St. Augustine grass (Stenotaphrum secundatum) is a coarse-textured, warm-season turfgrass highly valued for its dense, carpet-like appearance in southern climates. This species expands aggressively across the soil surface using specialized above-ground stems. The practice of overseeding, which involves sowing new grass seed over an existing lawn to improve density, is common for cool-season grasses. However, St. Augustine presents distinct biological and logistical hurdles for this method.
Why Seeding St. Augustine Is Not Standard Practice
St. Augustine grass is primarily propagated through vegetative means, such as sod or pre-grown sections called plugs, rather than by seed. The reason for this lies in its reproductive biology, as many common varieties do not produce a sufficient amount of viable seed for commercial use. While some seed is technically available, it is often difficult to find, expensive, and may not yield the desired quality or uniformity for a home lawn.
The species spreads horizontally using specialized stems called stolons, which root at nodes along the ground to establish new plants. This natural spreading habit means that a healthy St. Augustine lawn will fill in thin or bare areas on its own over time with proper cultural care. Relying on this vegetative growth is the most reliable method for establishing or thickening this turfgrass. Attempting to overseed with St. Augustine seed is impractical and often leads to disappointing results due to low germination rates or non-viable seed.
Recommended Methods for Achieving Density
Achieving a dense, healthy St. Augustine lawn requires focusing on cultural practices that activate and support the grass’s natural spreading ability. A soil test is a beneficial starting point to determine specific nutrient deficiencies and the soil’s pH level. Ideally, the pH should fall between 5.0 and 8.5 for St. Augustine.
The fertilization schedule directly fuels the lateral growth required for density. St. Augustine lawns require between two and four pounds of actual nitrogen per 1,000 square feet annually, with the higher rate needed in sandy soils. Nitrogen applications should begin in the spring after the grass has fully greened up and continue every six to eight weeks during the active growing season.
Correct mowing and watering techniques are equally important for stimulating density. The recommended mowing height is between 2.5 and 4 inches, as cutting too short can stress the turf and inhibit stolon growth. When irrigating, the goal should be deep and infrequent watering, supplying approximately one inch of water per week to encourage deep root development. For filling small to moderate bare patches, planting pre-grown plugs is the standard method, where small sections of sod are placed in a grid pattern to allow stolons to spread and connect.
The Process of Temporary Winter Overseeding
The only common scenario for overseeding a St. Augustine lawn is temporary winter overseeding, aimed at providing color when the warm-season grass goes dormant. This involves sowing a cool-season grass, typically annual ryegrass, over the dormant St. Augustine turf. The ryegrass provides a green appearance during the cool months when St. Augustine turns brown.
Timing is important, and overseeding should occur in the late fall, after the first frost, when the St. Augustine has fully entered dormancy. Before seeding, the St. Augustine should be prepared by mowing it at the lowest acceptable height range, around three inches, to ensure the seed makes good contact with the soil. Annual ryegrass is preferred over perennial ryegrass because it naturally dies out more quickly in the spring, reducing competition with the emerging St. Augustine.
The seed is applied using a broadcast spreader at a rate of seven pounds per 1,000 square feet for an established lawn. After application, the area requires light, frequent watering multiple times a day until the seed germinates, usually within a week. The final step is the spring transition, which requires removing the ryegrass as the St. Augustine begins to green up (typically when temperatures consistently exceed 50 degrees Fahrenheit). This is achieved by lowering the mowing height to stress the ryegrass or, if necessary, using a selective herbicide.