St. Augustine grass, a popular warm-season turf grown across the Southern United States, sometimes develops chlorosis, where the deep green blades turn pale green or yellow. This yellowing indicates the turf is struggling to absorb or utilize necessary resources, leading to diminished health. Since the color change can stem from nutrient deficiencies, improper watering, or pest damage, successful treatment depends entirely on accurately identifying the root cause. This article guides homeowners through diagnosing why their St. Augustine turf is yellowing and provides practical treatments to restore its vibrant green color.
Identifying Why the Grass is Yellow
Diagnosing the discoloration requires close inspection, as different nutrient deficiencies present distinct visual patterns. Iron deficiency, known as interveinal chlorosis, causes the tissue between the leaf veins to turn yellow while the veins remain green. This symptom is most noticeable on the newest growth near the stolon tips, indicating a problem with nutrient mobility.
A uniform pale green or lime-yellow color across the entire blade, affecting both veins and tissue, usually points to a nitrogen deficiency. Since nitrogen is highly mobile within the plant, this yellowing often appears first on the older, lower leaves as the plant prioritizes sending the nutrient supply to newer growth.
If the entire lawn appears uniformly yellow, especially after heavy rain or prolonged irrigation, the issue may stem from overwatering or poor drainage. Overly wet conditions suffocate the roots, causing uniform yellowing because the roots cannot properly function or absorb nutrients. Alternatively, visual cues like distinct spots, lesions, or irregular, spreading patches of damage can indicate a pest or fungal issue. For example, chinch bug damage often appears as yellow or brown, dying patches near sidewalks or driveways where heat is retained.
Applying the Correct Nutritional Treatment
Once iron deficiency is confirmed, the most effective treatment involves applying a chelated iron supplement. Standard iron fertilizers are often ineffective because high soil pH, common in many regions, binds the iron, rendering it unavailable to the grass roots. Chelated iron, however, utilizes an organic molecule to chemically protect the iron ion, allowing the roots to absorb it even in alkaline soil conditions.
These supplements are available in both liquid spray and granular forms and should be applied during the active growing season, typically late spring through summer. A liquid application provides the fastest green-up response, as the iron is absorbed directly through the leaf tissue, offering a noticeable color change within a few days. Granular options provide a slower, more sustained release into the root zone.
If the diagnosis indicates uniform yellowing from nitrogen deprivation, the lawn requires a high-nitrogen fertilizer. St. Augustine grass responds well to products with an analysis around 21-0-0 or similar ratios, where the nitrogen number is significantly higher than phosphorus and potassium. Select a fertilizer that incorporates a significant portion of its nitrogen as a slow-release component, such as sulfur-coated urea. Slow-release formulas deliver nitrogen gradually over several weeks, minimizing the risk of fertilizer burn and reducing nutrient runoff.
Before applying fertilizer, a soil test is highly recommended to confirm the exact pH level. High alkalinity (pH above 7.0) can limit iron uptake, requiring the use of chelated iron alongside the nitrogen treatment to ensure full recovery.
Managing Soil Health and Moisture
Cultural practices related to water and soil structure often contribute to yellowing and must be corrected for long-term health. St. Augustine grass should be watered deeply but infrequently to encourage strong, deep root growth. Apply enough water to soak the top 6 to 8 inches of soil, then allow the top layer to dry out completely before watering again.
Overwatering is a common cause of yellowing, as it displaces oxygen in the root zone, effectively drowning the roots. Reducing the frequency of watering can significantly improve root function and color.
For lawns experiencing heavy foot traffic or planted in clay soils, compaction can restrict water infiltration and oxygen flow, causing root stress. Mechanical aeration, which involves pulling small plugs of soil, helps break up this compaction and allows air, water, and nutrients to penetrate the root zone more easily.
While chelated iron provides an immediate fix for chlorosis, addressing the underlying soil pH is a long-term solution. If a soil test confirms high alkalinity, incorporating soil amendments like elemental sulfur can slowly lower the pH over time. This adjustment improves the soil environment, making naturally occurring iron available to the grass roots.
Ruling Out Pests and Fungal Issues
If nutritional and moisture issues have been ruled out, the yellowing may be the result of biological invaders that require specific chemical treatments.
Chinch Bugs
Chinch bugs are small, sap-sucking insects that inject a toxin into the grass blades, causing irregular, spreading patches of yellow-to-brown turf that often appear scorched. These damaged areas are typically concentrated in the hottest, driest sections of the lawn, particularly near concrete or pavement. Identifying them requires parting the grass near the edge of the damaged area and looking for the tiny, black-bodied adults or their reddish nymphs at the base of the blades. Confirmation necessitates the application of a targeted insecticide to stop the infestation.
Gray Leaf Spot
Another common biological threat is the fungal disease Gray Leaf Spot, which thrives in hot, humid conditions. This fungus initially appears as small, oblong spots on the grass blades that have dark borders and distinctive gray or tan centers. Severe infections cause widespread yellowing and browning as the lesions compromise the blade’s ability to photosynthesize. Treatment requires the application of a lawn-specific fungicide, often needing multiple applications to fully suppress the pathogen.