Lawn growth is a biological process governed by temperature, which dictates when grass plants are actively producing energy or simply trying to survive. The exact temperature at which grass stops growing is not a single number, but a range dependent on the grass’s genetic makeup and the local climate. Understanding these temperature thresholds is key to maintaining a healthy lawn throughout seasonal changes.
Cool Versus Warm Season Grasses
Turfgrasses are broadly classified into two major categories based on their native climate and photosynthetic mechanisms. Cool-season grasses, such as Kentucky bluegrass, fescue, and perennial ryegrass, use C3 photosynthesis, which is more efficient in cooler conditions. These grasses are adapted to regions with cold winters and hot summers, thriving during the mild temperatures of spring and fall.
Warm-season grasses, including Bermuda, Zoysia, and St. Augustine grass, utilize C4 photosynthesis, a pathway that conserves water and is more efficient in high heat and intense sunlight. These varieties are native to tropical and subtropical areas, meaning their active growth period occurs during the hottest months of summer.
The Dormancy Threshold
The moment grass stops growing is when it enters dormancy, a survival mechanism where the plant conserves energy by halting shoot growth. For cool-season grasses, leaf growth slows dramatically or ceases when air and soil temperatures dip below approximately 50°F. In these conditions, the grass enters a state of suspended growth, often retaining some green color through the winter months.
Warm-season varieties are more sensitive to cold and enter dormancy at higher temperatures, typically when air temperatures consistently drop below 65°F. This shutdown is usually triggered when the soil temperature falls below 50°F to 60°F, causing the entire plant to turn brown. Roots remain alive, storing reserves until warmer conditions return.
Optimal Growth and Restarting Temperatures
The temperature required to restart growth is slightly lower than the temperature needed for the grass to achieve its most vigorous growth rate. Cool-season grasses begin to emerge from their winter dormancy when soil temperatures reach 40°F to 50°F. The most active shoot growth occurs when daytime air temperatures are consistently between 60°F and 75°F.
Warm-season grasses require a much higher baseline temperature to break dormancy and begin their spring green-up, generally needing nighttime soil temperatures to be consistently above 60°F. Their peak growing season is defined by air temperatures ranging from 80°F to 95°F, which is when they aggressively produce new leaves and runners. Growth slows dramatically for both types when temperatures exceed their upper limits, such as when cool-season grass growth is stunted by air temperatures above 80°F or warm-season grass growth slows above 95°F.
Seasonal Maintenance When Growth Stops
Once temperatures indicate that grass growth has ceased, maintenance practices should be adjusted to support the plant’s dormant state. For both grass types, it is important to significantly reduce or stop watering, as the plant is not actively using moisture and excessive water can lead to disease. Foot traffic should also be minimized on any dormant lawn, especially one that is frozen or brown, because the plant tissue is brittle and easily damaged.
A final, lower mow is beneficial for cool-season grasses just before they enter deep winter dormancy, typically when air temperatures reach around 40°F. Applying a specialized fertilizer, often called a “winterizer,” in the late fall is also recommended, as it delivers nutrients that strengthen the root system for winter survival and promote early spring green-up. Removing all leaves and debris is important to allow air circulation and prevent the growth of winter weeds and fungi.