Crabgrass is a warm-season annual grass that grows best in full sun in the heat of midsummer when desirable lawn grasses are often semi-dormant and offer little or no competition. Crabgrass overwinters as seed, comes up about mid-May or later, and is killed by the first hard frost in fall. It 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 turfgrasses.
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 is 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-emergent crabgrass herbicides are also 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 that crabgrass seems to thrive when the weather heats up and drought conditions appear? 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 are present in these thinned areas are opportunistic and germinate. 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 and be prepared to apply a pre-emergent organic or synthetic herbicide in those areas early next spring.