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.
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