Why Is My St. Augustine Grass Turning Yellow?

A yellowing St. Augustine lawn is a common concern that signals the grass is under stress. This discoloration can stem from a variety of causes, each with its own distinct signs and solutions. Identifying the specific reason for the yellowing is the first step toward restoring your lawn’s health and vibrant green color.

Nutrient Deficiencies Causing Yellowing

Iron deficiency, or chlorosis, is a common cause of yellowing. This condition is identifiable when grass blades turn yellow while the veins remain green. This striping effect indicates your lawn is struggling to access enough iron to produce chlorophyll, the pigment that gives grass its green color.

To address iron chlorosis, apply an iron supplement, which is available in liquid and granular forms. Liquid iron provides a faster, shorter-lived green-up, while granular products offer a slower, more sustained release. For a long-term solution, address underlying soil conditions like high pH that can prevent iron uptake.

Widespread, uniform yellowing across the entire lawn often points to a nitrogen deficiency. Unlike the striped appearance of iron chlorosis, a lack of nitrogen causes the whole grass blade to turn a pale yellow or light green. The yellowing starts with the older, lower leaves before progressing up the plant, and St. Augustine grass has a high nitrogen requirement during its peak growing season.

Correct a nitrogen deficiency by applying a fertilizer formulated for Southern lawns with a slow-release formula. This provides a steady supply of nutrients without causing a sudden surge of growth. To confirm any nutrient deficiency, a soil test is the most accurate method, as it provides a detailed report on your soil’s pH and nutrient levels.

Improper Watering and Sunlight

Overwatering is a common mistake that prevents roots from getting enough oxygen. This results in weak or rotting roots that cannot absorb nutrients, causing the grass to turn yellow. Soggy soil or a spongy feel underfoot indicates overwatering.

Underwatering also causes stress and discoloration. Dehydrated grass blades wilt and take on a bluish-gray tint before turning yellow. The “footprint test” can confirm this; if footprints remain visible long after you walk across the lawn, it needs water.

Proper watering involves deep, infrequent sessions that encourage the roots to grow deeper into the soil, making the turf more resilient. Aim to provide about one inch of water per week, applied in one or two sessions depending on your soil type.

While St. Augustine has moderate shade tolerance, it still needs several hours of direct sunlight daily to thrive. If you suspect a lack of sunlight is the issue, observe how much direct light the affected areas receive. In areas with dense shade, the grass may thin out and develop a pale, yellowish color due to reduced photosynthesis.

Trimming tree branches or thinning the canopy can allow more light to reach the turf below. If the shade is too dense, you might consider a more shade-tolerant ground cover or a St. Augustine cultivar bred for low-light conditions.

Pest Infestations and Fungal Diseases

Pests and diseases are a primary cause of yellowing, often creating distinct damage patterns. The southern chinch bug is one of the most destructive pests for this turf. These insects pierce grass blades, suck out the juices, and inject a toxin that causes the grass to turn yellow, then reddish-brown, before dying.

Chinch bug damage appears as irregular patches, often in the sunniest parts of the lawn, and can spread quickly. To check for them, part the grass at the edge of a yellowing patch and look for small, black insects with white wings. Control them by applying an insecticide labeled for chinch bugs over the entire lawn to prevent their spread.

Fungal diseases also contribute to a yellowing lawn, with Take-All Root Rot being a common problem. This disease attacks the root system, causing it to become dark and rotted, which prevents it from absorbing water and nutrients. Above ground, this results in large, irregular patches of yellow grass that eventually thin out and die.

Other common fungal issues include Brown Patch and Gray Leaf Spot. Brown Patch creates circular patches of yellow or brown grass, while Gray Leaf Spot appears as small, oblong, gray-centered spots on the grass blades. Managing these diseases involves applying a targeted fungicide and adjusting cultural practices, like reducing irrigation and improving air circulation.

Incorrect Lawn Maintenance Habits

Routine lawn care practices can impact the health and color of your grass. Mowing the grass too short, or scalping, is a common error that causes extensive yellowing. This removes too much of the blade, stressing the plant and exposing stems to sun scorch.

This stress weakens the grass, making it more susceptible to weeds, pests, and disease. To avoid scalping, maintain a mowing height between 3 and 4 inches. Never remove more than one-third of the grass blade height in a single mowing session.

Soil compaction is another maintenance-related issue that can lead to a yellow lawn. Heavy foot traffic or equipment can compress the soil, removing air pockets that roots need. Compacted soil restricts the flow of water, air, and nutrients to the root zone.

A hard feel underfoot and pooled water after rain are signs of compaction. The solution is core aeration, which uses a machine to pull small plugs of soil from the lawn. Aerating your St. Augustine lawn every couple of years can improve its vigor and color.

Be cautious with herbicide applications, as using a product not specifically labeled as safe for St. Augustine grass can cause severe chemical burns. This can result in widespread yellowing or death of the turf.