St. Augustine grass, a warm-season turf prized for its dense, blue-green color, is a common choice for lawns in the southern United States. When it turns light green or yellow, it is exhibiting chlorosis. This discoloration signals that the plant is under stress and struggling to produce chlorophyll, the pigment responsible for its color. Chlorosis can be caused by nutritional imbalances, environmental stressors, or destructive pests. Diagnosing the exact cause requires careful observation of the discoloration pattern and the overall health of the turf.
Primary Nutritional Causes of Chlorosis
The most immediate cause of a pale lawn is often a deficiency in one of two major nutrients responsible for green coloration. Nitrogen is the nutrient required in the largest quantity by St. Augustine grass, acting as the primary building block for proteins and chlorophyll molecules.
A nitrogen deficiency typically results in a uniform, general yellowing across the entire lawn, with the oldest leaves showing the paleness first. Since nitrogen is a mobile nutrient, the grass reallocates the limited supply from older tissues to support newer growth. Applying a quick-release nitrogen fertilizer usually produces a rapid, noticeable green-up within a few days.
Iron deficiency, commonly called iron chlorosis, presents a visually distinct pattern. Iron is immobile in the plant, meaning it cannot be easily moved from old leaves to new ones. The symptom appears on the newest growth first, characterized by “interveinal chlorosis.” The tissue between the leaf veins turns light green or yellow, while the veins themselves remain dark green. This streaking pattern is a hallmark of iron deficiency, which is crucial for chlorophyll synthesis.
Management Mistakes Related to Water and Soil
Poor management practices often prevent the plant from accessing nutrients already present in the soil. Improper irrigation is a frequent culprit, as St. Augustine grass is susceptible to both overwatering and underwatering. Overwatering leads to saturated, oxygen-deprived soil, which suffocates the roots, encourages root rot, and inhibits nutrient absorption.
Under-watering causes the grass to enter a drought-stressed state, slowing growth and sometimes resulting in a faded or light green appearance. The proper technique is deep, infrequent watering. This encourages roots to grow deeper, making the plant more resilient. Water deeply (about three-quarters of an inch) and allow the soil to dry slightly before the next application.
Soil conditions also significantly affect nutrient availability, especially iron chlorosis. St. Augustine grass thrives in slightly acidic soil (pH 6.0 to 7.0). Alkaline soil (pH above 7.0) chemically binds with iron, making it insoluble and unavailable for root absorption. This high pH environment causes severe iron chlorosis even if the soil contains high total iron.
Soil compaction physically restricts root growth and reduces the pore space necessary for air and water movement. Dense soil prevents roots from effectively exploring the soil profile to take up water and nutrients, leading to a stressed, chlorotic appearance. Aerating the lawn breaks up this compacted layer, improving gas exchange and encouraging deeper, healthier root development.
Identifying Insect Pests and Common Diseases
The light green color can also be a symptom of damage caused by biological threats. The southern chinch bug is the most destructive insect pest of St. Augustine grass, particularly in hot, sunny areas. These insects feed by inserting a piercing-sucking mouthpart into the grass stem and draining the plant’s sap.
Chinch bugs inject a toxic substance that blocks the plant’s vascular system, preventing the movement of water and nutrients. Damage appears as irregular, spreading patches of light green or yellow turf that eventually turn straw-colored and die, even with watering. The active feeding zone is found at the border between the dead grass and the healthy green turf.
Fungal diseases also stress the turf, leading to discoloration. Gray Leaf Spot, caused by the fungus Pyricularia grisea, is prevalent during warm, humid conditions, especially when the grass is over-fertilized with nitrogen. It manifests as small, oblong lesions on the leaf blades with tan or gray centers and dark borders. Numerous spots cause the entire lawn to take on a faded, light green appearance.
Another serious fungal issue is Take-All Root Rot (TARR), which attacks the roots and stolons, hindering the grass’s capacity to absorb water and nutrients. TARR often appears in the spring and fall as irregular, large patches of pale green or yellow turf. Since the disease targets the root system, affected grass blades can often be easily pulled out of the soil.
A Step-by-Step Recovery Plan
The initial step in any recovery plan must be a professional soil test to gain precise data on the lawn’s environment. This test confirms nutrient levels, especially nitrogen and iron, and provides an accurate measure of the soil’s pH. Relying on visual symptoms alone can lead to misdiagnosis; applying nitrogen to an iron-deficient lawn, for instance, can worsen chlorosis.
Once the underlying issue is confirmed, correcting management practices is paramount. Targeted treatments depend on the specific diagnosis:
- If overwatering is suspected, adjust the irrigation schedule to water deeply but less frequently, allowing the top few inches of soil to dry out between applications.
- If the soil is compacted, perform core aeration during the grass’s peak growing season to improve root access to air and nutrients.
- If the soil test confirms nitrogen deficiency, apply a balanced, slow-release nitrogen fertilizer to provide a steady supply without causing excessive growth.
- For confirmed iron chlorosis, especially in high pH soil, a foliar application of liquid iron offers the fastest green-up, as the nutrient is absorbed directly through the leaf tissue, bypassing soil lock-up.
- If chinch bugs are identified, apply a labeled insecticide, focusing on the border between damaged and healthy turf.
- Manage fungal diseases like Gray Leaf Spot and Take-All Root Rot with a broad-spectrum fungicide, and reduce nitrogen fertilization during warm, humid periods.