The yellowing of St. Augustine grass is known as chlorosis, a visible symptom of an underlying stressor rather than a disease. This discoloration signals that the grass is struggling to produce chlorophyll, the pigment responsible for its green color. Identifying the specific cause—whether it is a nutrient imbalance, an environmental issue, or a biological threat—requires a careful inspection of the lawn’s visual patterns and environment.
The Visual Signs of Nutrient Deficiencies
Yellowing that is uniformly distributed across the lawn often points toward a nitrogen deficiency. Since nitrogen is the primary component of chlorophyll, a lack of it causes the entire blade of grass to turn a pale green or solid yellow. This deficiency typically appears first on the oldest blades, as the plant moves the limited nitrogen resource to support newer growth. Applying a high-nitrogen fertilizer usually corrects this widespread yellowing.
Interveinal chlorosis indicates a lack of iron. The newest grass blades turn yellow while the veins remain distinctly green, creating a striped appearance. Iron is often present in the soil but becomes chemically inaccessible to the plant in alkaline soils. Since iron is immobile within the plant, the deficiency shows up in the new growth first. The fastest correction involves applying a liquid chelated iron supplement as a foliar spray for quick absorption.
Water Management and Soil Environment Stress
Improper water management frequently causes St. Augustine grass yellowing, starting with under-watering or drought stress. This stress causes the grass blades to fold vertically inward along the mid-rib to conserve moisture. Affected areas first turn a dull, blue-gray color before yellowing. The grass will also fail the “footprint test,” where footprints remain visible because the blades lack the turgor pressure to spring back immediately.
Conversely, over-watering also causes yellowing because saturated soil pushes out necessary oxygen, leading to root suffocation and eventual root rot. This condition causes nutrient lockout, leaching away mobile nutrients like nitrogen, resulting in uniform yellowing across poorly draining areas. The soil in these spots often feels spongy underfoot. Soil compaction, caused by heavy traffic, exacerbates these problems by restricting root growth and reducing the air pockets roots need, which can be remedied through core aeration.
Identifying Damage from Pests and Fungal Diseases
Chinch bugs, tiny black insects with white diamond-shaped markings, damage the grass by sucking sap from the lower runners and blades. Their feeding injects a toxin that causes irregular, spreading patches of grass to turn yellow, then brown. These patches often appear first in hot, sunny areas along sidewalks or driveways. To confirm their presence, perform a float test using a bottomless coffee can submerged in water at the edge of a damaged patch; the insects will float to the surface.
Fungal diseases also cause chlorosis, but with unique identifiers. Take-All Root Rot (TARR) is a soil-borne fungus that causes the grass to thin out and turn neon-yellow in irregular patches. The defining symptom is a weakened root system, allowing the affected grass to be easily pulled up from the soil, often revealing dark, brittle, or rotted roots. Gray Leaf Spot, thriving in hot, humid conditions, causes small, oblong lesions on the grass blades. These lesions are gray or tan in the center and surrounded by a purple or brown border, leading to a general yellowing or scorched appearance.
Step-by-Step Lawn Diagnosis and Recovery
The first step in addressing yellow St. Augustine grass is to closely examine the pattern of the discoloration. Determine if the yellowing is uniform across the entire blade (nitrogen deficiency or over-watering), striped on new growth (iron deficiency), or confined to irregular, spreading patches (pests or disease). Next, check the soil moisture and drainage in the yellowed area, using a screwdriver to test for saturation or hardness, and look for signs of blade folding to rule out simple drought stress. If you suspect chinch bugs, perform the flotation test by sinking a can into the turf and filling it with water to see if the pests float up.
Implement the least invasive treatment first. For instance, correct an obvious watering error by switching to deep, infrequent irrigation, or apply a liquid iron supplement if striped yellowing is present. If symptoms persist after adjusting cultural practices, or if you confirm the presence of a specific pest or disease like Take-All Root Rot or Gray Leaf Spot, consider professional application of a targeted fungicide or insecticide.