Crabgrass is the most common type of weed in lawns and it’s nearly impossible to completely control this weed. Crabgrass is a warm season annual grass which grows best in the heat of midsummer when desirable lawn grasses are often semi-dormant and offer little or no competition. Crabgrass over winters as seed, comes up about mid-May or later, and is killed by the first hard frost in fall.
Crabgrass grows best in full sun. It does not grow in shady places. It will come up in clumps and look markedly different from the rest of the grass. As it grows older, it’s “arms” will flop over and radiate out from the center covering the ground.
Crabgrass can be controlled in a number of ways, but the best defense against crabgrass is a thick vigorously growing lawn that is mowed no closer than 2-1/2" for cool season grasses. Use a bagger to collect grass clippings while the weed’s seed heads are present.
However, you don’t want to have unsightly crab grass in your lawn, so to gain control, do not apply fertilizer in summer and get a post-emergent herbicide. This type of herbicide is applied after the crabgrass has already sprouted.