The question of when to put away the mower for the season is more complex than simply waiting for the first frost. Maintaining a healthy lawn through the colder months requires an understanding of the grass plant’s biology and its reaction to temperature changes. Cutting the lawn when it is too cold can undermine the turf’s natural defenses and compromise its ability to survive winter. Knowing the specific temperature thresholds helps prevent damage and ensures a vibrant return to green in the spring.
The Critical Temperature for Mowing
For most cool-season turf varieties, like Kentucky bluegrass or fescue, the recommended minimum air temperature for mowing is around 40°F (approximately 5°C). Mowing below this threshold is advised against because it suggests the grass is no longer actively growing. While the grass may not be entirely dormant, its metabolic processes have slowed significantly. Ignoring this temperature signal means expending energy on a task that offers little benefit to the turf.
The air temperature indicates that growth has ceased or is minimal, which is the primary reason to stop mowing. Active growth typically slows dramatically once the soil temperature dips below 50°F. A low-temperature cut is an unnecessary physical stress on a plant already preparing for the harshest part of the year.
Understanding Grass Dormancy and Cold Stress
When temperatures drop consistently, grass plants naturally enter a state of cold acclimation, often referred to as dormancy. This process is a survival mechanism where the plant shifts its focus from producing green leaf tissue to strengthening its root system and crown. During this time, the plant increases the storage of soluble carbohydrates, essentially sugars, in its crown and roots. These carbohydrate reserves provide the energy needed to survive freezing temperatures and fuel the first burst of growth in the spring.
Mowing during this stressed state forces the plant to divert its stored energy toward healing the cut leaf blades rather than storing it for winter survival. This energy expenditure weakens the plant’s overall cold hardiness and can induce premature or deeper dormancy. Interrupting the cold acclimation process reduces its resilience, making it more vulnerable to disease and winter injury.
Damage Caused by Cold Weather Mowing
Mowing grass when it is stiff from cold temperatures causes mechanical injury to the leaf blades. Instead of a clean slice, the chilled blades are more brittle, leading to tearing, shredding, and bruising by the mower. These ragged edges create open wounds that heal slowly, if at all, because of the plant’s low metabolic rate.
The damaged tissue provides an easy entry point for fungal pathogens, significantly increasing the turf’s susceptibility to cold-weather diseases like snow mold. If the grass is cut too low late in the season while growth has halted, the result can be scalping. Scalping exposes the vulnerable crown of the plant to the drying effects of winter winds and direct freezing temperatures, compromising the plant’s primary survival structure and potentially leading to plant death.
Practical Checklist Beyond the Thermometer
Relying solely on air temperature is insufficient, as other environmental factors can make mowing hazardous even if the thermometer reads above 40°F. The most important condition to avoid is the presence of frost or ice on the grass blades. When grass is frozen, the water inside the cells expands, making the blades extremely brittle. Pressure from a mower’s wheels or blade will cause the internal cell structure to shatter, resulting in irreparable damage that appears as brown or black tracks.
Another external factor is soil moisture. Mowing on saturated or muddy ground can lead to significant soil compaction under the weight of the equipment. Compacted soil restricts the movement of air and water to the roots, which is detrimental to root health and spring growth. It can also create ruts in the lawn that persist until spring thaw.
The decision to put the mower away should also be guided by the grass’s visible growth pattern. The final cut of the season is best performed when the grass has visibly stopped growing, typically when soil temperatures are consistently below 50°F. This final mow should aim for a height of about two to two-and-a-half inches for cool-season grasses, which is short enough to prevent matting that encourages snow mold, but tall enough to protect the crown throughout winter.