Garden News: Caring for crape myrtles in the summer

Crape myrtles are beginning to bloom now in south Louisiana, and they’ll continue into September. So LSU horticulturist Dan Gill has some suggestions for caring for your crape myrtles throughout the summer.

A common problem with crape myrtles is the careless use of mowers and string trimmers around the base of these thin-barked trees, Gill says. This type of equipment can easily damage the base of the trunk, leading to sickly, stunted trees.

To prevent this type of damage, don’t allow grass to grow within a foot of the trunk. To deter weeds, keep the area mulched with about 4 inches of pine straw pulled back slightly from the trunk.

Regularly remove shoots, called suckers, that grow up from the base of the trunk. Prune them all the way back to their point of origin at the trunk or root. Make a flush cut, and don’t leave a stub or several suckers will appear for every one you cut off.

A few insects and diseases attack crape myrtles, but they generally don’t cause significant damage.

Crape myrtle aphids are common, but control is really necessary only when populations are high. The aphids excrete a sugary liquid called honeydew, which accumulates on the leaves and branches, and a black fungus called sooty mold grows on it.

You can control aphids by spraying the tree with any commercially available insecticide labeled to control aphids, such as permethrin, or with a light, paraffinic oil, such as Year Round Spray Oil and other brands. The oils are your least-toxic option.

If you don’t want to spray, you can control the aphids by drenching the base of the tree with the systemic insecticide imidacloprid.

Powdery mildew is a common disease in early summer during hot, dry weather. It appears as a white, powdery coating or spots on leaves and flower buds. Treatment is rarely needed, although heavy infections of the flower buds can cause them to abort.

Cercospora leaf spot is a fungus disease that occurs more in mid- to late summer, particularly when weather is rainy. Dark spots show up on the leaves, which then turn yellow or orange and drop. Even though trees may lose a large portion of their leaves, the disease is not life-threatening and does not significantly affect the overall, long-term health of the trees.

Got a gardening question? Write to GardenNews@agcenter.lsu.edu.