St. Augustine grass is a popular warm-season turfgrass celebrated for its lush, carpet-like texture and deep green color when maintained properly. However, this dense growth habit also makes it susceptible to yellowing or dullness. Achieving a deep, uniform green is entirely possible, but it requires a targeted approach that addresses foundational care, nutrient delivery, and specific deficiencies.
Foundational Maintenance Practices
A deep green color begins with establishing the right physical conditions for the grass to thrive, starting with proper mowing height. St. Augustine grass has thick stolons and a coarse texture, meaning it should be mowed high, ideally between 3 and 4 inches throughout the growing season. Mowing at this height shades the soil, which cools the root zone, helps retain moisture, and encourages deeper root growth.
Mowing frequency is determined by the “one-third rule,” meaning you should never remove more than one-third of the blade height in a single session. During peak summer growth, this may require weekly mowing to prevent scalping, which stresses the plant and causes browning or yellowing. Maintaining a slightly acidic to neutral soil pH, in the range of 6.0 to 7.5, is also important for nutrient availability.
Soil structure must be conducive to healthy root function; compacted soil or a thick layer of dead organic matter, known as thatch, will restrict oxygen, water, and nutrient uptake. If the thatch layer exceeds about three-quarters of an inch, it should be addressed through dethatching or core aeration. Aeration physically removes small cores of soil, which improves air exchange and water penetration to the root zone.
Optimizing Nutrient and Water Delivery
Nitrogen (N) is the single most influential nutrient for promoting deep green color because it is a primary component of the chlorophyll molecule. A sufficient supply of Nitrogen stimulates vigorous leaf growth and the production of chlorophyll, the pigment responsible for capturing light energy. For the best color and growth, a high-Nitrogen fertilizer with a ratio such as 3-1-2 (e.g., 15-5-10 or 21-7-14) is recommended during the active growing season.
The preferred application method involves using a slow-release Nitrogen source, which provides a steady supply of nutrients over eight to ten weeks. This slow release minimizes the risk of burning the turf, which can happen with fast-release fertilizers, and prevents sudden, excessive growth flushes. Fertilization should begin in the spring after the grass has fully greened up and continue every eight to ten weeks through early fall.
Proper hydration is equally important, as water transports nutrients throughout the plant and is necessary for photosynthesis. St. Augustine grass requires deep, infrequent watering to encourage a robust root system, which improves the plant’s resilience. The general guideline is to provide approximately one inch of water per week, including rainfall, during the growing season.
Shallow, frequent watering should be avoided because it promotes weak, surface-level roots that make the grass susceptible to heat stress and discoloration. A deep watering session should moisten the soil to a depth of six to eight inches. Watering early in the morning, between 4 a.m. and 10 a.m., is ideal because it minimizes water loss from evaporation and allows the grass blades to dry quickly, reducing the risk of fungal diseases.
Targeting Chlorosis: Iron Supplementation
Even with adequate Nitrogen, St. Augustine grass may develop a yellowing, or chlorotic, appearance, often caused by an iron deficiency. This yellowing is distinct from Nitrogen deficiency because the newest leaves turn yellow first, while the leaf veins may remain green. Iron is a micronutrient essential for the production of chlorophyll, but unlike Nitrogen, it does not directly promote growth.
Iron deficiency is frequently tied to high soil pH (above 7.0), which chemically binds the iron, making it unavailable for root uptake. To provide a quick color boost, a foliar application of liquid iron, such as chelated iron or ferrous sulfate, can be sprayed directly onto the leaves for immediate absorption. This method offers a temporary solution, typically lasting only a few weeks, but provides a rapid greening effect without stimulating excessive growth.
For a more sustained solution, the underlying high pH issue must be addressed by applying soil amendments like elemental sulfur, which works slowly to lower the soil pH over time. Soil testing is recommended to confirm the pH level and iron content before applying amendments. Regular application of acid-forming fertilizers, which contain iron, can also help maintain the slightly acidic conditions St. Augustine grass prefers for optimal iron uptake.
Addressing Biotic Threats
Discoloration that appears suddenly or in localized patches may not be a nutrient issue but rather the result of pests or disease activity. The southern chinch bug is a common insect pest that feeds by sucking sap from the grass blades, injecting a toxin that causes the grass to turn yellow, then reddish-brown, and eventually die. Chinch bug damage often appears in irregular, expanding patches, especially in hot, sunny areas near concrete or pavement.
Fungal diseases are also a significant cause of poor color, particularly Brown Patch, which is active during periods of high moisture and cooler temperatures (below 80°F). Brown Patch typically causes circular areas of yellowed or brown grass, and the leaf blades can often be easily pulled from the runner due to rot at the base. Gray Leaf Spot is another fungus that causes small, diamond-shaped spots on the blades, leading to a scorched, yellowed appearance.
Correctly identifying the biological threat is necessary because the treatments for pests and diseases differ from those for nutrient deficiencies. For instance, treating Brown Patch involves reducing moisture levels and avoiding excessive soluble Nitrogen applications, while managing chinch bugs requires targeted insecticide application. Restoring a uniform green color depends on eliminating the pest or fungus that is actively damaging the leaf tissue.