How to Kill Existing Crab Grass

Crabgrass is an annual grassy weed. Crabgrass sprouts in established lawns in mid to late spring. These three to four blade sprouts quickly develop and spread. By the end of the summer, the mature plant's seed head turns purple as it prepares to deposit thousands of seeds in your lawn. One way to control existing crab grass is to kill it before late summer when it sets seed. Herbicides will work on immature plants that sprout early in the season. However, the mature plants that have already produced their spindly growth are fairly resistant to chemicals and must be physically removed.

...
Keep crabgrass out of your lawn with a combination of mechanical and chemical control methods.

Step 1

Water your lawn the day before you dig or spray crabgrass plants. Watering will help the established lawn stand up to any residual pesticide spray. It will also make your lawn easier to dig.

Step 2

Spray immature crab grass with a post-emergent herbicide scheduled for use on crabgrass. Immature crab grass is susceptible to herbicide at any point after germination until the plant produces more than five blades. Follow the herbicide's instructions and spray individual crabgrass plants until their foliage is well coated. Do not mow the lawn or water it for 24 hours after the application. Re-treat the crabgrass with the herbicide within seven to 14 days if necessary.

Step 3

Dig mature crabgrass plants that have begun to produce numerous stems. Use a spade to dig out the plant and its roots. Discard all plant material.

Step 4

Re-seed any bare patches left behind by digging or spraying. Seed warm-season grass in early spring and cool-season grass in early fall. Loosen the soil in the bare patch with a trowel. Then sprinkle a light covering of seed and pat it into the soil with your hands to ensure soil contact. Keep the soil moist with regular watering until roughly two weeks after the grass seed germinates.