Crabgrass is a relentless foe. One single plant can cast thousands of seeds across your lawn. They work their way down to the soil level and when the magic of sunshine and moisture come, the crabgrass begins its assault. It is tougher than most grasses, so in the heat when your lawn is withering, the crabgrass is spreading like a disease.
The best defense against crabgrass comes in the early spring. The seeds are dormant until the soil temperature gets warmer. If you spread a pre-emergent on the soil now, the seeds won't sprout. You give your lawn a good head start and the crabgrass will never get a foothold.
Once the soil gets warm and the seeds sprout a pre-emergent is no help.