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. 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.
Fertilize the lawn in late summer or fall and again in spring to develop a dense, healthy stand of grass. Fertilized bluegrass does not go into midsummer dormancy as soon as unfertilized bluegrass.
Pre-emergent applications made when soil temperature are still below 60 are the best prevention. Pre-emergent applications are not recommended for areas where new grass seed is going to be planted during the first half of the growing season. Pre emergent applications lose their effectiveness if the lawn is raked or disturbed during the first half of the growing season.
Post emergence crabgrass herbicides are now available. These are products that are applied after the crabgrass seed has sprouted. The herbicide (ACCLAIM) gives excellent crabgrass control with one application. This product should be applied when crabgrass is in the 3 to 4 leaf stage of development. Once crabgrass has become established during the growing season, it is best to leave it alone and it will die back in the fall. Next season, apply a pre-emergent herbicide to control it. Overseed areas that were heavily infested with crabgrass in early fall to help fill in those areas.
Ever notice when the weather heats up and drought conditions appear, that crabgrass seems to thrive? With the extreme heat and drought stress of summer and the loss of grass to things like billbug and brown patch, thin areas in many lawns are overtaken by crabgrass.
This is likely large crabgrass, which tends to germinate later than smooth crabgrass. With ample moisture in early summer, lots of sunny days and high temperatures, crabgrass seeds that were present in these thinned areas were opportunistic and germinated. There is nothing to do about crabgrass late in the season. Because crabgrass is an annual plant, it will die out with a killing frost. Take note of the infested areas, however, and be prepared to apply a pre-emergent herbicide in those areas early next spring; both organic and synthetic herbicides are available.