RoundUp Dual Action Weed & Grass Killer 1.5L - Ready-to-Use - Quickhand Root out the toughest weeds and grasses with Roundup Dual Action Weed & Grass Killer Plus 4 Month Preventer. This formula kills existing weeds and grass down to the root, and prevents new weeds from growing for up to 4 months by creating an invisible barrier in the soil. Apply only where you want to kill existing weeds AND prevent future weed growth for up to 4 months, such as 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. In order to prevent new weeds and grasses from growing, you must spray the entire area you want to control, not just the emerged weeds.