In the garden, weeds compete with desirable plants for water, nutrients, sunlight, and growing space. Weeds are controlled to prevent a reduction in yields of fruits and vegetables. Weed control also enhances the performance of annual and perennial flowers.
The first step in weed control is identification of the weed(s). The type of weed helps determine the best method of control. The two main types of weeds are annuals and perennials. Annual weeds germinate, grow, flower, set seed, and die within one year. Perennial weeds live for two or more years. Perennial weeds often die back to the ground in the fall, but sprout from their roots in the spring. Weeds can also be classified as broadleaf weeds or grasses.
There are three general methods of weed control in the home garden: cultivation (hoeing and rototilling) and hand pulling, mulching, and herbicides. The most common methods of weed control in the home garden are cultivation and hand pulling. Hoeing, rototilling, and hand pulling effectively controls most annual weeds. It often takes repeated cultivation to kill many perennial weeds. When cultivating the garden, avoid deep tillage. The roots of many vegetables, fruits, and flowers grow near the soil surface. Deep cultivation will cut off some of these roots. Also, deep cultivation will bring deeply buried weed seeds to the soil surface where they can germinate. Hoeing, rototilling, and hand pulling should be started early in season. Small weeds are easier to control than large weeds. This process must be repeated several times during the growing season to effectively control weeds. Hoe or rototill around plants or between rows and pull weeds close to the plants.
Some perennial weeds, such as quackgrass, are extremely difficult to control in the home garden. Quackgrass is a perennial grass that spreads rapidly by underground stems or rhizomes. Rototilling an area infested with quackgrass will simply cut up the rhizomes and spread the weed. To effectively control quackgrass, all the rhizomes must be carefully removed from the soil. Mulches control weeds by preventing weed seed germination. Mulches control most annual weeds. Perennial weeds, however, must be controlled by other methods. In addition to weed control, mulches help conserve soil moisture, reduce soil erosion, prevent crusting of the soil surface, keep fruits and vegetables clean, and may reduce disease problems.
Mulches may be divided into organic and synthetic materials. Organic mulches include ground corncobs, shredded leaves, grass clippings, straw, pine needles, wood chips, shredded bark, and compost. Synthetic or inorganic mulches include black plastic and landscape mats. Gardeners should base mulch selection on availability, cost, durability, appearance, and ease of application.
Grass clippings, shredded leaves, and straw are excellent organic mulches for vegetable gardens and annual flower beds. Destroy all weeds prior to mulching. Apply 4 to 6 inches of these materials in early June after the soil has warmed sufficiently. Plant growth may be slowed if organic mulches are applied when soil temperatures are still cool in early spring. Grass clippings, shredded leaves, and similar materials break down relatively quickly and can be tilled into the soil in the fall.
Wood chips, shredded bark, and ground corncobs, are excellent mulches for landscape plantings (trees, shrubs, and perennials). Apply 2 to 4 inches of material around landscape plantings. These materials decay slowly and should last several years. However, it will be necessary to periodically apply additional material to retain the desired depth.
Black plastic is an excellent mulch for the vegetable garden. The edges of the plastic are placed in trenches and covered with soil. Plants or seeds are set in holes made in the plastic. Black plastic controls weeds, but it also increases soil temperatures in the spring. Warmer soil temperatures promote plant growth and early yields of warm-season vegetables.
Black plastic is a poor mulch for landscape plantings. Plastics can trap excessive amounts of water in the soil during wet weather, damaging the roots of trees and shrubs. Landscape mats, however, can be used around trees and shrubs as these materials allow water and air to pass through them. The landscape mats should be placed on the ground and then covered with wood chips or shredded bark.
Herbicides can be used to help control weeds in the home garden. Cultivation, hand pulling, and mulches should be considered the primary means of weed control. Herbicides can be used to supplement these methods. Several limitations prevent their extensive use in the garden. (1) There is no single herbicide that can be used to control weeds in all vegetables and flowers. Different herbicides often have to be applied to the various crop areas. (2) Application methods and times may vary for different crops. (3) Herbicides often control some types of weeds, but may have little or no effect on others. For example, Preen (trifluralin) effectively controls most annual grasses, but doesn't control many broadleaf weeds.
Two herbicides that may be helpful in the home garden are Preen and Dacthal (DCPA). Both products are preemergence herbicides. They must be applied before the weeds germinate. Emerged weeds need to be eradicated prior to application. Dacthal can be used on yarrow, ageratum, snapdragon, columbine, aster, chrysanthemum, coleus, cosmos, dahlia, bleeding heart, purple coneflower, gladiolus, baby's breath, iris, lily, nasturtium, peony, geranium, petunia, rose moss, rose, sedum, marigold, and zinnia. Approved vegetable crops include broccoli, cabbage, cauliflower, cucumber, eggplant, muskmelon, onion, pepper, potato, snap bean, squash, sweet potato, tomato, and watermelon. Do not use DCPA on beets, ajuga, pansy, sweet william, and phlox. Dacthal is most effective on annual grasses.
Preen can be used for weed control on yarrow, ageratum, snapdragon, aster, calendula, chrysanthemum, cosmos, crocus, dahlia, nasturtium, oriental poppy, petunia, phlox, rose moss, rose, salvia, marigold, periwinkle, and zinnia. Trifluralin can also be used on asparagus, broccoli, cabbage, carrot, cauliflower, pea, pepper, potato, radish, snap bean and tomato. Do not use trifluralin on sweet corn, strawberry, raspberry, or blueberry as injury may result. Preen controls annual grasses and a few broadleaf weeds including purslane, lambsquarter, and pigweed. Preen needs to be incorporated into the soil mechanically or with water.
When selecting preemergence herbicides, see the herbicide label for the list of vegetables and ornamentals for which the herbicide has been approved. If a plant species does not appear on the herbicide label, it is not legal to use it on that plant.
A postemergence herbicide that can be used in the home garden is glyphosate (Roundup, Kleenup, Kleeraway, etc.) Glyphosate is a nonselective, systemic herbicide. It is absorbed by green, actively growing plants and translocated throughout the entire plant. Glyphosate kills virtually all plants that are directly sprayed. These products can be used to control perennial weeds around trees, shrubs, and in perennial beds. Do not spray or let it drift onto nearby desirable plants. Glyphosate can also be applied to vegetable gardens to control perennial weeds prior to planting. Read the label directions carefully for specific recommendations.
Plants that are considered weeds by home gardeners do play important roles in nature. However, their harmful effects on desirable plants in the garden often necessitates their destruction.
This article originally appeared in the May 3, 1996 issue, pp. 63-65.