I depends on where you live. The best way to time your crabgrass preventer application is by watching the yellow forsythia shrubs. When the flowers start to fall to the ground, apply your crabgrass preventer. That is when soil temperatures are high enough for the crabgrass to start sprouting. It is as they are sprouting taht the crabgrass preventer kills it. If it is put on too early, the crabgrass barrier can wear out before it has a chance to work. Using the forsythia as your timer, you’ll always put it on at the correct time no matter where in the area you are of if the season is running early or late. More on all this HERE.