An unfortunate side effect of crabgrass is its tendency to efficiently perpetuate itself in the form of thousands of dormant crabgrass seeds lying in your lawn and waiting to make an appearance in the summer. This makes spring a crucial time in the war on crabgrass, in that it is the best time to prevent its re-emergence by preventing the seeds from germinating. There is a short window to apply crabgrass preventer because if it comes down too early the prevention may lose effectiveness before the seeds germinate and if it comes too late, the damage is already done. The best time to apply prevention is between the second and third mowing of the spring.
Read the full article here: Time to Deal with Crabgrass!