When it comes to trimming your grass, the four most important words to remember are these: Mow high, mow sharp. “Mow sharp” simply means that you need to keep a razor edge on your mower blade. That way, it’ll slice cleanly through the grass, leaving as little wiggle room as possible for escaping moisture and invading germs.
As for the first part of that rule, the classic rule of green thumb — and an easy one to remember — is to never cut more than one-third of the height of the grass at any one time. What that means in terms of actual inches depends on the type of grass and the season. In the heat of summer, your grass is most vulnerable to sunburn and water loss. So always let it grow beyond its maximum height by one-third to slow its growth, conserve moisture, and inhibit weeds. Then cut it back to its maximum, not minimum, height when you mow.
Now that I’ve delivered my mow-high lecture, I must tell you that it is possible to let your grass get too high. Turfgrasses that grow much over 3 inches tall tend to get thin and stringy; they don’t form that nice, uniform look that we love in our lawns. Overly tall grass also tends to fall over and to get matted in wet weather. Then it may take it a long time to dry out, giving fungal diseases a chance to set in.