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Crabgrass a summer annual, is a member of the grass family. It is one of the most troublesome weeds in lawns. Crabgrass reproduces by seeds. It has a prolific tillering or branching habit. A single plant is capable of producing 150 to 700 tillers and 150,000 seeds. Crabgrass plants are very adaptive to mowing height. Plants can produce seeds at mowing heights as low as 1/2-inch. Crabgrass germination is related to soil temperature. When the soil temperature reaches 60 degrees F, crabgrass begins to germinate. However, the soil temperature must be in this range at least for a week. Seeds germinate best from early spring to late summer. Crabgrass seeds are dormant for a short period of time after they shed from plants. Crabgrass continues to grow until midsummer when days become shorter, and vegetative growth slows as plants enter their reproductive stage. Purplish seed heads form until frost kills the plants. Plants that emerge early in the season and have a long period of vegetative growth are much larger and more competitive than plants that germinate late in the season. Crabgrass is also known as hairy crabgrass, finger grass, crow foot, purple crab-grass and Polish millet.
Crabgrass is found in almost every situation. It is prolific in lawns, golf courses, athletic fields, gardens, orchards and waste places, and thrives well in lawn situations. Once established, it tolerates both high temperatures and dry weather conditions.
Crabgrass is very noticeable in lawns. It is a rapid growing, coarse textured yellowish-green grass that is conspicuous when found growing among fine textured, dark green cool-season turfgrasses. The stems spread outward and are very branched. Roots develop at nodes on the prostrate stems. The first leaf is only about twice as long as it is wide. It is tinged light purple and has a white strip ninning down the center. Both sides have silky, shiny hair. Leaves are 2/5- to 1/2-inch wide and 1/3- to 1-inch long. The leaf sheaths of large crabgrass seedlings are tinged purple and are covered with long stiff hairs. The ligule, a thin membrane or row of hairs at the top of the junction of the leaf sheath and the leaf blade, is membranous, flat at the top and smooth. Large and small crahgrass are the only species of the grass family which have a membranous ligule. Auricles, the appendages projecting around the stem from both sides of the collar, are absent.
The basic principle of a crabgrass control program is to prevent reinfestation by seeds. Controlling seed production for several years will help reduce the viable seed supply. Crabgrass cannot be controlled in one growing season because of the great number of viable seeds that accumulate in the soil from years of infestation. A good weed management program in lawns is one that consists of both recommended cultural practices and the use of herbicides as appropriate for the control of any given species. Satisfactory control may require several seasons of conscientious adherence to a good control program.
Establishing a dense and healthy stand of turfgrass is the best way to control crabgrass and other annual weeds, including grasses and broad-leaf weeds. The proper mowing height and frequency, fertilization and irrigation are part of the weed control program and should be practiced throughout the growing season.
Crabgrass has a shallow root system. As a result it is attracted to compacted soil. Annual aeration will reduce soil compaction allowing for desired grass species to grow in these areas.
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