Pre-emergent herbicides prevent the germination of seeds by inhibiting a key enzyme. They are used to prevent crabgrass from appearing in summer lawns.

Pre-emergent herbicides are applied to lawns in the spring and fall to prevent the germination of weed seeds. They will not affect any established plant. In the spring, they should be applied early to mid March and no later than early April if second treatment is suggested.

    Pre-emergent herbicides must be applied at a critical time. If they are applied to the soil too early, they get washed too deep into the soil or washed away by rainwater. If they are applied too late, the key enzyme inhibited is no longer active. The best control requires a second application about 6-8 weeks later. This provides coverage in mid-late summer when crabgrass can still germinate.