Most cool-season grasses should be cut when they reach heights of 3 to 3 1/2 inches, typically once a week. Cut warm-season grasses when they reach 2 to 2 1/2 inches. Cut no more than one-third of the grass height at each mowing to avoid damaging the plants.

If the lawn grows too high for you to cut off one-third the height and have an acceptable length, cut off one-third now and one-third again in two or three days.

Cutting more than one-third the height results in clumps of clippings that tend to lie on top of the lawn, decompose more slowly, and give the grass a less attractive, open, bristly appearance. In addition, short cutting will stunt or slow root grow and weaken the plants.

Courtesy Yardcare.com