Spectracide Weed & Grass Killer Concentrate is a nonselective weed and grass killer that kills weeds and grasses down to the root. It's formulated for spot treatment of weeds and grasses on patios, walkways and driveways around flowers, shrubs and trees and along fences and foundations. This formula is rainproof in 15 minutes. Spectracide Weed & Grass Killer Concentrate delivers visible results in as fast as 3 hours. Plant new flowers, trees and shrubs in the treated area after just 1 day. For best results, apply during warm, sunny weather when the temperature is over 60 degrees.