St. Augustine grass is a popular warm-season turfgrass, valued for its lush appearance. Yellowing often signals underlying problems. Identifying the cause is key to restoring its vibrant green color and maintaining a healthy lawn.
Understanding St. Augustine Grass Yellowing
St. Augustine grass can yellow due to environmental conditions, nutrient availability, or biological factors. Identifying the cause requires careful observation of symptoms.
Nutrient Deficiencies
Yellowing often stems from nutrient deficiencies. Iron deficiency (chlorosis) causes new grass blades to yellow with green veins, common in alkaline soils where iron is less available. Nitrogen deficiency results in uniform pale yellowing, affecting older leaves first.
Watering Issues
Improper watering, both over and under, can yellow St. Augustine grass. Overwatering creates waterlogged soil, depriving roots of oxygen and hindering nutrient absorption. Underwatering stresses the grass, causing dehydration and color loss. Under-watered grass may show a bluish tint or wilting.
Pest Infestations
Pests can yellow St. Augustine grass by damaging blades or roots. Chinch bugs suck sap, causing irregular yellow patches. Grubs, beetle larvae, feed on roots, leading to wilting and yellowing. Affected areas may feel spongy or pull up easily.
Fungal Diseases
Fungal diseases are a common cause of yellowing, favored by specific weather conditions. Brown Patch appears as circular yellow or brown patches, thriving in warm, wet environments. Gray Leaf Spot manifests as small, diamond-shaped lesions that can enlarge and cause widespread yellowing in hot, humid weather.
Environmental Stressors
Environmental factors can yellow St. Augustine grass. Soil compaction restricts root growth and nutrient absorption. Extreme temperatures, like heat or cold stress, can also cause discoloration. Incorrect soil pH levels limit nutrient availability, as St. Augustine grass thrives best in slightly acidic to neutral soil (6.0-7.5).
Chemical Exposure
Chemical exposure can also result in yellowed turf. Herbicide damage, especially from broadleaf weed killers not safe for St. Augustine grass, can cause widespread yellowing. Over-application of fertilizers can lead to chemical burn, where high salt content draws moisture out of the grass, resulting in yellowing or browning.
Diagnosing the Specific Cause
Identifying yellowing’s cause involves a systematic approach. Observe discoloration patterns. Uniform yellowing indicates nutrient deficiency or a watering problem. Patches of yellow point to pests or diseases.
Inspect grass blades for specific symptoms. For nutrient deficiencies, look for yellowing between leaf veins (iron) or overall pale yellowing (nitrogen). Examine blades for physical damage, like chewed edges or spots, indicating pest activity or fungal lesions. Gray Leaf Spot produces small, diamond-shaped lesions.
Checking the soil provides clues. Dig a section to assess moisture; wet or dry soil suggests watering issues. Compacted soil restricts root growth and nutrient uptake. Consider recent lawn activities like fertilization or herbicide application. A soil test accurately determines nutrient levels and pH, providing insights into imbalances.
Effective Solutions for Yellowed Grass
Addressing yellowed St. Augustine grass involves targeted solutions. For nutrient deficiencies, apply specific fertilizers. Iron deficiency is corrected with a chelated iron product, often effective within a week. For nitrogen deficiencies, a slow-release nitrogen fertilizer restores green color. Soil tests guide applications.
Correcting watering issues is fundamental. Deep, infrequent watering encourages deeper root growth and prevents stress. Aim for 1 to 1.5 inches of water per week, applied in one or two sessions, preferably in the early morning. Improving drainage in waterlogged areas can prevent root suffocation.
Managing pests requires specific treatments. For chinch bugs, apply labeled insecticides. Grub infestations are addressed with grub killers. Fungal diseases like Brown Patch and Gray Leaf Spot often require fungicides; select an effective product and rotate to prevent resistance. Improving air circulation and reducing thatch also help.
For environmental stressors, amending the soil is beneficial. Aeration relieves soil compaction, improving air, water, and nutrient penetration. Adjusting soil pH (optimal 6.0-7.5) enhances nutrient availability; elemental sulfur lowers pH, lime raises it. Chemical exposure damage may require flushing with water or natural recovery, while preventing future over-application.
Preventing St. Augustine Grass Yellowing
Preventing St. Augustine grass yellowing involves consistent lawn care. Establish a routine fertilization schedule, typically two to three times during its active growing season (spring through summer). A soil test guides balanced fertilizer selection, ensuring necessary nutrients without excess.
Maintaining proper mowing height is essential. St. Augustine grass should be mowed between 2.5 and 4 inches, with higher settings preferred for root development and stress tolerance. Mowing too short stresses the grass, making it susceptible to yellowing. Consistent watering (1 to 1.5 inches per week, deeply and infrequently) promotes strong root systems and helps withstand dry periods.
Regular aeration prevents soil compaction, allowing roots better access to water, nutrients, and oxygen. Dethatching (thatch over 0.5 inches) removes accumulated organic material that hinders water/nutrient penetration and harbors pests or diseases.