How to Kill Crabgrass in Kentucky Bluegrass

Apply a single layer of a ready-to-use liquid pre-emergent herbicide, containing the active ingredients bensulide, oryzalin, pendimethalin or trifluralin, two weeks before crabgrass germinates. Pre-emergent herbicide will kill any leftover seeds the crabgrass set the previous year. On average, crabgrass germinates when the nighttime soil temperatures reach 50 to 55 degrees Fahrenheit in the spring. Water the grass if there is no rain within 48 hours.

Pull young crabgrass out of the lawn by grasping it at the taproot and pulling up while twisting slightly. Put the crabgrass in a plastic bag, and dispose of it. The best time to pull crabgrass is after rain or watering. Scan the lawn weekly for crabgrass to pull before it sets seed.

Apply a ready-to-use liquid post-emergent herbicide containing the active ingredient disodium methanearsonate to eliminate any emergent crabgrass. Spray emergent weeds with the liquid. The best time to apply post-emergent herbicides is when the crabgrass is in the one- to five-leaf stage of development. Make a second application after one or two weeks if more crabgrass emerges.

Apply 1 to 2 pounds of a slow-release nitrogen fertilizer per 1,000 square feet in the spring, when the grass is green. Make an additional application of 2 to 3 pounds of a slow-release nitrogen fertilizer per 1,000 square feet in the fall. Fertilizer will increase the vigor of Kentucky bluegrass so it can crowd out crabgrass.

Mow Kentucky bluegrass at a height of 1 1/2 to 2 1/2 inches. Remove no more than one-third of the leaf at one time. Rinse off the mower to remove seed and prevent crabgrass from spreading to other areas. Regular, proper mowing will increase Kentucky bluegrass vigor and reduce chances of crabgrass germinating or growing.