The best weapon you have to know how to kill crabgrass is pre-emergence herbicide (also called crabgrass preventer). You apply this product in the spring before the crabgrass seed sprouts. This granular herbicide works by creating a chemical barrier at the surface of the soil. If crabgrass has become established in your lawn, proper lawn maintenance alone may not be enough. A pre-emergent herbicide is the next line of defense. Pre-emergent herbicides work by killing the crabgrass seedlings as they germinate. When applying the pre-emergent herbicide, always follow the manufacturer’s instructions on whichever crabgrass killer you choose below.