Crepe myrtles (Lagerstroemia indica) are popular ornamental trees known for their vibrant summer blooms and resilience. Seeing the leaves turn yellow, a condition called chlorosis, signals that the tree is under some form of distress. Chlorosis occurs when the leaves stop producing chlorophyll, the pigment responsible for the green color, and requires careful diagnosis to identify the underlying problem. While yellowing can sometimes be a normal, temporary occurrence, it often points to an environmental factor, a nutrient issue, or a pest or disease problem that needs attention.
Environmental Stressors
The most frequent cause of crepe myrtle yellowing is improper water management, as the tree’s roots can be compromised by too much or too little moisture. Overwatering or planting in poorly draining soil suffocates the roots by displacing oxygen, leading to a condition similar to drought stress. Root damage prevents the uptake of water and nutrients, causing the leaves to yellow and potentially drop prematurely. To diagnose this, insert a finger a couple of inches into the soil; if it feels saturated, you are likely overwatering or the drainage is inadequate.
Conversely, periods of extreme heat or drought stress from underwatering can also cause leaves to yellow as a survival mechanism. The tree will shed leaves to conserve moisture, often starting with the older, inner leaves. During hot summer months, deep watering once or twice a week is recommended, allowing the water to soak the root zone thoroughly. Applying a layer of organic mulch around the base of the tree can help retain soil moisture, but keep the mulch away from the trunk itself.
Nutrient Deficiencies
Beyond water management, yellowing often points to an imbalance in the tree’s nutritional intake, with iron chlorosis being a common culprit for crepe myrtles. This condition is characterized by a distinct pattern where the tissue between the leaf veins turns yellow, but the veins themselves remain green. This pattern is a sign that the tree is struggling to produce chlorophyll.
Crepe myrtles prefer slightly acidic soil, ideally with a pH between 6.0 and 6.5, which allows them to efficiently absorb micronutrients like iron. In alkaline soils (high pH), the iron and manganese become chemically “locked up” and unavailable for the plant to absorb. The best long-term solution is to conduct a soil test to confirm the pH level and determine any specific nutrient shortages.
Addressing Iron Chlorosis
A quick fix involves applying chelated iron, which is an iron molecule bonded to an organic compound that keeps it available for root uptake even in high-pH soil. For faster, temporary relief, a foliar spray of iron sulfate can be applied directly to the leaves, offering a rapid green-up. To address the underlying soil chemistry, elemental sulfur can be incorporated into the soil, which works gradually over time to lower the pH and improve the availability of iron and other micronutrients.
Pests and Fungal Issues
Living organisms like insects and fungi can also induce chlorosis and premature leaf drop. The most common pest affecting crepe myrtles is the aphid, a small, sap-sucking insect that feeds primarily on the underside of leaves. As aphids consume plant sap, they excrete a sticky, sugary substance called honeydew, which coats the leaves below.
Honeydew provides a perfect growth medium for a black fungus known as sooty mold. While sooty mold feeds on the honeydew, a heavy coating can block sunlight, reducing the leaf’s ability to photosynthesize. This leads to yellowing and early defoliation. Controlling the aphids is the primary solution, often achieved by washing them off with a strong jet of water or applying horticultural oil or insecticidal soap.
A common fungal problem is Cercospora Leaf Spot, which begins as reddish or yellow spots on the leaves, typically starting with the older leaves in mid to late summer. This fungus thrives in warm, humid conditions and is often exacerbated by poor air circulation or excessive moisture on the foliage. Eventually, the entire leaf may turn yellow before dropping off, sometimes leading to significant defoliation. Pruning the crepe myrtle to improve air flow through the canopy can help prevent the disease, and in severe cases, the application of a fungicide may be necessary.
When Yellowing Is Normal
Not all instances of yellowing leaves require intervention, as the tree has normal physiological processes that involve leaf color change. The most common benign cause is natural senescence, or aging, which occurs when the tree prepares for dormancy in the late summer and fall. During this time, the leaves turn shades of yellow, orange, or red before being shed completely. This natural drop typically affects the older leaves deeper within the canopy first.
Another temporary cause is adjustment yellowing, often seen shortly after a tree has been transplanted. This temporary chlorosis is a form of transplant shock, where the tree prioritizes root establishment over maintaining all its foliage. When the yellowing is confined to older, inner leaves and the new growth at the branch tips remains green, it is usually a sign of these natural processes, and no corrective action is necessary.