Crepe myrtle trees get their name from the crinkles in the petals of their flowers, which observers believed resembled crepe paper. Gardeners value certain species, such as the common crepe myrtle, as ornamental plants because of their showy blooms and thick green foliage. A tree with many positive qualities, the crepe myrtle also has a few weaknesses.
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Crepe myrtles don't always respond well to heavy pruning and gardeners must take extra care when trimming these trees. Cutting back all of the highest branches, or topping, is a particularly bad practice. The regrowth that results will come in scraggly and weak and might not bear flowers. Topping also harms the natural shape of tree. Rather than giving crepe myrtle a heavy annual pruning, merely cut back weak or damaged branches selectively. Avoid trimming branches larger than 1 inch wide, if possible.
Crepe myrtles flourish in the sun and heat. They don't generally like colder weather and rarely thrive any farther north than U.S. Department of Agriculture Hardiness Zone 7. A crepe myrtle planted in a climate colder than it prefers might not bloom. Young crepe myrtles typically react more negatively to cold than established trees. Several cultivars exhibit more cold tolerance than others. Check with your county extension for more information on the cold tolerance of various types of crepe myrtles.
Crepe myrtle has proved especially susceptible to a disease called powdery mildew. This fungus often attacks crepe myrtle in dry, hot summers. High humidity, low rainfall and poor air circulation increase a tree's vulnerability to this disease, according to the U.S. National Arboretum. The best method of prevention is to select varieties with resistance to powdery mildew. Treat infected trees with fungicide or horticultural oil and follow the label directions.
If you spot yellow-green insects causing damage to crepe myrtle foliage, then a pest called the crepe myrtle aphid is attacking your tree. Under normal circumstances, beneficial insects such as the lady bug keep this pest under control, but spraying pesticides on your plant might reduce the population of beneficial insects and leaves the aphids without any natural predators. You can control the aphid by reducing the use of pesticides, which increases the predator insects, washing the leaves with a spray of water or treating the plant with a horticultural soap,