Roundup for lawns1 is a tough weed killer for use on Northern grasses that kills weeds, not the lawn. It won't harm lawns when used as directed on Northern grasses, including Bluegrass, Fescue, Zoysia, Perennial Ryegrass. Roundup for lawns1 kills the root so weeds won't come back. Apply Roundup for lawns1 to actively growing weeds during the early growth stages for the best results. Apply in spring or fall when daytime temperatures are between 45 degree fahrenheit and 90 degree fahrenheit.