Crepe myrtles are popular flowering trees and shrubs known for their vibrant summer blooms. While generally resilient, yellowing leaves often signal underlying issues. Understanding the reasons behind this discoloration helps maintain plant health.
Normal Leaf Yellowing
Not all yellowing leaves indicate a problem; some are natural. As autumn approaches, crepe myrtle leaves naturally transition through shades of yellow, orange, and red before falling, preparing the plant for winter dormancy. Older leaves, particularly those in the inner canopy or on lower branches, may also yellow and drop throughout the growing season. This is normal senescence, where the plant sheds less efficient leaves to prioritize new growth. Additionally, a newly planted crepe myrtle might display temporary yellowing due to transplant shock as it adjusts to its new environment.
Environmental Stress
Environmental factors frequently contribute to leaf yellowing in crepe myrtles. Both insufficient and excessive watering can lead to discoloration. Overwatering, often due to poorly draining soil, can suffocate roots and lead to root rot, causing leaves to yellow. Conversely, underwatering or drought stress causes dehydration, leading to wilting and yellowing.
Light conditions also play a role; crepe myrtles thrive in full sun, typically requiring at least six hours of direct sunlight daily. Inadequate light can result in pale, yellowing leaves as the plant cannot perform sufficient photosynthesis. Extreme temperatures, such as sudden cold snaps or prolonged heat waves, can stress the plant, leading to yellowing. Compacted soil or heavy clay, which impede drainage and oxygen flow to the roots, can also mimic overwatering symptoms, causing leaves to yellow.
Nutrient Deficiencies
Crepe myrtle leaves may turn yellow due to a lack of specific nutrients in the soil. Iron chlorosis is a common issue, particularly in alkaline soils where iron is present but unavailable to the plant. This deficiency typically presents as interveinal chlorosis, where the leaf tissue between the veins yellows while the veins themselves remain green.
A general yellowing of older leaves, often starting from the tips and progressing inward, can indicate a nitrogen deficiency. Nitrogen is crucial for overall plant growth and green foliage. Magnesium deficiency might cause yellowing of the leaf margins or between veins, sometimes accompanied by reddish or purplish tints. Over-fertilization can also lead to yellowing by creating an imbalance of nutrients or by burning the roots.
Pests Causing Yellowing
Several common pests feed on crepe myrtles and can cause their leaves to yellow. Aphids are small, soft-bodied insects that cluster on new growth. Their feeding causes leaves to yellow and sometimes distort, also producing a sticky substance called honeydew, which can attract sooty mold. Scale insects appear as small, immobile bumps on stems and leaves, resulting in yellowing foliage. Spider mites can cause fine stippling on leaves, eventually leading to overall yellowing and a bronzed appearance.
Diseases Causing Yellowing
Various diseases can also be responsible for yellowing leaves on crepe myrtles. Leaf spot fungi, such as Cercospora leaf spot, appear as dark, circular spots, often with a yellow halo. Severe infections can lead to widespread yellowing and premature leaf drop, particularly in humid conditions.
Powdery mildew, characterized by a white, powdery growth on leaf surfaces, can also cause leaves to yellow and drop prematurely. This fungal disease is more prevalent in shaded, damp areas with poor air circulation. Root rot, typically a consequence of overwatering or inadequate drainage, causes roots to decay, leading to general yellowing and wilting.
Preventing and Addressing Yellow Leaves
To minimize leaf yellowing, proper site selection is important. Plant crepe myrtles in locations that receive full sun for at least six hours daily and have well-draining soil. Consistent watering practices are key; water deeply but infrequently, allowing the top few inches of soil to dry out between waterings to prevent both overwatering and drought stress.
Regular soil testing identifies nutrient deficiencies. Apply a balanced, slow-release fertilizer in early spring, and address specific deficiencies like iron chlorosis with chelated iron supplements if needed.
Pruning for air circulation, by removing crossing or crowded branches, can reduce humidity within the canopy, thereby mitigating fungal issues such as powdery mildew and leaf spot. Regularly inspecting plants for early signs of pests or diseases enables prompt intervention. Applying a layer of organic mulch around the base of the plant regulates soil temperature, conserves moisture, and improves soil health. If symptoms persist or worsen despite these efforts, consulting with a local arborist or extension office can provide further diagnostic assistance.