I was recently asked about pruning crape myrtles. Light pruning in the dormant season is normally all that is needed. The following is a good article on the subject from Clemson University.
We often use crape myrtles in the landscape because they bloom all summer long. We also value them for their peeling bark, fall color and the grace of their natural form. It is as tough as it is beautiful.
The practice of chopping off the tops of crape myrtle has become very commonplace. Many people believe that it is required to promote flowering; some prune because the plant is too large for the space provided; others see their neighbors doing it and feel the need to follow suit. There are some instances in which heavy pruning is necessary, but light pruning is usually all that is needed. The type and amount of pruning depends on the desired shape and size of the plant.
Crape myrtle can be a low-maintenance plant, and the best way to ensure this is to choose the cultivar that best suits your landscape needs before planting. There are many new cultivars in different sizes and colors. The dwarf (3 to 6 feet) and semi-dwarf (7 to 15 feet) selections now available make it easy to choose the right size plant for a certain space.
Crape myrtles that mature between 5 and 15 feet include ‘Acoma’ (white flowers), ‘Hopi’ (light pink), ‘Comanchee’ (dark pink), ‘Zuni’ (lavender) and ‘Tonto’ (red). These are also resistant to powdery mildew, a fungus that attacks and distorts the leaves. Compact crape myrtles between 3 and 6 feet include ‘Hope’ (white), ‘Ozark Spring’ (lavender) and ‘Victor’ (red). Unfortunately, the compact crape myrtles are not resistant to powdery mildew.
If careful consideration is given to the projected size of the mature plant, a selection can be found that will not outgrow its boundaries and can be allowed to display its graceful beauty with minimal pruning. Crape myrtle does not require heavy pruning to promote bloom. Flowers are produced on new growth. It will produce flowers without any pruning, although it will produce larger flowers and bloom more profusely if at least lightly pruned. Pruning in late winter or early spring will stimulate vigorous new growth in the spring. Encourage a second bloom in summer by pruning flowers immediately after they fade.
This plant prefers hot, sunny climates and in Delaware will grow to tree-size proportions. It is important that tree types are sited where they have enough area to spread. When given an ideal location, these small tree types should be allowed to develop their natural style without whacking off their tops.
To develop a tree shape, remove all limbs growing from ground level except the three to five strongest limbs. As the tree matures, remove lower, lateral branches (“limbing up”) one-third to halfway up the height of the plant. Remove branches that are crossing or rubbing against each other and shoots growing into the center of the canopy. Make your cuts to a side branch or close to the trunk. Head back wayward and unbranched limbs. As it grows taller, remove lower branches as needed. Remove any future growth from the ground to retain the desired tree shape. This basal sprouting may occur whether the tree is being pruned or not. Pull these out when succulent instead of pruning them.
You may feel the need to improve the appearance by removing the seed heads in late winter or early spring before growth begins. This is recommended only if they are within reach. Once this becomes a tall, mature plant, allow nature to take its course – the seeds will drop, the plant will bloom, and the natural grace of the plant has been retained.
Consider all your options when confronted with a large, old crape myrtle in a space meant for a different shaped tree or shrub. To create clearance under the canopy, limb up old trees that have spread their lower limbs where they interfere with people or cars. Limb up above the roofline of a single story home to clear obstruction of a window or door. Eliminate one of the major trunks if it is leaning too close to a building. Only as a last result should you top a beautiful, old specimen to squeeze it into a confined space.
To keep a crape myrtle at a manageable height, prune moderately by removing all twiggy growth back to lower growing side branches. This will give the plant a more uniform appearance. As mentioned earlier, the best way to maintain a crape myrtle at a particular size is to plant a known cultivar that will mature at the desired height and spread.
If you have a crape myrtle in a spot where you want a low, compact plant, you have two options: (1)Dig it up and plant a new dwarf cultivar that will require little or no maintenance; (2)Prune the stems back to about six inches above the ground each year. Severe pruning will not kill or injure a healthy crape myrtle.
Practice corrective pruning to remove defective or dead branches. This should be done at the time the problem is detected. Otherwise, prune to remove lateral branches, small twigs or branches in the center to create more open space for sun and air movement while the plant is dormant (winter or early spring).
Reprinted with minor changes from "Crape Myrtle Pruning" prepared by Bob Polomski, Extension Consumer Horticulturist, and Debbie Shaughnessy, HGIC Information Specialist, Clemson University from the South Carolina Master Gardener Training Manual