Crabgrass preventer should be applied between February and April, depending on the warmth of the climate, according to Scotts. Crabgrass starts to germinate when the soil is between 55 and 60 degrees Fahrenheit, and the preventer must be applied before the seedlings appear.
Scotts recommends applying the preventer after the first mowing and before the third mowing of the season. Crabgrass preventer should be applied when the lawn is dry, and it should be watered thoroughly after application. If the lawn needs to be aerated, this should be done prior to application. Crabgrass preventer should also be applied to new sod.