The Crepe Myrtle (Lagerstroemia species) is a highly valued ornamental tree, celebrated for its long-lasting summer blooms and attractive exfoliating bark. When this otherwise robust plant begins to display leaves turning yellow, a condition known as chlorosis, it signals that the tree is under stress and unable to produce sufficient chlorophyll. Diagnosing the precise cause of this discoloration is the first step toward restoring the plant’s vibrant green foliage and overall health. Chlorosis is a symptom, not a disease itself, and its pattern across the plant provides the necessary clues to identify the underlying environmental, chemical, or biological issue.
When Yellowing Is Normal or Environmental
Not all yellowing indicates a problem, as the Crepe Myrtle naturally sheds its leaves in preparation for dormancy. This process, called senescence, occurs in the late summer or fall, where foliage uniformly changes color before dropping. If the discoloration occurs in the cooler months, it is typically a natural, seasonal phenomenon.
Yellowing during the spring or summer often points to an issue with water management. Both overwatering and underwatering can induce chlorosis because both conditions impair the roots’ ability to function. Overwatering, particularly in poorly draining soils, suffocates the root system by displacing oxygen, leading to root rot. This damage prevents the uptake of nutrients and water, often resulting in leaves that turn yellow and drop quickly.
Conversely, underwatering or drought stress causes the plant to conserve moisture by sacrificing foliage. The leaves may wilt and feel dry or crispy before turning yellow, signaling dehydration. Crepe Myrtles require consistent moisture, especially during establishment, and extreme heat or recent transplanting can temporarily induce yellowing as the tree adjusts.
A lack of sufficient sunlight is another environmental factor causing yellowing and weak growth. Crepe Myrtles need a minimum of six to eight hours of direct sun daily to thrive. When planted in excessive shade, the tree will produce fewer flowers, and its lower, shaded leaves may turn pale and yellow as the plant prioritizes resources for the upper canopy.
Yellowing Caused by Soil Nutrient Imbalances
The most frequent nutritional cause of yellowing is iron chlorosis, occurring when the tree cannot absorb adequate iron. This condition is visually distinct, appearing as interveinal chlorosis: the area between the leaf veins turns bright yellow, while the veins themselves remain dark green. Symptoms appear most prominently on the newest growth because iron is an immobile nutrient within the plant.
The underlying chemical problem is almost always a high soil pH, typically above 7.0. Crepe Myrtles prefer a slightly acidic environment, ideally between pH 5.5 and 6.5. In alkaline soil, the iron mineral changes into an insoluble compound that the roots cannot absorb, effectively locking the nutrient away.
Other deficiencies present with different visual patterns that help distinguish them. Nitrogen deficiency causes a general, uniform yellowing across the entire leaf blade, but it begins on the older, lower leaves. This occurs because nitrogen is highly mobile, allowing the plant to move the nutrient from older foliage to support new growth.
Magnesium deficiency also presents as interveinal chlorosis, similar to iron, but it affects the older, lower leaves first. The yellowing is more gradual and can leave a distinct green “arrowhead” shape at the base of the leaf. A soil test provides the only definitive way to confirm the soil’s pH and exact mineral levels before treatment.
Identifying Pests and Diseases Causing Chlorosis
Biological factors, such as insects and fungi, cause yellowing that is typically localized or spotted, unlike the uniform chlorosis seen with nutrient issues. The Crepe Myrtle aphid is a common pest that feeds on sap, causing yellow spots to form on the leaves. As they feed, these insects excrete honeydew, which encourages the growth of black sooty mold.
The sooty mold does not directly infect the plant but coats the leaves, blocking sunlight and reducing photosynthesis, leading to stress-induced yellowing. Scale insects are another sap-sucking pest that produces honeydew and sooty mold, often found clustered on the branches and stems.
Fungal diseases also create distinctive patterns of discoloration. Cercospora leaf spot begins as small, circular yellow spots, often on the lower leaves where humidity is highest. These spots may enlarge, causing the leaves to prematurely drop in mid-to-late summer. Powdery mildew appears as a grayish-white, dusty coating on new leaves, buds, and shoots, causing the tissue to become stunted and distorted, which inhibits chlorophyll production.
Step-by-Step Guide to Restoring Leaf Color
Restoring the Crepe Myrtle’s leaf color begins with correcting the underlying cause identified by the yellowing pattern. If the issue is water stress, adjust the irrigation schedule to ensure deep, infrequent watering that maintains consistent soil moisture. Water the root zone thoroughly, allowing the soil to dry slightly between sessions, and avoid overhead watering, which can encourage fungal growth.
Addressing Nutrient Deficiencies
For nutrient-related chlorosis, immediate and long-term strategies are necessary. A quick fix is a foliar spray using a solution of ferrous sulfate. This produces a rapid greening effect, often within days, but the result is temporary, as the iron cannot move to new growth.
For a more lasting solution in high-pH soils, apply a specialized chelated iron product containing the FeEDDHA molecule to the soil. The chelating agent keeps the iron soluble so the roots can absorb it, but this treatment typically lasts only one season. The permanent solution is to lower the soil pH by applying elemental sulfur.
Elemental sulfur is converted slowly by soil bacteria into sulfuric acid, which acidifies the soil. This is a gradual process that can take one to two years. The application rate should not exceed 20 pounds per 1,000 square feet to prevent plant damage.
Managing Pests and Fungi
For biological problems, ensure the tree is planted in a location that receives full sun and has excellent air circulation, which helps manage fungal issues. Aphid and scale infestations can be treated directly with horticultural oil, neem oil, or insecticidal soap.