St. Augustine grass is a popular choice for warm-climate lawns, known for its dense, blue-green texture and broad leaf blades. When this turf turns yellow—a condition known as chlorosis—it signals an underlying problem. Yellowing is a common symptom resulting from nutritional shortages, environmental stress, or biological threats. Diagnosing the specific cause is the first step toward restoring the lawn’s health, with primary culprits categorized as deficiencies, cultural practices, and biological factors.
Nutritional Deficiencies Causing Chlorosis
Two distinct nutrient shortages are the most frequent causes of yellowing in St. Augustine lawns, and their symptoms help differentiate them. Iron deficiency, often called iron chlorosis, occurs even when iron is present in the soil. Highly alkaline soil (pH above 7.0) binds the iron, making it chemically unavailable for the roots to absorb. This deficiency appears as a yellowing of the newest growth, particularly the young blades, while the veins remain distinctly green; this is known as interveinal chlorosis.
Nitrogen deficiency presents differently, affecting the entire lawn more uniformly and starting with the oldest blades first. Nitrogen is a mobile nutrient, meaning the grass plant will move it from older leaves to support new growth, causing the older, lower leaves to turn a pale yellow. This deficiency usually results from insufficient fertilization or nutrient leaching, especially in sandy soils following heavy rain or excessive watering. For immediate relief from iron chlorosis, apply a foliar spray of chelated liquid iron, which the grass absorbs directly through the leaf blades for a quick green-up. To address a nitrogen shortage, apply a slow-release nitrogen fertilizer to provide a steady supply of this nutrient over several weeks, encouraging a return to a deep green color and vigorous growth.
Water and Cultural Stressors
Improper watering practices are a leading cause of yellowing that mimics nutrient deficiencies. Overwatering saturates the soil, reducing oxygen available to the roots, which leads to root suffocation and the inability to absorb nutrients. This issue is often compounded by poor drainage or compacted soil, where water lingers. Conversely, drought stress from underwatering causes the grass blades to curl inward to conserve moisture, resulting in a dull, grayish-yellow appearance before the turf turns brown.
Mowing height is another cultural factor that directly affects the turf’s health and color. St. Augustine grass requires a high cut, typically between three and four inches, and mowing it too short, or “scalping,” removes too much of the leaf surface. This severe cutting stresses the plant and compromises its ability to photosynthesize, leaving the exposed, pale lower stems visible and making the lawn look yellowed and stressed. Addressing cultural stressors involves switching to deep, infrequent watering, aiming for one to one-and-a-half inches of water per week in the early morning. For compacted soil, core aeration creates small holes that allow oxygen, water, and nutrients to penetrate the root zone, while raising the mower deck will allow the grass to maintain the necessary leaf surface area.
Pests and Fungal Diseases
Biological agents like insects and fungi can cause yellowing that often appears in distinct, spreading patterns. The southern chinch bug is a destructive pest that feeds by inserting its piercing-sucking mouthpart into the base of the grass blade to extract sap. Their feeding introduces a toxic saliva that initially causes the grass to turn yellow, then red-brown, and eventually die. Damage typically appears as irregular, spreading patches in the hottest, driest areas of the lawn, such as along sidewalks and driveways.
Fungal diseases also cause widespread chlorosis, particularly when environmental conditions are favorable. Take-All Root Rot (TARR) is a soil-borne fungus that attacks the roots and stolons, causing them to become dark and brittle. The disease manifests as irregular patches of yellowed or bronze grass that are easily pulled up due to the damaged root system. Another common fungal issue is Gray Leaf Spot, which thrives in warm, highly humid conditions and high nitrogen levels, recognized by small, diamond-shaped lesions on the leaf blades. Treatment for chinch bugs requires a targeted insecticide application, while fungal diseases are managed with cultural practices, such as reducing nitrogen and improving drainage, and the application of a labeled turf fungicide.