St. Augustine grass (Stenotaphrum secundatum) is a warm-season turfgrass widely used in the southern United States, prized for its dense growth and tolerance for moderate shade. Its popularity in tropical and subtropical climates stems from its ability to thrive in warm, humid conditions. However, its rapid, aggressive spreading habit often leads to misunderstandings about its ecological classification, confusing its vigorous growth with true invasiveness.
Ecological Status Versus Horticultural Aggression
A true invasive species is formally defined as a non-native organism whose introduction causes or is likely to cause economic or environmental harm. St. Augustine grass is generally not classified this way in its primary turf regions, such as the Southeastern United States. The species is considered native to some coastal areas of North and Central America, including parts of Texas, where it naturally occurs.
While it may be non-native in some international locations, in its primary turf market, its status is a landscape management issue rather than an environmental threat. The aggressive nature of the grass is a horticultural challenge. This turfgrass forms a thick, dense sod that quickly outcompetes weeds and other grasses, which is desirable for turf uniformity.
The term that accurately describes its behavior is horticulturally aggressive, meaning it spreads vigorously beyond the boundaries intended by the gardener or homeowner. This aggressive growth allows it to rapidly encroach into adjacent flower beds, sidewalks, and neighboring lawns. The grass’s ability to quickly colonize bare soil makes it a successful turf, but also the source of management headaches.
The Mechanisms of St. Augustine Grass Spread
St. Augustine grass achieves its aggressive spread almost exclusively through specialized above-ground stems called stolons, often referred to by homeowners as runners. These modified stems creep horizontally along the surface of the soil and are the primary way the plant expands its territory.
As the stolon grows, new roots and shoots form at its nodes, which are the joints along the stem. Each node that successfully roots becomes a clone, or daughter plant, capable of sustaining itself and further extending the spread. This vegetative propagation method allows the grass to rapidly colonize adjacent ground and invade garden beds.
In contrast to grasses like Bermuda grass, which also use underground runners called rhizomes, St. Augustine grass is primarily stoloniferous. The turf varieties used in landscaping rarely produce viable seeds, meaning the plant relies almost entirely on this above-ground stem system for lateral expansion. Understanding the surface nature of the stolons is important because it dictates the most effective physical containment strategies.
Strategies for Managing Unwanted Growth
Managing the aggressive spread of St. Augustine grass requires proactive maintenance and the use of physical barriers. Installing edging is one of the most effective mechanical controls, as it physically blocks the surface-level stolons from creeping into adjacent areas. Barriers made of plastic, metal, or concrete should be installed with a portion extending above the soil line to intercept the runners and prevent them from growing over the boundary.
Cultural practices related to mowing and fertilization can also influence the grass’s growth rate and density. Maintaining a higher mowing height, typically between 3.5 and 4 inches, encourages a deeper root system and helps the grass shade out weeds. Excessive nitrogen fertilization can accelerate the aggressive growth and contribute to the build-up of thatch.
For runners that have already encroached into garden beds, targeted removal methods are necessary. Manual removal by hand-pulling the runners is a common, labor-intensive practice. When using chemical controls, a non-selective herbicide can be applied directly to the unwanted runners in the garden area. Extreme caution must be used during application to prevent the herbicide from drifting or leaching onto desirable plants.