Mowing Height and Crabgrass
Sean Breckin, AOLCP

By now you should be mowing at a height of 3 ½.  Mowing height is an important cultural practice to help control crabgrass.  Mowing at a height of 3 ½ helps on two fronts:

  1.   Higher height of desirable turf grass creates a dense turf canopy which helps shade the soil.  Weed seeds need light in order to germinate.  This is why crabgrass will germinate in areas of your lawn where your turf grass is thin.
  2.   Higher mowing height also helps to keep the soil temperature cooler.  Crabgrass is a warm season, or C4, grass.  Crabgrass seeds germinate when soil temperatures are above 60° for a week or more.

In order to attempt to combat crabgrass you need to understand how it grows.  Crabgrass is an annual weed that thrives in conditions that desirable grasses do not do well in. Primarily soils that are low in calcium, low in organic matter, compacted and acidic.  Since crabgrass needs warmer soil conditions, you will find it in sunny areas of the lawns. It is especially prevalent along walks and driveways where radiant heat from the asphalt or brick warms the soil. These areas typically have poor soil conditions because of the stone pack along the edges.

Attempting to control crabgrass by lowering mowing height is perhaps the worst thing to do.  Crabgrass plants are very adaptive to mowing height. Plants can produce seeds at mowing heights as low as 1/2-inch, while the desirable cool season grasses get burned out at a low height of cut.  Crabgrass reproduces by seeds and a prolific tillering or branching habit. A single plant is capable of producing 150 to 700 tillers and 150,000 seeds that can lay dormant in the soil for up to 20 years.

So what to do about it this season?  The best thing is to improve the soil conditions come late summer and early fall. Middle of August to Middle of September is the best time to aerate, top-dress, and heavily overseed affected areas. If your soil test results warrant it: make certain that you lime. If appropriate, plant a tree to provide some shade.  All these activities will encourage a healthier stand of turf for next season.  Remember to keep mowing high throughout the season! 

Establishing a dense and healthy stand of turf grass is the best way to control crabgrass and other annual weeds. The proper mowing height and frequency, fertilization and irrigation are part of the weed control program and should be practiced throughout the growing season.