Ask Dr. Hort: Replace unhealthy hibiscus with another plant

Inspect your sprinklers

Dry times are ahead. It's time for a sprinkler tuneup. Here's what to have on the to-do list: Replace broken or worn-out sprinkler heads, redirect sprinklers to keep water on-site (and not in the street), check the clock for appropriate run times and retrofit plant beds to drip systems instead of sprays and rotors to save water. As plants age, sprinkler types and placements need to change accordingly. Also check for leaky or broken pipe. The goal is an efficient system that supplements rainfall.

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Time to give these hibiscus the old heave-ho

Q: This problem with our hibiscus started sometime last fall and has gotten progressively worse. One plant is totally dead and parts of others are too. It appears something is eating the leaves, but I cannot find anything on them. I have sprayed numerous times and nothing seems to work. At this point I am ready to pull out all the existing plants and replant new ones. However, I do not want to do this if it is a problem in the soil and would attack new plants. What to do and what is this? None of the other surrounding plants (ixoras, crotons, etc.) are affected.

Jan Adams, St. Petersburg

A: You will most definitely want to replace these plants and a smart bet would be something other than hibiscus. It is really hard to tell how or what started the problem in the fall. Because of the totally stunted growing points, I suspect pink hibiscus mealy bug, a small (1/8-inch) pinkish white sucking insect that likes to colonize buds (growing points), hence the stunting. Andy Wilson, my colleague at the extension service, also suggested a possible boron toxicity due to the distorted, yellowed leaves (perhaps from a termite treatment, something in the numerous sprays you used or maybe gray water from a washing machine containing borax). This problem was difficult to diagnose; replacement, however, is the answer.

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Keep crape myrtle trim and full of blossoms

Q: Are there any specific rules about pruning a crape myrtle? Does trimming increase blossoms? Should they be kept at a certain height?

David E. Thomas Sr., Dunedin

A: I'm so glad you asked this question. Ideally, crape myrtles only need maintenance pruning (dead wood and shoots arising from the base of the tree) and criss-crossing branches). It is best to know what kind of crape myrtle you have by variety, shrub form or tree form. Shrub forms just need to be thinned and shaped. Tree forms need their interior branches and old seed heads removed, cutting back to nothing more than pencil sized; this gives a pleasing ice cream cone shape. Pruning should be done in winter so you can see the overall shape without leaves. To increase bloom in the summer, spent blooms can be dead headed (cut back just behind the flower). Cutting large trunks causes excessive succulent growth, resulting in increased insect and disease problems. Pruning with the natural form of a plant results in much less maintenance.

Need help?

Enter Greg "Dr. Hort" Charles, who for more than 30 years educated gardeners through the Pinellas Technical Education Centers. He answers questions about landscape and garden pests. E-mail your questions to drhort@tampabay.rr.com or to features@sptimes.com (put Dr. Hort in the subject line). Mail questions to HomeLink, Features Department, St. Petersburg Times, P.O. Box 1121, St. Petersburg, FL 33731. Describe your problem in full, and include your name and contact information. If possible, include a photo. We will print his advice on Saturdays in HomeLink.

Ask Dr. Hort: Replace unhealthy hibiscus with another plant 03/18/10 [Last modified: Thursday, March 18, 2010 11:32pm]

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