How high should you cut the grass when you mow in fall? Well, there's a general rule of thumb for determining grass height (regardless of the season), and in fall it's generally easy to stick to that rule of thumb.
An optimal height for a cool-season grass in fall is about 2 1/2 inches. And at each mowing, you should only be removing about the top 1/3 of the grass blade. Consequently, a good time to mow lawns is when your grass is about 3 2/3 inches high.
Why is 2 1/2 inches an optimal grass height? Well, what youre striving for is a balance. On the one hand, mowing the grass is like "pinching" a garden plant to stimulate rugged growth. So if youre letting the grass get tall, youre not mowing very much, which means youre passing up opportunities to make your lawn healthier and more attractive. But if you cut grass too short, you stress it.
And let me stress that it is the height the grass has attained that should determine when it's time to mow. It would be arbitrary to establish a schedule for mowing based on length of time between mowings. For instance, advising someone to "mow every other week" fails to take into account the growing conditions during that period of time. For instance, rainfall stimulates growth in lawns, so the presence or absence of rain will have a great impact on mowing frequency.
While the 2 1/2-inch height for a cool-season grass is optimal for the fall, it is hardly an absolute figure that must be adhered to throughout the mowing season. If you're trying to get rid of crabgrass, for instance, you may wish to make an adjustment in spring and summer. That's because it is during these seasons that this annual weed (here's what crabgrass looks like) competes for space with your lawn grass.
Some experts recommend keeping lawn grass around 3 inches tall during spring and summer. Why? Because the extra shading that results may inhibit crabgrass enough to keep it in check. In fall, you can go back to maintaining the 2 1/2-inch height, since, by then, frost will have killed the tender crabgrass.