Taller Grass, Healthier Lawn
When I started gardening years ago, we were told the ideal mowing height for a cool-climate lawn was 2 inches (5 cm). This would, we were told, give you the effect of a golf green and who wouldn't like that? The problem was low-mown lawns also had huge health problems. They tended to dry out in the summer and fill up with weeds.
We now know that was bad advice. A taller lawn shades the ground and protects its own roots from the burning sun. Also, the root length is roughly proportional to the height of the turf. So short grass has short roots that can't reach additional water underground in times of drought while a high mown lawn has long roots that can find deep water if necessary. Finally, weed seeds have great difficulty germinating in a high turf: sun, essential for their germination, simply does not reach the ground. So for cold climate grasses at least, a mowing height of 3 inches (8 cm) is ideal.