St. Augustine grass (Stenotaphrum secundatum) is a warm-season turf favored across the southern United States and other subtropical regions. It is prized for its coarse, attractive blue-green color and ability to form a dense, carpet-like lawn. Homeowners often choose this species for its tolerance to heat and performance in partially shaded conditions. Its vigorous growth rate allows it to quickly colonize open areas, making understanding the specific mechanisms and speed of this expansion crucial for management.
The Mechanism of Horizontal Growth
The expansive nature of St. Augustine grass is due to its unique method of vegetative reproduction. This grass primarily spreads through above-ground runners known as stolons, which are thick, horizontal stems that creep along the soil surface, radiating outward from the main turf area.
At regular intervals along these stems, specialized joints called nodes develop. When a node contacts the soil, it roots downward and produces new vertical shoots, establishing a new plantlet. This continuous process of extending, rooting, and shooting allows the grass to rapidly fill in bare spots and create a dense mat.
Typical Spreading Rates and Timelines
The rate at which St. Augustine grass spreads is rapid under optimal conditions during the growing season. Stolons of a healthy lawn typically expand outward at a rate of 6 to 12 inches per year. During the peak growing months of late spring and summer, new runners can extend several inches in a single week.
When starting a lawn from small plugs, this aggressive growth provides a quick timeline for full coverage. Plugs planted 12 to 18 inches apart may show outward spread within two weeks. With consistent care, a lawn planted from plugs can achieve complete, dense coverage in six months to one year, depending on the initial spacing.
Key Environmental and Maintenance Factors
The spreading rate is significantly influenced by environmental and maintenance variables. The turf performs best in full sun, where photosynthetic activity fuels the creation of new stolons. Although St. Augustine tolerates some shade, lateral spread slows considerably in heavily shaded areas.
Consistent moisture is a major factor, as the grass requires more water than other common turf types. Adequate irrigation encourages pliable stolons and facilitates rooting at the nodes. Soil health is also important, with the grass preferring well-drained soil with a pH between 6.0 and 7.5.
Nutrient availability, particularly nitrogen, directly correlates with expansion speed. A regular fertilization schedule, often involving 3 to 6 pounds of nitrogen per 1,000 square feet annually, sustains vigorous growth. Maintaining a tall mowing height (3 to 4 inches) promotes a strong canopy and encourages the grass to spread laterally to fill voids.
Controlling Aggressive Lawn Expansion
Because of its vigorous nature, St. Augustine grass requires strategies to prevent it from invading adjacent garden beds or walkways. The most effective method of containment is installing a physical barrier. Edging materials, such as metal, stone, or heavy-duty plastic, create a defined boundary that surface-creeping stolons must cross. The edging should extend 1 to 2 inches above the soil line to physically block runners from growing over the edge. Although St. Augustine does not spread via deep rhizomes, installing the barrier 4 to 6 inches deep provides a secure demarcation.
Regular maintenance involves using a string trimmer or edger to sever any runners attempting to grow across the barrier before they root. Another containment strategy is minimizing fertilizer application near the lawn’s borders, as reducing nitrogen availability slows outward growth. Maintaining the recommended high mowing height across the main lawn, but using a lower cut or frequent string-trimming along the border, helps stress and weaken the grass in the containment zone.