Roundup 5100910 ready to use extended control weed and grass killer 1 gallon is a dual action formula that kills existing weeds to the root and prevents new weeds from growing for up to four months. kills weeds down to the root and starts working immediately with visible results in 24 hours. rainproof in 30 minutes. pre-treats weed prone areas such as driveways and sidewalk cracks, patios and paths, tree rings and mulched areas and along fence lines.