Crabgrass has earned its name because in summer, the dense mat of low-growing grass resembles the shell of a crab. By August, crabgrass begins to reach upward with claw-like seed heads. Although crabgrass does not develop in early spring, this is the best time to control it. It can be controlled with a pre-emergent weed spray, but the best defense against crabgrass is to keep your lawn thick and healthy so it can't compete.
Apply a pre-emergent herbicide on your lawn in spring, before the soil heats to temperatures above 50 degrees. Since most crabgrass lives dormant as a weed over the winter months and sprouts in spring, this is one of the best ways to prevent summer crabgrass from becoming established.
Repeat the pre-emergent herbicide every 30 days throughout the spring. Crabgrass seeds can germinate up to 50 years after they have been produced, so you must continue to apply pre-emergent herbicides to keep them from sprouting.
Treat crabgrass that emerges with a post-emergent herbicide, which are typically sold in spray form so they can be applied directly. Be careful that the herbicide does not coat surrounding grass, or it will also be killed. Wait for crabgrass to turn brown and then remove with a garden hoe.
Pull up any crabgrass you find either by hand or with a garden hoe. Crabgrass sprouts from seed, so eliminating the plants before they produce seed heads will prevent future outbreaks.
Set your lawn mower to a height of 2½ to 3 inches tall. Taller grass prevents light from reaching crabgrass seeds and crabgrass from germinating.