Garden Detective: Crape myrtle pruning has her puzzled

06/28/2014 12:00 AM

06/27/2014 9:45 PM

What is the real story on pruning a crape myrtle? I have a pink blossom crape myrtle that is about 4 or 5 years old, approximately 11 to 12 feet tall, with a trunk that’s just over 2 inches in diameter. Last year, I pruned it. This year, it grew up and out and it seems fairly happy, although I would like the trunk to get a bit stronger. A gardener told me if I do not prune it, it will get taller and if I do prune, it will stop growing up and only get bushier. When I look around town, most gardeners do prune the ones in people’s yards, but the ones that are in the center strips of roads don’t get anything. What is the best idea? I’d like it to get both taller and bushier. Also, the sticky aphids are awful. I was told to use the systemic rose fertilizer-insect control product; several cups of it.

–Laura Petersen, Sacramento

The tree’s final height and shape is determined more by its variety than pruning methods. To avoid unnecessary pruning, match the mature growth, height and width, with the desired needs at the time of purchase, according to UC master gardener Carol Hunter. There are currently dwarf (3 to 6 feet) and semi-dwarf (7 to 15 feet) crape myrtle selections available. Judging by your tree’s age and development, you may have a semi-dwarf, and it won’t get much taller.

Most popular crape myrtle varieties (Natchez, Miami, Sioux, Dynamite, Muskogee, Watermelon Red) grow at least 25 to 30 feet tall. Among the semi-dwarf, compact varieties are Acoma, Centennial, Hopi, Prairie Lace, Victor and Zuni.

For existing trees, only light pruning is needed. Pruning in late winter to early spring stimulates new growth in the spring. Dead or defective branches can be pruned when observed. Some light pruning in the center of the tree allows for more air circulation and light penetration. Removal of spent blooms can be done but is not necessary as the tree will still bloom and will retain its natural graceful form.

Aphids seldom damage mature trees; however, the sticky honeydew can warrant control.

The best method to eliminate aphids is with a strong spray of water. Once the temperature increases sufficiently, the aphids will leave.

In some situations, ants tend aphids and feed on the honeydew aphids excrete. At the same time, ants protect the aphids from natural enemies. If you see ants crawling up aphid-infested trees or woody plants, put a band of sticky material (such as Tanglefoot) around the trunk to prevent ants from climbing up.

If insecticides are needed, insecticidal soaps and oils are recommended. The systemic pesticides can have a negative impact on predators, parasitoids and pollinators, so should be avoided where soaps and oils will provide adequate control.

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