When mowing the lawn, what is the proper mowing height?

FAQ
Question: 

When mowing the lawn, what is the proper mowing height?

Answer: 

Kentucky bluegrass lawns should be mowed at a height of 2½ to 3 inches in the spring and fall months.  Mow bluegrass lawns at a height of 3 to 3½ inches in June, July, and August.  A higher mowing height in summer helps to cool the crowns of the turfgrass plants, encourages deeper rooting, and provides more leaf area for photosynthesis during the stressful summer months. 

Mowing below the recommended range may scalp the turf and cause the turfgrass to deteriorate.  Extremely low mowing heights decrease the total leaf surface area, carbohydrate reserves, and root growth, creating a situation where the turfgrass plants are unable to produce enough food to meet their needs.  This makes the plants more susceptible to drought, high temperature, and wear injury.  In addition, the bare areas created by a decrease in turfgrass density increase the likelihood of weed problems. 

Mowing too high can also create problems.  Mowing above the recommended range reduces tillering and causes matting of the grass.  Reduced tillering results in fewer and coarser plants, while matted grass creates a micro-environment that encourages disease development.