Don’t bite the bark
A common problem is the careless use of mowers and string trimmers around the base of these thin-barked trees. This type of equipment can easily damage the base of the trunk leading to sickly, stunted trees. To prevent this type of damage, do not allow grass to grow within a foot of the trunk. Keep the area mulched with about 4 inches of pine straw pulled back slightly from the trunk. The reason the bark is so smooth and thin becomes apparent when you see a crape myrtle shedding its outer layer of bark. Shedding the outer bark often reveals different shades and colors from tan to rusty brown, which add to the beauty of these trees. There’s no need for concern; simply gather up the shed bark to keep things looking neat.
A sucker’s born every minute
It seems that crape myrtles are trees with aspirations to be big shrubs. Leave a tree alone and eventually most will send up lots of shoots from the base and turn into something more akin to a large shrub. To maintain a pleasing tree shape, we regularly remove these shoots – called suckers – from the base of the trunk.
Remove suckers regularly as soon as they appear. They are easier to deal with and cause less damage to the tree when they are removed early. Prune them all the way back to their point of origin at the trunk or a root. Do not leave a stub, or several suckers will appear for every one you cut off. To prevent suckers from growing back so fast, treat the freshly cut spots with a product, such as Monterey Sucker Stopper or other brands, to prevent sucker regrowth.
A few insects and diseases attack crape myrtles, but they generally don’t cause significant damage. Common problems are crape myrtle aphids, powdery mildew and Cercospora leaf spot. Crape myrtle aphids are quite common, and most trees get them during summer. Only when populations are high every year is control really necessary. The aphids feed on the sap and excrete tiny droplets of a sugary liquid called honeydew. This accumulates on the leaves and branches and provides a food base for growth of a black fungus called sooty mold. Heavy infestations of aphids can lead to so much sooty mold the tree turns almost all black. You can control aphids by spraying the tree with any commercially available insecticide labeled to control aphids. A light, paraffin-based oil, such as Summit Year Round Spray Oil or Bonide All Seasons Oil, is your least-toxic option. If the trees are too large to spray, you can control the aphids by treating the base of the tree with a systemic insecticide. It is better to apply a systemic insecticide early in the season, around April or May, but you should get some late summer benefit if you apply one now. Choose a product like Bayer Advanced Tree and Shrub Insect Control with Merit. Powdery mildew and Cercospora leaf spot are occasional problems. Powdery mildew is particularly common in early summer during warm, humid 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 occurs more in mid- to late summer, particularly when weather is rainy. Dark spots show up on foliage, which then turns yellow or orange and drops. Again, control is not generally needed. Even if a tree loses all if its leaves to this disease, it will not kill it. If you wish, you can control both diseases with applications of chlorothalonil (Daconil and other brands).
Many new crape myrtle varieties are the result of hybridizing the traditional crape myrtle, Lagerstroemia indica, with a different species, Lagerstroemia fauriei. This has provided them with increased disease resistance (especially for mildew) and also more colorful bark.
Lack of flowers is a complaint I hear occasionally. Common reasons for lack of blooms include young trees, trees getting too much shade, trees that are sickly or low in vigor, and heavy insect or disease problems. Given good care and proper growing conditions, crape myrtle flowers are as certain as summer.