Healthy, vigorously lawns can easily out-compete a crabgrass infestation. But, in the heat of July, your lawn may be especially stressed. These hot summer months are exactly when crabgrass thrives. Unfortunately, this is also the toughest time to treat crabgrass. It is often resistant to post-emergent herbicide, and the herbicide will not kill the crabgrass seed that will produce future outbreaks.
Hand weed your crabgrass with the help of a small spade. Water the grass first to loosen the soil.
Spot treat the crabgrass with a post-emergent herbicide listed as safe to use on crabgrass and the type of grass in your lawn. Mow or cut the crabgrass low and allow it to re-grow around 1 inch before spraying it. Coat the crabgrass with the herbicide (according to the manufacturer's instructions) early in the morning on a day when there is no rain forecast for the next 48 hours. Re-spray at the intervals dictated by the manufacturer until the crabgrass is gone.
Mow your lawn to a height of 2 1/2 to 3 inches after the crabgrass is gone to help guard against an outbreak the following season.
Apply a pre-emergent crabgrass herbicide in early spring. Spray or spread liquid or granular pre-emergent herbicide over your lawn. Re-apply the herbicide at the intervals dictated by its manufacturer.