A fast-growing annual that sprouts from seed each year about the time forsythia blooms, crabgrass crowds out other plants with its flat-lying blades. In the garden, the best preventives are early and repeated cultivation or application of a mulch to keep young plants from maturing.
In the lawn, apply a pre-emergence herbicide such as benefin, bensulide, DCPA, oxadiazon, or siduron before seeds sprout. Except for siduron, these herbicides will retard or possibly prevent germination of some desirable grasses as well, so use them with care. Later on, after shoots have appeared, use a postemergence herbicide, such as a methanearsonate (DSMA, MAMA, or NSMA). Read and follow package directions carefully.