St. Augustine grass is a warm-season turf variety prized for its dense, aggressive growth and lush, coarse texture, making it a favorite in Southern climates. Managing this turf requires careful attention to nutrient supply to support its rapid lateral spread via above-ground stems called stolons. The success of a St. Augustine lawn depends far more on the precise timing of fertilizer applications than the total amount used. Applying nutrients at the wrong time can be ineffective, wasteful, or severely damage the grass by promoting vulnerable growth.
Understanding St. Augustine’s Growth Cycle
St. Augustine grass has a distinct active growing season, running from late spring through early fall, which dictates its nutritional needs. During this period, the grass efficiently absorbs and utilizes nutrients to produce its characteristic thick, green turf. Outside of this time, the grass enters a period of reduced activity and growth.
The start of the active season is triggered by “spring green-up,” which is closely tied to soil temperature. St. Augustine roots begin to actively take up nutrients when the soil temperature consistently reaches about 65°F. Applying fertilizer before this point is ineffective because the roots are not yet ready to absorb the material.
Once temperatures drop in the fall and winter, the grass slows its metabolism significantly. Fertilizing during this dormant phase is wasteful and harmful, as it encourages tender new growth highly susceptible to cold damage. Unused nitrogen can also leach into the soil, posing environmental risks. This is why following the grass’s natural cycle, determined by temperature, is the most reliable method for optimal lawn health.
Selecting the Appropriate Nutrient Blend
Fertilization involves understanding the N-P-K ratio, where Nitrogen (N), Phosphorus (P), and Potassium (K) are the primary macronutrients. Nitrogen is important for St. Augustine grass, driving the vegetative growth that creates its dense canopy and deep green color. Recommended ratios for established lawns often reflect this need, such as 3-1-2 or 4-1-2 formulas (e.g., 15-5-10 or 16-4-8), which are high in nitrogen relative to the other elements.
The physical form of the nitrogen is as important as its quantity. St. Augustine grass is prone to “burning” from quick-release nitrogen sources, so a slow-release nitrogen fertilizer is recommended. This formulation releases nutrients gradually over six to eight weeks, providing a steady food source and preventing excessive surge growth or chemical burn. Phosphorus, which supports root development, is usually only necessary for newly planted sod or if a soil test indicates a deficiency, as established lawns often have adequate levels.
Potassium contributes to overall plant health, strengthening the grass’s tolerance to heat, drought, and disease. St. Augustine grass also benefits from micronutrients, particularly iron. Iron supplementation enhances the deep green color by aiding in chlorophyll production. This is especially helpful in alkaline soil conditions where iron uptake can be inhibited, which helps prevent the yellowing known as chlorosis.
The Optimal Annual Fertilization Schedule
A maintenance schedule for St. Augustine grass involves three to four applications during the active growing season. The precise timing is guided by the grass’s growth and the season’s progression, rather than fixed calendar dates. The goal is to provide consistent nutrition every six to ten weeks from spring green-up until early fall.
The first application is timed approximately two weeks after the lawn has fully emerged from dormancy, usually when the soil temperature reaches 65°F. This initial feeding should contain a balanced blend, sometimes including a pre-emergent herbicide. This promotes root recovery and provides a base of nutrients for the coming months, ensuring the grass is fully active before being stimulated with high nitrogen.
The second and third applications occur throughout the summer, approximately six to eight weeks apart, to sustain the grass during its most aggressive growth phase. These mid-season feedings should utilize a high-nitrogen, slow-release product. This supports the lateral spread of the stolons and maintains a dense, vibrant turf despite peak heat stress. Consistent feeding is necessary to match the grass’s high demand for nitrogen during the summer.
The final application is the most time-sensitive, designed to prepare the grass for winter dormancy. Applied in early fall, this treatment should be lower in nitrogen and often higher in potassium. The increased potassium encourages carbohydrate storage in the roots, which enhances cold hardiness and disease resistance over the winter. Stop all nitrogen fertilization at least four to six weeks before the first expected frost date in your area. Applying nitrogen too late stimulates new, tender growth that is easily killed by freezing temperatures, causing significant damage to the lawn.