St. Augustine grass is prized for its dense, coarse texture and ability to form a thick, carpet-like lawn in warm climates. This density is achieved through vigorous above-ground runners, called stolons, that creep across the soil surface, filling in any gaps. When this turf begins to thin, it is a clear indication that the grass is under significant stress, often due to a combination of environmental factors, pests, or disease. Understanding the precise cause of the decline is the only way to implement an effective treatment plan to restore the lawn’s original vigor.
Diagnosing the Specific Cause of Thinning
The first step in restoration involves closely inspecting the pattern and symptoms of the thinning patches to distinguish between possible causes. Look for irregular, expanding areas of yellowing grass that seem to defy watering efforts, especially in the hottest, sunniest parts of the lawn, such as along concrete sidewalks. This damage strongly suggests an infestation of Southern chinch bugs, which suck the sap from the grass and inject a toxin, rapidly killing the turf.
To confirm their presence, you can perform a “float test.” Cut out the bottom of a coffee can and push it into the soil at the edge of a damaged area. Filling the can with water for several minutes will cause the tiny, black-bodied insects, which are about one-fifth of an inch long, to float to the surface if they are present in damaging numbers.
If the thinning appears as distinct, circular patches that can range from a few inches to several feet in diameter, a fungal disease is the more likely culprit. If the leaf easily detaches from the stolon when gently tugged, leaving the runner intact, the issue is likely Brown Patch disease. Alternatively, if the grass blades themselves show small, oblong spots with tan or gray centers and dark borders, Gray Leaf Spot is probable.
You should also examine the stolons and roots beneath the thinning areas by gently pulling up a small section of turf. If the stolons have dark, rotted lesions, or if the grass pulls up easily because the roots are decayed and mushy, you may be dealing with a root rot disease like Take-All Root Rot. Finally, if the thinning is widespread and uniform across the entire lawn, the stress is most likely related to a cultural issue, such as inadequate sunlight, poor soil, or improper watering practices.
Addressing Pests and Fungal Diseases
Once chinch bugs are confirmed via the float test, immediate action with a targeted insecticide is necessary to prevent complete lawn loss. These pests are highly destructive, and their damage will not recover even after the initial stressor is removed. Apply a contact insecticide, such as one containing bifenthrin, to the affected areas and the surrounding healthy grass where the insects are migrating. A follow-up application two to three weeks later is often needed to manage newly hatched generations of nymphs.
Fungal diseases require a different approach, often involving both chemical and cultural management. Brown Patch, which is generally most active during mild temperatures in the spring and fall, thrives in excessive moisture and overly fertilized conditions. Treating this disease involves applying a broad-spectrum fungicide containing an active ingredient like azoxystrobin.
Gray Leaf Spot is a summer disease that flourishes in warm, wet conditions and is often exacerbated by high nitrogen fertilization. Fungicide application should target the warm, humid months, but specific cultural adjustments are also necessary. Avoid applying high-nitrogen fertilizers during the peak summer heat when the fungus is active. For both diseases, water the lawn only in the early morning hours, before 9:00 AM. This practice reduces the duration of leaf wetness, which is necessary for fungal spores to germinate and infect the grass.
Correcting Environmental and Cultural Stressors
Incorrect maintenance practices are a frequent cause of chronic thinning in St. Augustine grass. This turfgrass requires deep and infrequent watering to encourage the development of a resilient, deep root system. The goal is to apply approximately one to one and a half inches of water per week, allowing the soil to dry out between watering events. Shallow, daily watering promotes weak, shallow roots that are highly susceptible to drought stress and disease.
St. Augustine grass, while more shade-tolerant than many other warm-season varieties, still needs a minimum of four to six hours of direct sunlight each day to maintain its dense growth habit. If thinning is concentrated under large trees or structures, the lack of solar energy is likely the primary stressor. Pruning tree limbs to allow more light penetration can help, but in heavily shaded areas, even the most shade-tolerant cultivars will eventually thin out.
Soil conditions also play a significant role in turf density and overall health. Compacted soil restricts oxygen and nutrient flow to the roots, leading to a weak, thin stand of grass. If the soil is hard and dense, mechanical aeration should be performed to relieve compaction and improve drainage. Furthermore, St. Augustine grass should be consistently mowed at a height of three to four inches, as mowing too low severely stresses the grass and reduces its ability to generate new stolons.
Promoting Recovery and Preventing Recurrence
After the immediate cause of thinning has been successfully addressed, the focus shifts to promoting vigorous recovery. St. Augustine grass spreads by stolons, meaning it will naturally fill in damaged areas over time, but this process can take six to ten weeks depending on the severity of the damage. To accelerate the recovery of bare patches, a light top-dressing of compost or quality topsoil can be applied to provide a favorable rooting medium for new stolons.
Fertilization should be managed carefully to support growth without triggering a recurrence of fungal issues. Provide a balanced fertilizer application in the spring to encourage initial green-up and again in the early fall to prepare the grass for the cooler months. Avoid heavy nitrogen applications during the hottest, most humid summer months, as this practice encourages the development of Gray Leaf Spot disease. Monitoring the lawn weekly for the earliest signs of stress, such as a bluish-gray color that signals drought, allows for timely intervention before a minor issue becomes a widespread thinning problem.