Garden News: It’s not great year for crape myrtles

Photo by RICK BOGREN -- Lichens growing on crape myrtles are a sign of poorly drained soil and low soil fertility.
Photo by RICK BOGREN -- Lichens growing on crape myrtles are a sign of poorly drained soil and low soil fertility.

Unusual summer weather has caused crape myrtle trees to behave unusually.

In most years crape myrtles start blooming between mid-May and early June and continue flowering for 90-120 days, depending on the variety. This year, however, some crape myrtles aren’t blooming well.

LSU AgCenter horticulturist Allen Owings offers some factors to consider:

Leaf spot. Foliar diseases decrease plant vigor and flowering, especially in the absence of new growth in spring.

The main cause of leaf spot in crape myrtles is the fungus Cercospora, and it’s bad this year.

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.

New growth. Crape myrtles need to have new growth each spring in order to produce summer flowers.

Shade. Crape myrtles require eight hours of direct sun daily to bloom well.

Variety. Some varieties don’t flower as vigorously as others.

Hybrid crape myrtles usually flower first. And semi-dwarf varieties follow later.

Insects. Heavy infestations of aphids decrease flowering. This is the most common insect problem on 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, can eliminate or delay flowering.

“In terms of weather, cool springs can slow crape myrtle foliage growth, and this will result in later flowering,” Owings says.

In addition, hot summers usually provide more flowers while average to above-average rainfall lessens the amount of time that flowers look good. Ultimately, he says, dry weather favors improved flowering.

If you have gardening questions, please send them to GardenNews@agcenter.lsu.edu.