For most Nebraska lawn grasses – Kentucky bluegrass, perennial ryegrasses, and fine-leafed fescues – mow at a height of 3 - 3 1/2" inches, with the taller height recommended. This will usually be the highest or second highest setting on most mowers.
Tall mowing produces a deep root system that is important during periods of summer heat and drought. The larger leaf surface shades the soil and keeps it cooler. This shading effect makes lawn grasses more competitive with weeds that germinate from seed every year.
On the other hand, close mowing is detrimental to lawn grasses. It reduces the mass of roots beneath the grass plant and also the depth of the roots. Close mowing also produces less shading of the soil because there are fewer leaves on grass plants, resulting in higher soil temperatures and weaker lawn grass plants.
The general rule of thumb for mowing lawn grasses is to mow high, mow frequently, and allow the clippings to return to the lawn where they can decay and help recycle nutrients.