Understand Pre-emergent weed control. Click here.

The only way to control annual grasses like crabgrass, and annual bluegrass is with pre-emergent herbicides.  For a pre-emergent to work effectively, their must be a layer of chemical in the soil.  The suns UV, moisture, and disturbances (aeration, rodents, animals etc..) can disrupt this layer.  Usually 2 applications can provide adequate control.  Preemergent must be in the soil when soil temperatures reach 60 degrees.  That is when most annuals will begin germination.