How to Kill Crabgrass in Kentucky Bluegrass

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Kentucky bluegrass (Poa pratensis) makes an uniform, dense turf that covers U.S. Department of Agriculture plant hardiness zones 2 through 9. Although Kentucky bluegrass is considered a tough turfgrass, it may struggle with crabgrass infestations. Crabgrass (Digitaria spp.), an annual weed, invades turfgrass with its aggressive growth. The ability of Kentucky bluegrass to recover quickly makes it easy to eradicate crabgrass. Promoting healthy growth of Kentucky bluegrass combined with chemical control will help get rid of crabgrass quickly.

Things You'll Need

  • Plastic bag
  • Mower
  • Slow-release nitrogen fertilizer
  • Ready-to-use liquid post-emergent herbicide containing the active ingredient disodium methanearsonate
  • Ready-to-use liquid pre-emergent herbicide, containing the active ingredients bensulide, oryzalin, pendimethalin or trifluralin
  • 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.

Tips & Warnings

  • Keep herbicides out of reach of children and animals. Always read the herbicide label for safe use. Handle the herbicides with the proper equipment. Always wear a hat, face mask, protective eye gear, gloves, pants and a long-sleeve shirt.
  • Herbicides containing disodium methanearsonate may discolor Kentucky bluegrass but will not injure it.
  • Kentucky bluegrass is considered invasive in some regions. It may invade rapidly by spreading seed and rhizomes.

References

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