Roundup For Lawns Crabgrass Destroyer, 1 Gallon is formulated with a proprietary active that kills even the toughest crabgrass. Whitening occurs 4-7 days after application. Product may be applied when daytime temps are between 45 - 90 degrees. May be applied before seeding, including the say of seeding. For use on Kentucky bluegrass, fine fescue, perennial ryegrass and tall fescue.Won't harm the lawn.