The use of chemicals to treat lawns Or, in the spring, spot-treat weeds that are actively developing by using a liquid, selective, postemergent, broadleaf weed killer. This should be done before the weeds go to seed (photo at top of page). You should look for a product that has at least one of the following active ingredients: 2, 4-D, MCPP (mecoprop), Dicamba*, or Triclopyr.