Roundup Dual Action Weed and Grass Killer Plus 4 Month Preventer 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 of well-established desirable plants, shrubs, or trees, and on gravel areas. All ornamental bedding plants, trees, shrubs, sod, and seed can be planted 4 months after application.