Late winter (February-March) is the best time to prune crape myrtles in eastern North Carolina. Like all trees, the correct way to prune a crape myrtle involves enhancing its natural form rather than trying to force it to grow in a space that is too small or an artificial shape. Crape myrtles naturally grow as small upright or vase shaped trees with multiple trunks. In a well pruned crape myrtle, the trunks grow upward and outward, with branches fanning out rather than growing inward into the center of the tree.
Remove crossing and inward growing branches.
To determine if your crape myrtle needs to be pruned, examine the direction in which the trunks and branches grow. Starting at ground level, follow the trunks upward to where they begin to branch, focusing on the interior of the tree rather than the outer edges. Prune branches that grow into the center of the tree. Branches crossing over other branches or trunks, should be removed.
To prune a branch, follow it back to where it joins a larger branch or trunk. Take a close look at the point where the branch joins the trunk. You will notice at the point where the two join the branch is swollen or enlarged. This area is known as the branch collar. Using a pruning saw, remove the branch by cutting just above the branch collar rather than flush with the trunk. If the branch was removed at the correct place the branch collar left behind will extend out a centimeter or two from the trunk.
This branch has been removed, preserving the branch collar.
If it has been years since the tree was last pruned you may have to remove several branches. Whole trunks can be removed by cutting them off as close to ground level as possible, but this results in profuse sprouting of suckers during the summer and should be done only as a last resort.
If you have the time and the tree is not too large, you can clean up the interior of the tree by cutting off small twiggy branches that grow from the main trunks. This is best done using hand pruners. You can also remove seed pods and trim off the ends of branches that are less than pencil sized in diameter. While these practices will help the tree look neater and may increase the size of flower clusters, they are not necessary to keep the tree healthy.
See before and after pictures of a crape myrtle I pruned this past weekend:http://pender.ces.ncsu.edu/pruning-crape-myrtle/
Reducing Tree Size
In a properly pruned crape myrtle, branches grow up and out from the center of the tree.
Crape myrtle varieties like ‘Natchez’, ‘Muskogee’ and ‘Tuskegee’ easily reach twenty five feet tall or higher with a similar spread. If you have planted a large growing variety in a space that is too small to allow it to mature your best option is to move the tree to a different spot and replace it with a more compact selection such as ‘Tonto’, ‘Sioux’ or ‘Catawba’, which grow to fifteen feet or less. For help selecting the right variety of crape myrtle for you site, visit this Clemson Extension website.
If moving the tree is not an option, you can reduce tree height and width by cutting back the tallest and most wide spreading branches at the point where they join another branch. Known as reduction pruning, this method preserves the natural shape of the tree and is infinitely preferable to topping. Learn more about reduction pruning from this Florida Extension website.
Keeping a large growing variety confined to a small space will require annual pruning. Minimizing irrigation and fertilizer applications can also help keep these trees in bounds.
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Charlotte Glen, Extension Agent, Agriculture – Horticulture Chatham County, NC