Hi Sue, not sure where you're located but anytime in the fall is a good time. A few of ours are still in bloom, we've always had great success in cutting them back after blooms have faded. But before it frosts! Best of luck!
I found a couple of articles that I thought were interesting:
Late winter (right now) is the best time to prune a crepe myrtle, because it's leafless and you can easily see all of the branches. It also blooms on new growth, so pruning now won't reduce blooming. In fact, it may increase it.
Crape myrtles (Lagerstroemia spp.) signal the beginning of summer with a profusion of colorful flower clusters. Some cultivars continue to bloom for as long as 120 days, making this shrub a popular landscape plant. They are hardy in U.S. Department of Agriculture plant hardiness zones 7 to 9, and are available in a variety of cultivar heights from 18-inch bushes to 40-foot trees. Crape myrtles rarely need pruning, other than occasional maintenance cutting or the removal of unhealthy branches. Because these shrubs flower on new growth, it is important to prune at the correct time to avoid ruining flower development.
Winter
Crape myrtles are dormant through the winter months of December, January and February. Although the leaves begin falling from the shrub in late fall, it may take a month or more before all the leaves have fallen. The best month to prune crape myrtles is February, when the shrub is fully dormant and the branches are leafless and exposed. The naked branches give you a view of the structure and frame of the shrub. This is the month to prune out the interior of the plant, if needed, to improve air circulation. Check the shrub for any dead, diseased or broken branches and remove them. If you are training your crape myrtle to a tree shape, remove any suckers at the base of the plant and prune off lateral branches on the lower trunk. Cuts made during February often develop new growth in spring and flowers in summer.
Spring
Dormancy continues into March and sometimes April. By May, crape myrtles are showing new growth and beginning to set buds for flowers. Limit aesthetic pruning to March or early April, cutting only those branches that do not yet show new growth. Cutting new growth at this time decreases the summer’s flower production. If you didn’t remove them in February, go ahead and prune any broken, dead or diseased branches. This type of maintenance pruning should be performed throughout the year to prevent the spread of plant diseases. Continue removing any suckers or lateral branches on the trunk, if you are pruning for a tree shape.
Summer
June signals the start of crape myrtle’s blooming season, although it may begin a few weeks before or after the start of summer. The only pruning you should do at this time is to remove branches for the health of the shrub. If you are pruning your crape myrtle to a tree shape, remove any suckers or lateral branches, as well. As the summer passes through July and August, you may want to trim off dead flowers. Doing this sometimes results in a second bloom period, but the upcoming winter may damage the tender new growth.
Fall
As the weather cools, growth stops and the leaves begin to fall from the trees. Pruning crape myrtles in the fall may make the plant susceptible to freeze damage. Pruning stimulates new growth that may not survive cold temperatures. As in the rest of the year, it is fine to remove branches for the health of the tree as well as suckers and lateral branches, if you want your crape myrtle to develop a tree shape. Otherwise, allow your crape myrtle to rest during September, October and November. Any shape pruning or pruning to open up the interior of the tree should wait until late winter in February.
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immediately after flowering
This guy seems to have the answer.
https://www.youtube.com/watch?v=8JFDE9A184U
Hi Sue, not sure where you're located but anytime in the fall is a good time. A few of ours are still in bloom, we've always had great success in cutting them back after blooms have faded. But before it frosts! Best of luck!
I found a couple of articles that I thought were interesting:
Late winter (right now) is the best time to prune a crepe myrtle, because it's leafless and you can easily see all of the branches. It also blooms on new growth, so pruning now won't reduce blooming. In fact, it may increase it.
Crepe Myrtle Pruning Step-by-Step - Southern Living
www.southernliving.com/garden/grumpy-gardener/what-concerns-p
The following website explains each season pertaining to crepe myrtles:
http://homeguides.sfgate.com/month-prune-crepe-myrtle-59407.html
Crape myrtles (Lagerstroemia spp.) signal the beginning of summer with a profusion of colorful flower clusters. Some cultivars continue to bloom for as long as 120 days, making this shrub a popular landscape plant. They are hardy in U.S. Department of Agriculture plant hardiness zones 7 to 9, and are available in a variety of cultivar heights from 18-inch bushes to 40-foot trees. Crape myrtles rarely need pruning, other than occasional maintenance cutting or the removal of unhealthy branches. Because these shrubs flower on new growth, it is important to prune at the correct time to avoid ruining flower development.
Winter
Crape myrtles are dormant through the winter months of December, January and February. Although the leaves begin falling from the shrub in late fall, it may take a month or more before all the leaves have fallen. The best month to prune crape myrtles is February, when the shrub is fully dormant and the branches are leafless and exposed. The naked branches give you a view of the structure and frame of the shrub. This is the month to prune out the interior of the plant, if needed, to improve air circulation. Check the shrub for any dead, diseased or broken branches and remove them. If you are training your crape myrtle to a tree shape, remove any suckers at the base of the plant and prune off lateral branches on the lower trunk. Cuts made during February often develop new growth in spring and flowers in summer.
Spring
Dormancy continues into March and sometimes April. By May, crape myrtles are showing new growth and beginning to set buds for flowers. Limit aesthetic pruning to March or early April, cutting only those branches that do not yet show new growth. Cutting new growth at this time decreases the summer’s flower production. If you didn’t remove them in February, go ahead and prune any broken, dead or diseased branches. This type of maintenance pruning should be performed throughout the year to prevent the spread of plant diseases. Continue removing any suckers or lateral branches on the trunk, if you are pruning for a tree shape.
Summer
June signals the start of crape myrtle’s blooming season, although it may begin a few weeks before or after the start of summer. The only pruning you should do at this time is to remove branches for the health of the shrub. If you are pruning your crape myrtle to a tree shape, remove any suckers or lateral branches, as well. As the summer passes through July and August, you may want to trim off dead flowers. Doing this sometimes results in a second bloom period, but the upcoming winter may damage the tender new growth.
Fall
As the weather cools, growth stops and the leaves begin to fall from the trees. Pruning crape myrtles in the fall may make the plant susceptible to freeze damage. Pruning stimulates new growth that may not survive cold temperatures. As in the rest of the year, it is fine to remove branches for the health of the tree as well as suckers and lateral branches, if you want your crape myrtle to develop a tree shape. Otherwise, allow your crape myrtle to rest during September, October and November. Any shape pruning or pruning to open up the interior of the tree should wait until late winter in February.