When to Apply Crabgrass Preventer. Crabgrass seeds germinate in the early Spring when the temperature of the surface soil rises into the 50s and 60s. Once the seeds germinate, the opportunity for prevention is passed. So, applying the preemergent early is key.rabgrass seeds germinate in the early Spring when the temperature of the surface soil rises into the 50s and 60s. Once the seeds germinate, the opportunity for prevention is passed. So, applying the preemergent early is key.