To help prevent crabgrass, reseed and thicken up your lawn in the fall, mow at 3 inches, and fertilize in the fall. In the spring, apply a crabgrass preemergent. For season-long crabgrass control, consider applying your preemergent twice (but make sure the product you choose does not contain fertilizer). Apply first in early spring when the forsythia blooms begin to fall from the shrub. Crabgrass germinates at 54°F, and this is coincident with the forsythia blossom drop. A second application of the product is necessary about 6 weeks later, usually in May. Click on the following publication on crabgrass for herbicides: Herbicides for Crabgrass and Goosegrass Control in Turf