Diversified
Crape Myrtles (Lagerstroemia spp.) are outstanding flowering trees that are generally fast-growing and vigorous plants in U.S. Department of Agriculture plant hardiness zones seven through nine.
There are a few characteristic problems that plague crape myrtles, such as aphids, powdery mildew and the failure to set flowers, which is the hallmark of this plant. Their natural growth is healthier than topping, which is a pruning that is unnecessary and unsightly.
Aphids
Many crape myrtles are such aphid magnets that trees are planted around commercial nurseries to lure aphids away from nursery plants. Trees attract different types of aphids, but the crape myrtle aphid (Tinocallis kahawaluokalani) feeds only on crape myrtle trees.
As aphids feed on cell sap they excrete a sticky residue called honeydew, upon which sooty mold grows, coating leaf surfaces with a grayish cast. Because the damage is cosmetic and does not injure trees, insecticides are unwarranted.
Powdery Mildew
A primary disease of crape myrtles is powdery mildew. This fungal disease is caused by the pathogen Erysiphe lagerstroemia, which leaves white growths resembling talcum powder on leaves, flowers and twigs.
The disease is spread most easily in damp, shady locations on trees that do not have good air circulation around them. Some cultivars, such as Apalachee and Fantasy are resistant to this disease.
If tree size does not prohibit you from applying a fungicide to all plant parts, select a copper-based product or one that contains thiophanate-methyl.
Failure to Flower
Crape myrtles may fail to flower for a number of reasons. They must receive six to eight hours of sun each day to produce many flowers. Trees that receive less than this may flower sparsely or not at all. Crape myrtles that receive excessive nitrogen from fertilizer will have lush green growth at the expense of not producing profuse flowers. This typically happens to crape myrtles that are planted in lawns. Turf fertilizer, which contains nitrogen to green up lawns, also promotes lush foliage on crape myrtles but hinders flower growth.
Pruning
One of the biggest problems with crape myrtle trees is improper pruning.
The only recommended pruning practice for these trees is to remove dead wood, dense branches and crossing limbs.
Topping trees, which is commonly called “crape murder,” lops off all branches and leaves disfiguring knobby ends.
This practice weakens trees and increases their susceptibility to insect and disease penetration. Severely topped crape myrtles expend most of their energy toward growing new limbs instead of flower production.