Crepe Myrtles (Lagerstroemia indica), prized for their vibrant summer blooms, are a staple in many landscapes, but the sudden appearance of red leaves outside of autumn can signal a problem. The standard green color is due to chlorophyll, the pigment responsible for photosynthesis. When a leaf turns red, the chlorophyll is breaking down, revealing other pigments like anthocyanins that were previously masked. This degradation can be triggered by natural seasonal changes or by environmental and chemical stressors that cause the tree to prematurely shut down energy production. Understanding the cause requires looking closely at the timing and pattern of the discoloration.
The Expected: Natural Color Change and Cultivar Specifics
The most reassuring reason for red foliage is that the tree is simply following its natural seasonal cycle. As daylight hours shorten and temperatures cool in late summer or fall, the tree prepares for dormancy by degrading its chlorophyll. This process makes the yellow and orange carotenoid pigments visible, alongside newly produced red and purple anthocyanin pigments, resulting in the brilliant fall color display the species is known for.
Not all red leaves are a sign of the season’s end; some varieties naturally exhibit red or bronze foliage during the growing season. Certain Crepe Myrtle cultivars, especially those bred for dark foliage or intense fall color, will flush with a reddish hue when new leaves emerge in the spring. Varieties like ‘Red Rocket’ or those with “Burgundy” in their name often display this reddish-green color, which then fades to dark green by mid-summer. If the discoloration is occurring only on new leaf tips or during the cooler autumn months, it is likely a normal, non-problematic event.
The Environmental Culprits: Stress from Water and Temperature
When red leaves appear in the heat of summer, the cause is often a physical stressor forcing the tree into a defensive state. Drought stress is a frequent culprit, as the tree attempts to conserve water by shedding foliage, triggering the same chlorophyll degradation process that occurs in autumn. Before the color change, leaves may lose their glossy appearance and look dull or slightly wilted, indicating the tree is struggling to pull moisture from the soil. For established trees, deep, infrequent watering during dry periods is the best corrective action, which encourages a robust root system capable of handling minor droughts.
Extreme temperature fluctuations can also shock the leaves into changing color. Sudden cold snaps in late spring or early fall can damage leaf tissue, leading to localized red or purple patches, particularly on exposed branches. This is often a temporary reaction, and the tree will usually recover once stable temperatures return. In addition, intense, direct sunlight, especially on leaves that were previously shaded, can sometimes trigger a protective reddish or bronzed pigment to form in the leaf surface. Providing adequate water helps the plant manage both heat and sun intensity.
The Chemical Culprits: Nutrient Deficiencies and Soil pH
If the tree is receiving adequate water and the weather is stable, the red coloration may point to an issue with soil chemistry. A common nutritional deficiency that manifests as red or purple leaves is a lack of phosphorus (P). Phosphorus is a macronutrient essential for energy transfer, root growth, and flower development, and a shortage often causes older, lower leaves to develop a reddish-purple tint, especially on the undersides and margins.
The soil’s pH level also dictates how easily the tree can absorb available nutrients. While Crepe Myrtles tolerate a wide range, excessively alkaline soil can effectively “lock up” certain micronutrients, even if they are present in the soil. This nutrient unavailability, such as iron or manganese, stresses the plant and contributes to overall leaf discoloration. To accurately diagnose a nutrient imbalance, a soil test is the most reliable tool. If a phosphorus deficiency is confirmed, applying a high-phosphorus fertilizer can help correct the issue, while adjusting soil pH with elemental sulfur can improve nutrient uptake in alkaline conditions.