If you are in an area where temperatures are below 85°, then it is a perfect time to apply Green-Up Weed & Feed. Apply the product to a wet lawn, which allows the herbicide to stick to the weed leaves and destroy the root for good. Make sure that there is no rain in the forecast for 48 hours. (Do NOT apply Green-Up Weed & Feed when temperatures are above 85° or if it is very humid because it could injure the lawn. ) What Else Can I Do to Prevent Weeds? Weeds thrive in soils with pH is below 6.2 and above 7.5.