To kill crabgrass, pull out young plants as soon as they are visible. If crabgrass roots are deeply embedded in the soil, spray with a post-emergence herbicide. If there are more weeds than desirable grass in a lawn, kill all vegetation, soak the soil, and reseed.
If light green blades begin to sprout up in a lawn early in the season, they are likely young crabgrass. If the small, green seed head is closed and folded against the leaves, pull the plant. After the seed head tines have spread out like a fork, leave it alone to avoid spreading crabgrass seeds. In the fall, reseed bare and patchy areas with desirable grass.
If roots are deeply embedded in the lawn, it may be difficult to pull them without uprooting chunks of desirable grass. Apply a post-emergence herbicide on stubborn patches of crabgrass, following specific instructions on the label to ensure safety and effectiveness. Most post-emergence herbicides work best when the soil is moist and plants are dry. Spray on a hot day with light wind.
Crabgrass pre-emergence herbicide is the most effective way to avoid crabgrass infestation. Apply in the spring before crabgrass seed sprouts. Do not reseed the lawn if using a pre-emergent herbicide, as it also kills desirable grasses, such as ryegrass, bluegrass and fescue.