The best way to prevent crabgrass, is to apply a pre-emergent barrier in the spring, before soil temperatures reach 55 degrees. This chemical barrier will effectively stop approximately 80% of germinating crabgrass from becoming established. In the northern US, this means the application should be made by mid-April. To be safe, an earlier application is recommended. The most effective preventive program includes two pre-emergent applications; one in early spring and a second application later in the spring. By applying per-emergent twice, you are extending the protection into early summer. And, since crabgrass will continue to germinate into June, the second application makes good sense. Application timing should be adjusted according to your geographical location.