Roundup Concentrate Poison Ivy Plus Tough Brush Killer kills the toughest brush and allergy activating weeds without you having to handle them, with results becoming visible in 24 hours. For best results, apply when the weeds are actively sprouting and during calm, sunny, and warm days, where the temperature is above 60°F. This gives the Roundup a chance to get into the ground, as it will be rainproof in thirty minutes and keep your desirable plants safe from the herbicide, as it will kill them. The weeds will start to die in two days and will completely die within one to two weeks.