Whether grass grows in the fall depends entirely on the type of grass and its biological programming. Turfgrass species are categorized based on the temperatures at which they perform their best, creating distinct seasonal growth cycles. Autumn represents a period of peak growth for some species, while for others, it signals the beginning of a shutdown phase in preparation for winter dormancy.
Understanding Cool-Season Versus Warm-Season Grasses
The growth pattern of any lawn is determined by whether it is composed of cool-season or warm-season grasses. Cool-season grasses, which include varieties like Kentucky Bluegrass, Tall Fescue, and Perennial Ryegrass, thrive in the northern two-thirds of the United States and have two peak growth periods. Their preferred temperatures range from 60 to 75 degrees Fahrenheit, making spring and fall their times of most active growth.
Warm-season grasses, such as Bermuda, Zoysia, and St. Augustine, are biologically adapted to warmer climates, with optimal growth occurring at temperatures between 80 and 95 degrees Fahrenheit. These grasses experience their main growth surge during the summer months. Consequently, when fall arrives and soil temperatures consistently drop below 65 degrees Fahrenheit, warm-season varieties slow their metabolism and begin the process of turning brown and going dormant for the winter.
For cool-season lawns, fall is often considered their second spring, offering a reprieve from the summer heat that often causes them to slow their growth. The combination of cooler air temperatures and soil that remains warm from the summer sun provides an ideal environment for them to flourish.
The Physiological Focus of Autumn Growth
Even if the visible growth of the grass blades appears to slow down in the fall, significant biological activity continues beneath the soil surface. As temperatures moderate and daylight shortens, the grass plant shifts its primary focus from vertical leaf growth to subterranean development and energy storage.
During this period, the plant prioritizes developing a deeper, more robust root system, a process known as root extension. These new roots help anchor the plant and are far more efficient at absorbing nutrients and water than older roots. Simultaneously, the grass begins the process of cold acclimation, or “hardening off,” by accumulating non-structural carbohydrates in its root crowns and rhizomes.
These stored carbohydrates, primarily in the form of fructans and starches, act as the plant’s food reserves throughout the winter months. They provide the necessary energy for the grass to survive freezing temperatures and power the rapid “spring green-up” when active growth resumes.
Optimizing Fall Lawn Management
The autumn growth cycle for cool-season grasses provides a window of opportunity for specific lawn care practices that have a long-term impact on turf health. Because the soil remains warm while the air cools, fall is the single best time to plant new cool-season grass seed or overseed an existing lawn. Optimal germination occurs when soil temperatures are between 50 and 65 degrees Fahrenheit, a condition typically met in early to mid-fall.
Applying a specialized fertilizer late in the season is another beneficial practice that directly supports the carbohydrate storage process. These “winterizer” products often contain a higher ratio of potassium and nitrogen to promote root growth and strengthen the plant’s cell walls. The nitrogen is stored in the root system and crowns, providing a reserve supply that helps the grass survive the winter and contributes to a quick start the following spring.
Finally, adjusting the mowing height is necessary to prepare the grass for winter. While the grass should be kept relatively long throughout the fall to maximize photosynthesis, the final cuts of the season should be gradually lowered. The objective is to achieve a final height of about two to two-and-a-half inches before the first hard frost. This shorter length prevents the grass from matting down under snow cover, which reduces the likelihood of fungal diseases like snow mold developing during the winter months.