Different grasses have different ideal heights, and you should mow to keep the lawn within that optimum range. Cool-season northern grasses, such as Kentucky bluegrass, fescue and ryegrass, grow upright and should be cut higher than southern grasses, like Bermuda and zoysia, that spread out as they grow. More often than not, homeowners mow too low. There are three good reasons for mowing higher. First, grass, like all green plants, lives mostly on food manufactured in its leaves.
Taller grass produces more food, so it's more vigorous and disease-resistant.
Second, the longer the top growth, the deeper the roots. Deep roots are better able to withstand drought and extreme cold. Finally, tall grass shades weed seeds, keeping them from sprouting. Lower the cutting height for these grasses during the cool weather of spring and autumn. And make the last few cuts of the season at the bottom end of the ideal range.