Cool-season lawn grasses perform best when they’re cut to about 3-4 inches during the growing season. Taller grass keeps the soil cooler in a hot summer, slows moisture loss, and denies sunlight to lawn weeds, not to mention providing additional chlorophyll to manufacture energy for the grass roots.
But there’s an exception to this mow-high rule of thumb – the end of the growing season.
Shorter cuts at the end of fall are a good way to discourage early-season lawn disease.
Why a shorter end-of-fall mow?
Gray snow mold and pink snow mold are two fairly common fungal diseases that grow under the protection of snow cover in winter. Pink snow mold also can grow during snow-free periods of cool, damp winter weather. Both show up as matted-down patches of dead grass blades in late winter to early spring.
Fortunately, the grass crowns – from where new blades emerge – usually survive and go on to grow new grass, albeit with a little raking help from the lawn owner to speed up the process.
Since both of these mold diseases favor damp and shaded conditions, one of the best ways to prevent them is by mowing the grass slightly shorter heading into winter. That helps the lawn dry a little faster and opens it to better sunlight penetration.
The lower mowing not only lessens the shading effect of the grass blades themselves but chops up fallen leaves and other organic debris that can contribute to trapping moisture over winter.
Cutting shorter doesn’t mean scalping. Going too low is a stress on the lawn, especially if the lawn is shortened drastically in one fell swoop. An ideal height for the last cut of the season on the typical Kentucky bluegrass, perennial ryegrass, and/or fescue cool-season lawn is between 2 and 2½ inches.
How to finish the mowing season
To avoid stressing the lawn without removing more than one-third of the blade height at a time (another good rule of thumb), shorten the height gradually over the last two or three mowings of the season.
If the additional clips are enough to make mats or channels that threaten to suffocate the remaining turfgrass, either bag them while mowing or rake them afterward. The combination of mowed leaves and grass clippings creates an ideal mix for the compost bin.
Avoid mowing the lawn when the grass is wet, and make sure the blades are still sharp. Most mower manufacturers recommend sharpening mower blades every 25 hours.
One other potential benefit of cutting at this slightly lower height heading into winter is that it’s less inviting to voles that make surface tunnels in the lawn during winter.
See more tips on how to correctly mow a lawn