Roundup® Dual Action Weed & Grass Killer Plus 4 Month Preventer ConcentrateRoundup Dual Action Weed & Grass Killer Plus 4 Month Preventer Concentrate kills existing weeds and grass down to the root and prevents new weeds from growing for up to 4 months This weed killer and preventer is rainproof in as fast as 30 minutes, and produces visible results in as fast as 6 hours Apply this herbicide to hardscapes and landscapes cracks and crevices in driveways, sidewalks, and patios, along fences, in mulched beds or around the base