St. Augustine grass (Stenotaphrum secundatum) is a warm-season turfgrass known for creating a thick, dense, blue-green lawn across the Southern United States. It establishes quickly through above-ground runners called stolons. Understanding its “growing zone”—the specific geographic area where it can survive and thrive—is the most important factor for successful installation, as this zone is determined by the plant’s tolerance to cold temperatures.
Defining the Hardiness Range
The primary determinant for successfully growing St. Augustine grass is its susceptibility to prolonged freezing temperatures. This warm-season grass is generally best suited for USDA Plant Hardiness Zones 8 through 10, which encompasses much of the Gulf Coast, Florida, Texas, and the Southern California coast. The USDA zone map is the standard metric for determining a plant’s winter survival, based on the average annual minimum winter temperature.
Zone 8 represents the northern limit of the recommended range, experiencing average minimum temperatures that rarely drop below \(10^{\circ}\text{F}\) to \(20^{\circ}\text{F}\). St. Augustine grass can typically survive in this zone, but it is highly susceptible to winterkill if temperatures drop below \(20^{\circ}\text{F}\) for extended periods, especially if the ground is not insulated by snow or is exposed to wind chill. The threat of lethal damage becomes substantial if the soil temperature drops to around \(10^{\circ}\text{F}\).
More temperate zones, such as Zone 10 and 11, provide ideal conditions where the risk of winterkill is minimal. In these regions, the grass may not fully enter a deep dormancy phase, allowing it to maintain its green color for a longer period throughout the year.
Ideal Climate and Site Requirements
While cold tolerance defines the survival boundary, optimal growth for St. Augustine grass is dictated by high heat and humidity. This turfgrass thrives in tropical and subtropical climates, with its most vigorous growth occurring when temperatures range between \(80^{\circ}\text{F}\) and \(95^{\circ}\text{F}\). The grass’s natural adaptation to warm, humid environments highlights its need for consistent moisture and warmth.
St. Augustine grass possesses better shade tolerance than many other warm-season grasses, like Bermuda grass, but it still requires at least four to six hours of unfiltered sun exposure daily to flourish. Deep shade conditions are detrimental to its long-term health. The grass is adaptable to a wide variety of soil types, including sandy and clay soils, but the soil must be well-drained to prevent fungal issues. It also exhibits a good tolerance for salt, making it a common choice for lawns in coastal areas.
Seasonal Zone Management and Dormancy
Successful management requires adapting care practices to the specific zone, especially concerning seasonal changes. As a warm-season grass, it enters dormancy when soil temperatures consistently drop below \(55^{\circ}\text{F}\). This natural protective mechanism causes the grass to turn golden-brown and cease active growth until warmer weather returns.
In the colder Zone 8, preparation for winter begins in the fall. Homeowners should avoid late-season nitrogen fertilization, which encourages tender new growth highly susceptible to frost damage. Instead, a final application of a low-nitrogen, high-potassium fertilizer increases cold hardiness. During dormancy, the grass still requires occasional deep watering to maintain root health, though frequency is significantly reduced compared to the growing season.
In the warmer Zone 10 and 11, dormancy may be brief or non-existent, meaning the grass can remain green year-round. This continuous growth requires a different management approach, often involving year-round irrigation and fertilization to sustain the lush appearance. Mowing frequency also decreases significantly in all zones during cooler months, and the mowing height should be maintained at a higher level, typically between three and four inches, to protect the stolons and root system from cold stress.
Zone Limitations and Cultivar Selection
Attempts to grow St. Augustine grass outside of established Zones 8 through 10 carry a high risk of failure due to temperature extremes. Planting in Zone 7 often results in the grass succumbing to hard freezes, as it lacks the cold tolerance to consistently survive the winter. Conversely, in areas warmer than Zone 10 (such as Zone 11 or 12), the extreme heat and humidity increase the maintenance burden, leading to a higher incidence of chinch bugs and fungal diseases like gray leaf spot.
The selection of a specific cultivar can adjust the effective growing zone. Cultivars such as ‘Raleigh’ and ‘Palmetto’ are known for improved cold hardiness, making them better choices for the northern fringes of Zone 8. ‘Raleigh’ was developed for northern transition zones and has a reputation for winter survival. Other varieties like ‘Floratam’ are less cold-tolerant but are popular in the deepest South due to their rapid growth and heat tolerance.