Digitaria spp.

Large crabgrass
Large crabgrass. Photo courtesy of Rasbak. Licensed under CC BY-SA 3.0.
Crabgrass is a branching annual grass weed. A single plant can produce 150,000 seeds. Besides seeds, crabgrass also spreads through tillering. Seeds germinate best in very warm soil, and can be prevented from germinating by thick sod or thatch.