The transition to winter dormancy marks the most significant period for lawn preparation, and the final mowing of the year is crucial for the turf’s survival. Ending the season at the correct time and height dictates how well the grass resists diseases and cold stress. Stopping too early leaves the grass vulnerable to matting and mold under snow cover, while continuing too late damages the plant’s ability to store energy for the spring. This final cut protects the lawn before it enters its long winter rest.
Determining When Growth Stops
The decision to put the mower away is based on observing environmental signals, not a specific calendar date. Grass growth is directly governed by soil temperature, not just the air temperature. Chilly nights slow top growth, but the plant’s energy-storing processes in the roots remain active until the ground cools significantly.
For most cool-season grasses, such as fescues and Kentucky bluegrass, shoot growth effectively ceases when the soil temperature consistently drops below 40 degrees Fahrenheit. This 40-degree mark is the true indicator of dormancy. Monitoring the soil temperature with a thermometer is the most precise way to know when to stop, especially where temperatures can fluctuate.
This biological trigger provides a more reliable guide than air temperature or the first frost date. Even after the first frost, the soil retains heat, allowing the grass to continue photosynthesis and store food reserves. Continuing to mow until the grass visibly stops growing ensures that the lawn enters dormancy at the proper height.
The Optimal Height for the Final Cut
The final mow requires a height adjustment, aiming for a slightly lower cut than regular fall maintenance. For most cool-season turf, the ideal height range for the final cut is between 2 and 2.5 inches. Achieving this target requires lowering the mower deck by one or two settings from the usual autumn height.
Cutting the grass to this precise range is a preventative measure against winter diseases and pests. If the grass remains too long (exceeding three inches), the blades can mat down easily under snow and moisture, creating an ideal environment for snow mold. Long grass also provides shelter and a food source for small rodents, such as voles, which can cause significant damage to the turf over winter.
Conversely, cutting the grass too short (below 1.5 inches) exposes the plant’s crown and root system to excessive cold and desiccation injury. The grass needs remaining leaf blade to insulate the crown and continue minimal energy production before the ground freezes solid. This specific height balances the need to reduce disease risk with protecting the plant’s vulnerable parts.
Preparing the Lawn for Winter
The final mowing should be followed immediately by other maintenance tasks that work in conjunction with the cut height to ensure successful dormancy. One of the most important actions is the complete removal of all leaf litter and debris from the lawn surface after the final trim. Trapped leaves create a dense, wet layer that can smother the turf and further encourage the development of fungal pathogens like snow mold.
A critical step is the application of a specific late-fall fertilizer, often termed a “winterizer,” once the grass has stopped growing but the soil is still workable. This specialized application focuses on delivering nutrients that the grass can store in its root system for use during winter and for a quicker green-up in the spring. Winterizer formulas typically contain a higher proportion of nitrogen and potassium, with the nitrogen helping to build carbohydrate reserves and the potassium strengthening cell walls to improve cold tolerance.
This application should be timed to occur before the ground completely freezes, allowing the nutrients to be absorbed by the still-active root system. Lastly, ensuring the lawn has adequate moisture before the deep freeze is important to prevent desiccation. The turf should not be saturated, but a final, deep watering may be necessary if the fall season has been particularly dry, protecting the roots from drying out when the ground hardens.