Roundup for Lawns Crabgrass Destroyer Ready-to-Spray kills even the toughest crabgrass without harming the lawn. Whitening occurs 4-7 days after application. Product may be applied when daytime temps are between 45-90 degrees. May be applied before seeding. Ideal for use on Kentucky bluegrass, fine fescue, perennial ryegrass and tall fescue.