Crabgrass is a stubborn weed and can be very difficult to get rid of. The fastest and simplest way to get rid of this weed is to use a pre-emergent herbicide, and to monitor the lawn closely after the crabgrass is gone for the growing season in the late spring and summer.
Prepare to treat the crabgrass in early spring, after the last frost but before the grass growing season. This is when the crabgrass seeds begin to germinate.
Follow the herbicide instructions strictly. Crabgrass herbicides can be found any any local hardware or gardening store. It is important to follow instructions because even the smallest amount of too much may be fatal to other plants, grass, animals and people.
Apply the herbicide according to the instructions. For example, one pre-emergent herbicide is Sidurion. This must be applied to the grass when the soil reaches a temperature of 50 to 55 degrees Fahrenheit, right around the time lilacs and forsythia bloom. Right after, thoroughly water your lawn.
Leave your lawn alone, and don't mow it or apply anything else to it. About two weeks after the application, apply the herbicide again in the same manner and water the lawn well.
Maintain your lawn properly during the summer growing season. Do this by watering it once a week, and mowing it regularly. When the lawn is strong enough, it will get rid of crabgrass by itself by choking out the roots.