St. Augustine grass (Stenotaphrum secundatum) is a warm-season turfgrass commonly found across the Southern United States and other tropical or subtropical regions. This grass spreads aggressively under favorable conditions, a growth habit that allows it to form the dense, thick lawns it is known for. Understanding how this turf expands is the first step toward managing it effectively, whether promoting rapid coverage or preventing encroachment into other areas.
The Mechanics of St. Augustine Spread
The expansive nature of St. Augustine grass is attributed to specialized above-ground stems called stolons. These runners grow horizontally along the surface of the soil, radiating outward from the parent plant and creating a dense turf canopy. This lateral growth pattern allows the grass to quickly colonize open areas and is the reason for its aggressive spreading reputation.
As the stolons elongate, they feature periodic swellings known as nodes. When a node contacts the soil, it initiates the formation of new roots downward and new shoots upward. This process creates a new, independent grass plant genetically identical to the parent, leading to rapid area coverage and lawn thickness.
St. Augustine grass rarely produces rhizomes, which are similar spreading structures that grow underground. Furthermore, St. Augustine seed is generally not viable for establishing a lawn. Almost all natural spread and propagation occurs exclusively through the action of these surface runners.
Optimizing Growth for Rapid Coverage
Promoting the natural spreading ability of St. Augustine grass is the most effective way to achieve a uniform, thick lawn and rapidly repair damaged areas. Cultural practices are the primary drivers that encourage the stolons to grow faster and root more successfully.
Mowing Height
Maintaining an appropriate mowing height is a significant factor, ideally between three and four inches. Mowing the grass too short stresses the plant and exposes the stolons to damage from the sun and heat, potentially slowing expansion. A higher cut provides necessary shade for the runners, protecting them while encouraging horizontal growth across the soil surface. This practice conserves energy, which the plant dedicates to producing more nodes and longer runners instead of focusing only on vertical leaf growth.
Fertilization
Fertilization plays a direct role in fueling the expansive growth of the runners. Nitrogen is the nutrient most directly linked to vegetative growth, and applications should coincide with the grass’s peak growing season. Applying a slow-release, balanced fertilizer in late spring and early summer provides the sustained energy necessary for the plant to push out new stolons and accelerate coverage across bare patches.
Irrigation
Proper irrigation techniques support this aggressive spreading mechanism by encouraging deep root development. Watering deeply but infrequently promotes the growth of strong, deep roots rather than shallow ones susceptible to drying out. This deeper root system is better equipped to support the energy demands of a large network of spreading stolons, especially during periods of heat stress.
Soil Health
Ensuring adequate soil health assists the rooting process of the newly formed nodes. Periodic aeration can reduce compaction, making it easier for the stolons to anchor their new roots into the ground. A well-aerated soil environment facilitates the necessary contact between the nodes and the soil surface, which is a prerequisite for forming the new plants that complete the coverage cycle.
Containing Aggressive Spreading
While promoting spread is desirable for lawn coverage, the aggressive nature of St. Augustine grass requires proactive measures to prevent encroachment into unwanted areas like garden beds and walkways. The most reliable method of containment involves installing physical barriers, or edging, along the lawn’s borders. Materials such as plastic, metal, or concrete can be used, but they must be installed correctly to be effective.
Because the stolons grow above ground, the edging does not need to be deeply buried to block underground rhizomes. However, it must rise high enough to intercept the surface runners. Edging installed approximately two to four inches above the soil line is sufficient to create a physical obstacle that the runners cannot easily cross. Regular maintenance is necessary because the stolons will attempt to grow over any barrier if left unchecked.
The runners must be routinely trimmed back from the barrier using edging shears or a vertical trimmer to maintain a clean border. Allowing the runners to cascade over the edging will eventually permit them to root on the other side, negating the barrier’s purpose. This mechanical removal, done every two to three weeks during the peak growing season, is the safest method for long-term control.
For hard-to-manage areas, a non-selective herbicide can be used along sidewalk or paver cracks, but this requires caution. These chemicals kill any plant material they touch, so they must be applied with precision to avoid drift onto desirable turf or ornamental plants. Follow all label instructions and treat only the immediate, unwanted runner growth.