St. Augustine grass is a warm-season turf that establishes a dense, attractive lawn in tropical and subtropical regions. To achieve the deep green color and vigorous growth characteristic of this grass, specific nutrient management is required. The frequency of fertilization is a major factor in maintaining the health and density of St. Augustine grass throughout its active growing period.
Standard Seasonal Application Timing
The timing of the first application is directly related to soil temperature, not the calendar date. St. Augustine grass begins its active growth phase when the soil temperature consistently reaches approximately 65 degrees Fahrenheit at a four-inch depth. Fertilizing before this point is inefficient because the grass cannot absorb the nutrients, and the fertilizer may be wasted or leach away. The ideal time to apply the first fertilizer is typically in the late spring, after the grass has fully greened up and has been mowed at least twice, signifying it is out of dormancy and actively growing.
Once the growing season is underway, the next application usually occurs in mid-summer (June or July) to support the high nutritional demands of peak growth. St. Augustine grass requires significant nitrogen, with total annual application often ranging from three to five pounds per 1,000 square feet. This mid-season feeding helps the turf withstand the stress of high summer temperatures and heavy foot traffic.
The final application prepares the lawn for cooler weather and potential dormancy. This application should be timed for early fall, generally six to eight weeks before the first expected frost. Avoid applying high amounts of nitrogen late in the season, as this stimulates tender new growth easily damaged by frost.
Instead, the final feeding should focus on potassium, often using a low-nitrogen, high-potassium blend. Potassium is beneficial for enhancing the grass’s overall health, including its disease resistance and cold tolerance, allowing the turf to store energy for the winter months. Fertilization should be avoided during peak winter dormancy and intense summer heat or drought stress, as the grass is not actively growing and can be damaged.
Adjusting Frequency for Fertilizer Formulation
The chemical makeup of the fertilizer is the primary factor modifying the standard seasonal schedule into a specific application frequency. Fertilizers containing water-soluble nitrogen (quick-release) provide an immediate surge of nutrients for rapid green-up. Because these nutrients are quickly consumed or washed away, they require application every four to six weeks to maintain consistent color.
Using quick-release products too frequently or at too high a concentration can lead to fertilizer burn, which appears as brown, damaged turf, because the salts pull moisture out of the grass blades. The rapid availability of nutrients increases the risk of nutrient leaching, especially nitrogen, into the water table, necessitating careful application to avoid environmental impact and turf damage.
Conversely, slow-release fertilizers use coatings that control nutrient dispersal, supplying a steady feed over an extended period. These formulations can significantly extend the time between applications, often lasting eight to twelve weeks or longer, depending on the coating and environmental conditions. The use of slow-release nitrogen reduces the risk of burning and minimizes the flush of excessive growth, resulting in less frequent mowing.
Many turf management programs utilize a combination of both types to achieve optimal results. A slow-release product forms the foundation of the feeding program, while quick-release applications can correct nutrient deficiencies or provide a rapid color boost. This blended approach allows for both a sustained nutrient supply and targeted adjustments, customizing the frequency based on the lawn’s immediate visual needs.
How Soil Testing Determines Nutrient Needs
Soil testing is the most accurate tool for determining the precise nutrient requirements of a St. Augustine lawn, moving beyond general recommendations. A standard soil test provides a detailed analysis of the soil’s chemistry, including pH level and existing concentrations of phosphorus, potassium, and various micronutrients. These results are used to customize the fertilizer composition, which in turn influences the frequency of application.
The test results indicate sufficient levels of phosphorus (P) and potassium (K), which are stable in the soil and typically do not need annual reapplication. If the soil test shows high or very high levels of these nutrients, the subsequent fertilization program should utilize a product containing mostly or only nitrogen, such as a 21-0-0 formulation. This prevents the unnecessary buildup of phosphorus, which can lead to deficiencies in other nutrients like iron and contribute to water pollution.
While soil tests do not typically measure nitrogen (as it is highly mobile), they determine if the soil pH is within the optimal range of 6.0 to 7.5 for St. Augustine grass. An imbalanced pH can lock up existing nutrients, making them unavailable to the grass roots, regardless of how often fertilizer is applied. The presence of visual cues, such as general yellowing, can indicate an iron deficiency, which is often made worse by high soil pH or excessive phosphorus, signaling a need for a diagnostic test.