crabgrass, (genus Digitaria), genus of about 220 species of grasses in the family Poaceae. Several species, notably hairy crabgrass (Digitaria sanguinalis) and smooth crabgrass (D. ischaemum), are very troublesome weeds in lawns, fields, and waste spaces because they have decumbent stems that root at the joint and form tenacious patches. Arizona cottontop (D. californica) is a useful forage grass in southwestern North America.
Crabgrasses are annuals or perennials perpetuated by seeds that overwinter in the ground. Most spread vegetatively and root at the joints of the stems. The eradication of weedy species is difficult, especially if they have seeded into existing lawns, as mowing merely induces new flowering and the shedding of more seed. Because the seed germinates later than that of desirable competitors, preemergence herbicide sprays may be useful as a means of eliminating the plants. Hand weeding is also useful, but the most effective control is a hardy lawn that smothers crabgrass seedlings.