Crabgrass dies during the winter, germinates in the late spring, and thrives in the summer months. The best way to kill crabgrass is to prevent the seeds from germinating in late spring, but you can use a pre- or post-emergent herbicide to get rid of it in late spring and throughout summer. It is also necessary to monitor your lawn closely to keep crabgrass away in the summer months.
Use a pre-emergent herbicide on your lawn to prevent crabgrass from germinating, if possible. Apply it in mid-spring to early summer, depending on your climate. A good rule of thumb is to apply the pre-emergent herbicide right after the forsythia bushes cease blooming.
Apply a post-emergent herbicide if the crabgrass has already germinated, most likely in the summer. This type of herbicide is meant for young crabgrass. It can be used on more mature crabgrass weeds, but it will be very difficult for one treatment to kill it all. Follow the directions on the label exactly when you apply post-emergent herbicide or you may end up killing your lawn.
Remove any individual blades or small patches of crabgrass that emerge by hand or by spot treating with a general herbicide. Apply herbicide with a large soft brush to avoid affecting desirable grasses. If large patches occur, you can spray the herbicide directly onto the crabgrass.
Maintain your lawn and keep it as healthy as possible in order to prevent crabgrass growth in the summer. Mow your lawn at the highest setting to keep sunlight from reaching the crabgrass seeds. Also make sure to reseed bare spots quickly before the crabgrass takes over.