crabgrass, name for any of several grass species of the genera Digitaria, Eleusine, and Panicum, especially the species D. sanguinalis. Crabgrass is a common lawn weed, especially in the S and E United States. The grass has branching stems that may reach a length of 3 ft (91 cm) and flowers borne on purple spikes. It is sometimes cut for hay.
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