A red or reddish-purple lawn suggests the turf is under stress. This discoloration signals stress caused by either a living organism, such as a fungus, or non-living factors like weather or nutrient imbalances. Although the appearance can diminish the lawn’s aesthetic appeal, the underlying causes are usually manageable. Understanding the difference between a fungal infection and environmental stress is the first step toward restoring a vibrant green color.
The Primary Suspect: Fungal Disease
The most common cause of true red or pink grass is Red Thread, a fungal infection caused by the pathogen Laetisaria fuciformis. This disease typically affects cool-season grasses like perennial ryegrass and fescues, appearing when conditions are cool and wet, generally between 60° to 75°F.
The fungal growth is visible as distinctive pink or red thread-like structures, called stromata, protruding from the tips of the grass blades. These threads can be up to an inch long and may look like small needles or pink, gelatinous crusts. The disease often presents as irregular patches of tan or pink turf, usually between 2 and 8 inches in diameter. Red Thread is primarily a foliar disease that rarely attacks the crown or roots, meaning the damage is mostly cosmetic.
Management of Red Thread often involves cultural controls, since the fungus thrives on under-fertilized turf. Applying a nitrogen-rich fertilizer helps the grass outgrow the disease by stimulating new, healthy growth. Improving air circulation and adjusting watering practices, such as watering only in the early morning, reduces the fungus’s ability to spread. Fungicide application is rarely necessary for residential lawns, as the turf typically recovers once growing conditions improve.
Deficiency and Stress Related Causes
If the grass has a purplish-red hue rather than a bright pink or red thread-like appearance, the cause is often a physiological response to stress or a nutrient deficiency. This discoloration is due to the production of anthocyanin, a reddish-purple pigment that plants create as a protective mechanism. The change in color is a natural reaction, similar to what happens in the fall leaves of deciduous trees.
One of the most frequent causes of purpling is cold stress, especially in early spring or late fall when soil temperatures drop below 50°F. When the soil is cold, the grass struggles to take up phosphorus, which is necessary for energy transfer and root development. The combination of cold soil and bright sunlight can induce the plant to produce anthocyanin to protect its photosynthetic system. This is a temporary condition, and the grass will usually return to its green color once the soil warms up.
A true phosphorus deficiency can also cause older grass blades to turn reddish-purple, particularly at the tips, often accompanied by stunted growth. This is more common in newly seeded lawns or those on sandy soils, but a soil test is necessary to confirm a lack of the nutrient. Drought stress can also trigger a purplish or reddish-brown tint in the grass blades as the plant attempts to conserve resources.
Diagnosis and Immediate Steps
Determining the exact cause of the red grass requires a close visual inspection to distinguish between a fungal issue and a stress response. If you can see distinct pink or red threads on the individual grass blades, the problem is almost certainly Red Thread. If the grass is uniformly purplish or reddish-brown without any visible threads, the cause is likely cold stress or a nutrient problem.
For definitive answers on nutrient imbalances, submitting a soil sample to a local extension office for testing is the best course of action. This test will confirm if your lawn is deficient in phosphorus or any other nutrient and provide a precise recommendation for a balanced fertilizer.
While waiting for results, you can take low-risk, immediate actions to improve the lawn’s general health. Ensure you are mowing at the proper height, as cutting the grass too short stresses the plant and makes it more susceptible to disease. Adjust your watering schedule to water deeply and infrequently, preferably in the morning, to minimize the time the grass blades are wet. Avoid immediately applying heavy fertilizer, especially if you suspect a nutrient deficiency, until you have confirmed the specific cause.