A crabgrass preventer should be applied between the May 1 and June 1. Crabgrass seeds start germinating when the soil reaches 60 degrees Fahrenheit, which usually happens during May in Minnesota.
Most crabgrass preventers are effective for about 30 days. They interrupt the germination process of crabgrass. They contain pre-emergent herbicides and fertilizers. In addition to killing crabgrass, they strengthen the turf grass.
Crabgrass cannot be dealt with in a single season. A combination of pre-emergent and post-emergent herbicides minimizes the amount of crabgrass annually. Pre-emergent herbicides stop the germination process. Post-emergent herbicides deal with the remaining crabgrass and stop it from reaching the seeding phase.