How low should you go on the first cutting? On tall fescue and Kentucky bluegrass, you can mow as low as 1 to 1½ inches. Be careful you don’t go so low that you scalp the turf though. Actually, low mowing can speed green-up by removing old, dead grass and allowing the soil to warm up faster. But, the mowing height should be raised to normal after the first or second cutting to discourage crabgrass.
Crabgrass seed must have light to germinate, and a high mowing height will shade the soil. Also, root depth and mowing height are related on upright growing grasses such as tall fescue and Kentucky bluegrass — the higher the height of cut, the deeper the root system. A deeper root system means a more drought-resistant turf. Normal mowing height for Kentucky bluegrass is 2 to 3 inches and for tall fescue is 3 to 3½ inches.
Information provided by Kansas State Extension Service regarding early spring mowing height.