Preparing a lawn for the harsh Minnesota winter is necessary to ensure the turf survives dormancy and emerges healthy in the spring. The final mowing of the season is important, as the height and timing of this cut directly influence the grass’s ability to resist disease and damage from prolonged snow cover. This winterization step addresses the unique challenges of the northern climate, setting the stage for a successful start to the next growing season.
Determining the Optimal Final Cut Height
The question of how short to cut the grass before winter is answered by balancing the need for disease prevention with plant protection. For the cool-season grasses common in Minnesota, such as Kentucky bluegrass, the ideal final cut height is between 2 and 2.5 inches. This specific height range is a compromise that addresses two major biological concerns.
Cutting the grass shorter than 2 inches, or “scalping” the lawn, removes too much of the leaf blade, stressing the plant’s crown and depleting its stored energy reserves. This lack of insulation and energy leaves the turf vulnerable to winterkill from extreme cold. Conversely, leaving the grass longer than 2.5 inches creates a significant problem with matting once heavy snow falls.
Longer grass blades fold over and become dense under the weight of snow, trapping moisture and creating an anaerobic environment conducive to fungal growth. To achieve the optimal final height without shocking the turf, the mower deck should be gradually lowered over the last few cuts of the season. This prevents removing more than one-third of the blade length at any one time, maintaining the grass’s health right up to dormancy.
Timing the Final Mowing
The correct timing for the final mow is determined by the grass’s physiological state, not a specific calendar date. Cool-season grasses slow their growth substantially when ambient temperatures consistently dip into the 50-degree Fahrenheit range. The final cut should occur when the turf has nearly stopped growing, signifying its approach to winter dormancy.
In Minnesota, this usually places the final mowing window around mid-to-late October, though local weather patterns can shift this timing. Continue mowing as long as the grass blades are actively growing to prevent them from entering winter too long. Cutting too early allows a flush of new growth that can be damaged by cold and subsequent snow.
The final cut must be completed before the first permanent snow cover, which would prevent a clean cut, and ideally just before the ground freezes solid. If temperatures drop low enough to cause a hard frost, which freezes the moisture inside the grass blades, mowing should be avoided until the grass thaws and dries. Waiting for the grass to stop growing ensures the plant has successfully moved its energy stores into the root system for winter survival.
Preventing Snow Mold and Winter Damage
The primary reason for the shorter final cut is to reduce the risk of snow mold, a fungal disease that can severely damage lawns in cold climates with extended snow cover. Two types, Gray Snow Mold (Typhula blight) and Pink Snow Mold (Microdochium nivale), thrive in the cold, moist conditions under a blanket of snow, especially if the ground was not yet frozen when the snow fell.
Long grass that has matted down under snow traps air and moisture against the soil surface, creating the microclimate for these fungi to proliferate. By reducing the height to 2 to 2.5 inches, the surface area available for the mold to colonize is minimized, and the chance of severe matting is decreased. This allows for better air circulation once the snow begins to melt in the spring.
Beyond mowing height, removing all lingering debris is also a preventative measure. Fallen leaves and excessive thatch serve as a food source for the fungal pathogens and provide shelter for small rodents like voles, which can cause significant damage while tunneling under the snow. A clean, shorter lawn reduces the environmental conditions that support disease and pest activity throughout the winter months.