Prune crape myrtles carefully

Daniel Gill, Bogren, Richard C.  |  3/2/2009 10:38:26 PM

For Release On Or After 03/06/09

By Dan Gill
LSU AgCenter Horticulturist

Late January through early March is an appropriate time to prune most summer-flowering trees and shrubs. Especially in the case of trees, pruning generally should be done to enhance their natural shape while correcting any problems.

When it comes to pruning, one of the most abused trees in Louisiana’s residential and commercial landscapes is the crape myrtle. Crape myrtles need occasional pruning to obtain the desired landscape effect, but many times these plants are butchered for no good reason.

An unfortunate trend in crape myrtle pruning is to lop off the tops, which results in a tree reduced to large branches ending in stubs. The lush growth that occurs at these cut sites appears vigorous but is actually structurally weak and more susceptible to fungus diseases such as powdery mildew. And when pruning is conducted improperly over several seasons, unsightly large, swollen knobs form at the point where pruning is done each year.

Horticulturists, arborists and landscape architects across the southeastern United States are appalled that this method of pruning continues to increase. Why should we do this to crape myrtles? Don’t tell me it looks attractive. It looks amputated. And once it’s done, it ruins the tree’s graceful natural shape for the rest of its life. There’s not another small flowering tree in Louisiana landscapes that is treated this way. For the overwhelming majority of us, pruning to enhance the natural shape of our crape myrtles is most appropriate.

I think the hardest thing to understand is that many times I see crape myrtle trees that were pruned this way by professional crews. No wonder home gardeners see this technique done on commercial properties maintained by professionals and think they should do the same thing. But maintenance crews that disfigure crape myrtles this way are not the example you should be following.

If gardeners understand the problems this type of pruning can cause and still decide that lopping off the tops creates the appearance they desire for their trees, well, that’s their choice. But I often encounter gardeners who somehow have gotten the idea that they are supposed to prune their crape myrtles that way, and nothing could be farther from the truth.

Some gardeners have been told that crape myrtles need to be cut way back to bloom well. This is not accurate. The flower clusters may be larger on lopped trees, but the added weight on the ends of long branches causes them to bend over awkwardly, especially after it rains. And since the tree is smaller, it actually produces fewer flower clusters.

Sometimes crape myrtles are pruned improperly in an effort to create a different shape. A wide selection of crape myrtle varieties is available today. Some grow tall and upright like a vase, while others are shorter and spreading, more like a mushroom. You cannot make an upright-growing crape myrtle grow in the shape of a mushroom by cutting it back. The new growth will simply grow upright again over time. So if you want a crape myrtle that will mature to the shape you desire, make sure you choose one that naturally grows that way.

Sometimes young crape myrtles are cut back to make them look “fuller.” Young trees often appear more spindly and less substantial, but this is a matter of age – not something that needs to be corrected with pruning. Young crape myrtles are not supposed to look like old crape myrtles. Over time young trees will attain the shapely, full canopies of older trees without drastic pruning.

I also hear people say they need to cut a tree back because of its size. If the height of the crape myrtle is not causing a problem with a nearby structure or power lines, however, there is little reason to reduce the tree’s height. To cut a crape myrtle back for the vague reason of “it just seems too large” ignores the fact that these plants are trees. They are supposed to be large.

To prune a crape myrtle properly, first decide if it needs to be pruned. As with any pruning project, you must have a specific, valid purpose in mind before you begin. In other words, if you can’t come up with a good reason to prune your tree – leave it alone. If you do see something that calls for pruning, study the tree carefully and determine what needs to be pruned to accomplish the specific purpose identified.

Every crape myrtle will need some pruning in its life to grow properly and fit in well with its surroundings. One important reason to prune is to eliminate crossed and rubbing branches because rubbing branches can lead to open wounds. Over time, branches that are too low on the trunk will need to be pruned to raise the canopy. We often need to remove weak, thin branches from the inner part of the tree to produce a cleaner-looking tree. Selected branches may need to be pruned back to a side branch or the trunk to create a shapelier tree. Of course, you need to prune to keep suckers removed from the base of the trunk. Generally, avoid cutting back or shortening branches much larger than your finger, although cutting larger branches back to a side branch or to the trunk when needed is fine.

You also may need to redirect the direction of a branch’s growth. This can be done by studying the branch carefully and looking for a side branch that grows in the desired direction. Prune back to that branch, and you have redirected the growth of the branch. This can be helpful where trees are too close to a structure, such as a house. Branches can be redirected to grow away from or up and over the roof line.

Northern visitors to Louisiana during the summer marvel over our crape myrtles. Their colorful flowers, long blooming season, attractive bark and beautiful shape make them among our most valuable landscape plants. I wish more professionals and home gardeners would appreciate this – and stop the unfortunate trend of hacking them back.

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Contact: Dan Gill at (225) 578-2222 or dgill@agcenter.lsu.edu  

Editor: Rick Bogren at (225) 578-5839 or rbogren@agcenter.lsu.edu
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