Roundup® Dual Action Weed & Grass Killer Plus 4 Month Preventer RefillRoot 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