St. Augustine grass, botanically known as Stenotaphrum secundatum, is a warm-season turfgrass highly valued for its distinct broad, flat leaf blades and deep blue-green color. This grass naturally creates a dense lawn through its signature creeping growth habit, utilizing above-ground stems called stolons to spread across the soil. Achieving a truly thick, lush lawn requires encouraging this lateral growth and managing the specific challenges that can cause thinning. The foundational principles for maximizing St. Augustine density lie in precise cultural practices, targeted nutrient application, and proactive troubleshooting of common issues.
Mowing Height and Watering Techniques
The height at which St. Augustine grass is maintained directly affects its ability to spread and thicken. Mowing too short, a practice known as scalping, stresses the grass, inhibits root growth, and leaves the soil vulnerable to weeds and sun exposure. The optimal height for a dense, healthy St. Augustine lawn is consistently between 3.5 and 4 inches, which allows the blades to photosynthesize effectively, shade the soil, and deter weed germination.
To avoid stressing the plant, always adhere to the “one-third rule,” which dictates that no more than one-third of the total blade height should be removed in any single mowing session. For example, if your preferred height is 4 inches, you should mow when the grass reaches 6 inches tall. Maintaining a sharp mower blade is also important, as dull blades tear the grass, creating ragged edges that appear brownish and make the plant more susceptible to disease and water loss.
The method of watering is important for promoting a thick turf. St. Augustine grass requires deep and infrequent watering, which encourages the development of a resilient, deep root system. Aim to provide approximately one inch of water per week, factoring in natural rainfall, to moisten the soil to a depth of six to eight inches.
Shallow, frequent watering sessions train the roots to stay near the surface, making the grass susceptible to heat stress and thinning during dry periods. The best time for irrigation is early morning, ideally between 4 a.m. and 10 a.m. This minimizes water lost to evaporation and allows the leaf blades to dry before nightfall, reducing the risk of fungal disease development.
Nutrient Management for Lateral Spread
Once proper cultural practices are established, the next step is supplying the nutrients that fuel lateral spread. St. Augustine is a heavy feeder, requiring an annual application of three to six pounds of actual Nitrogen (N) per 1,000 square feet. Nitrogen is the primary driver of vegetative growth, directly stimulating the production of new stolons that thicken the turf.
Fertilization should begin in the late spring, three weeks after the grass has fully greened up, and continue every eight to ten weeks through early fall, using a slow-release nitrogen formula. A common strategy for active thickening is to apply 1 to 1.5 pounds of slow-release nitrogen per 1,000 square feet during each application period. Cease nitrogen application six to eight weeks before the first expected frost to prevent tender new growth that can be damaged by cold temperatures.
Soil health plays a significant role in nutrient uptake and overall density. St. Augustine grass performs best in a slightly acidic to neutral soil environment, with an ideal pH range between 6.0 and 7.5. Conducting a soil test before applying heavy inputs ensures that the grass can efficiently absorb available nutrients and prevents over-application that can lead to problems like thatch build-up or disease.
For a deep green color without stimulating excessive vertical growth, supplemental applications of Iron (Fe) may be required. Iron is a micronutrient that promotes chlorophyll production, correcting the yellowing (chlorosis) that occurs in high-pH soils without the growth surge associated with nitrogen. Applying a chelated or liquid iron product throughout the growing season provides a rich color that enhances turf density.
Identifying and Treating Thinning Causes
Even with the best maintenance, a St. Augustine lawn can thin out due to pests, disease, or soil-related issues. One of the most destructive pests is the Southern Chinch Bug, which causes irregular yellow to brown patches, particularly in hot, sun-exposed areas near pavement. These tiny insects damage the grass by sucking the sap and injecting a toxin, causing the grass to die outright and not recover even with watering.
If chinch bugs are suspected, check the margin where the dead grass meets the living turf for the small, black-and-white adults or orange-red nymphs. Treating with a contact insecticide labeled for chinch bugs is necessary, focusing the application on the active feeding areas. Fungal diseases are another common cause of thinning, particularly Gray Leaf Spot and Brown Patch.
Gray Leaf Spot is prevalent in warm, highly humid conditions and is often exacerbated by excessive nitrogen. It appears as small, oval, tan-to-gray lesions on the leaf blades with distinct purple or brown borders. Brown Patch is a cool-weather fungus that leaves circular patches of yellowed grass, where the blades easily detach from the stolon. Both diseases are managed by avoiding late-day watering and improving air circulation, with fungicides required for severe outbreaks.
Finally, soil compaction can hinder lateral spread by restricting root growth and nutrient delivery, leading to thinning. While St. Augustine is sensitive to aggressive soil disturbance, core aeration can be beneficial in severely compacted areas by removing plugs of soil to improve water and air penetration. This should be performed during the active growing season in late spring or early summer, allowing the grass to quickly recover and stolons to fill in the small holes, promoting a denser turf.