapes and are "push-pull" weeders that require no lifting. One of the most popular is the Dutch version, which is an open stirrup with a blade. Others consist of triangular plates. Most scuffle hoes are lightweight and are effective on small weeds. The Warren hoe has an arrowhead shape and can be used to scratch a furrow with one end; when turned upside down the two shoulder points fill in the furrow. The onion hoe is a thin bladed hoe that is recommended when removing weeds near vegetable plants, as it is easier to handle than large, heavy hoes. Onion hoes can be made by grinding a common hoe so that the blade is thin, allowing more precise maneuvering near vegetable plants. The Garden Weasel has three sets of wheels with spikes that you "push-pull" to cultivate weeds. It is recommended where numerous small weeds are present. The Garden Weasel is also useful in breaking soil crust to aid vegetable crop emergence.

Several implements can be used to cultivate weeds. Push plows, also called push cultivators, are inexpensive and also very effective in killing small weeds. The large wheel versions are usually easiest to push. For large weeds, a garden tiller or small tractor is most effective. Regardless of the implement, cultivate no deeper than 2 inches deep to prevent root damage to vegetable plants. Cultivators should normally be adjusted to throw soil around the base of crop plants and over any emerged weeds that are present in the row. Row spacing can be adjusted to allow close mowing near the soil surface to control weeds. Self-propelled rotary or sicklebar mowers and/or mowers with large rear wheels are easiest to maneuver. Weedeaters fitted with plastic string can also be used to cut weeds near the soil surface. Extreme care should be used for crop and personal safety. See manufacturer's warnings prior to operating all equipment.

Adapted from David W. Monks and Larry Bass, North Carolina Cooperative Extension Service, 2000

Pesticides are poisonous! Read and follow all safety precautions on labels. Handle carefully and store in original containers out of reach of children, pets or livestock. Dispose of empty containers immediately, in a safe manner and place. Pesticides should never be stored with foods or in areas where people eat.

When trade names are used for identification, no product endorsement is implied, nor is discrimination intended against similar materials. Be sure that the pesticide you intend to use is registered for the state of use.

The user of this information assumes all risk for personal injury or property damage.

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For more information, call the URI CE Gardening and Food Safety Hotline at 1-800-448-1011 or (401)874-2929 from outside Rhode Island; Monday-Thursday between 9 am and 2 pm.

 

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