AgCenter: Why crape myrtles struggle to flower

Crape myrtle flowers can last longer in drier weather.
--Allen Owings photo

HAMMOND – What’s the most popular summer-blooming tree in Louisiana? Crape myrtles. Pretty easy question. Louisianans plant many crape myrtles in their landscapes every year. The lovely, long-lasting blooms make them attractive.
Most years, crape myrtles start blooming between mid-May and early June. Flowering continues for 90 to 120 days depending on the variety. Horticultural practices play the most significant role in how much a crape myrtle blooms. Weather conditions also play a role, too.
Here are some factors to consider if your crape myrtle trees did not bloom well this year:
—New growth. How much new growth did your crape myrtles have this spring? Crape myrtles need to have new growth each spring in order to produce summer flowers. These flowers come on current-season growth, so late winter/early spring fertilization can aid crape myrtle flowering in the summer. It is too late to fertilize trees now. Wait until next year.
—Shade. Crape myrtles require eight hours of direct sun daily to bloom well. Crape myrtles planted in areas that receive less than six hours of direct sun do not get enough sunlight for adequate bloom development. Many times you will see a crape myrtle that appears to be “leaning toward the sun.” This indicates inadequate sunlight for proper plant growth.
—Variety. Some varieties don’t flower as vigorously as others. Hybrid crape myrtles usually flower first. Natchez, Tuscarora, Basham’s Party Pink and Muskogee are the easiest-flowering varieties. The semi-dwarf varieties such as Tonto, Acoma and Sioux follow a week or two later. Burgundy and black-foliage varieties start blooming a little later than green-foliaged varieties.
—Insects. Heavy infestations of aphids decrease flowering. This is the most common insect problem on crape myrtles. Ever feel like you’re being “rained” on under the canopy of a crape myrtle? That “rain” is actually bodily fluid being excreted from aphids. White flies and other insect also can cause problems for crape myrtles.
—Improper pruning. Drastic pruning or pruning after new spring growth can delay summer flowering. Drastic pruning, in fact, may promote excessive growth and less flowering. Sometimes the “crape murder” method of pruning can initiate too much growth at the expense of flowering.
—Too much fertilizer. Excessive fertilization, especially high amounts of nitrogen, in conjunction with other factors, primarily improper pruning, can eliminate or delay flowering.
—Leaf spot. Foliar diseases decrease plant vigor and flowering, especially in the absence of new spring growth. The main cause of leaf spot in crape myrtles is the fungus Cercospora. Long term, this disease is not detrimental to the plant. Using fungicides for control has not been very effective because they would have to be applied repeatedly throughout the growing season, and getting adequate coverage on larger trees is difficult.
—Wet soil. Crape myrtles need well-drained areas to grow well. Lichens growing on bark is common on crape myrtles growing in shady areas accompanied by poorly drained soils and low levels of native soil fertility.
In terms of weather, cool springs can slow crape myrtle foliage growth, and this will result in later flowering. Also, hot summers usually yield more flower production than summers that have normal to slightly below-normal temperatures. Average to above-average rainfall lessens the amount of time that flowers look good. Dry weather favors improved flowering.
So, that’s the list. Consider these reasons if your crape myrtles are not performing to their potential. Hopefully, your crape myrtles will bloom and bloom some more for you this summer.
You can see more about work being done in landscape horticulture by visiting the LSU AgCenter Hammond Research Station website at www.lsuag center.com/hammond.

By ALLEN OWINGS LSU AgCenter horticulturist

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