Wednesday, May 23, 2018
Home and Garden

Ask Dr. Hort: Out with hibiscus black spot, oleander worms, in with tough, pretty shrubs

Black spot is harming hibiscus

Q: We have our special hibiscus in pots in the screened-pool area (deer problems). While they bloom nicely, they all seem to have the same problem with the leaves getting black spot and then falling off or being picked off. What is causing the spots and what can be done to eliminate this problem? William N. Turanin, Palm Harbor

A: During this time of year with the cool nights and increased dew sets, many varieties of hibiscus come down with a black spotting fungus that eventually leads to leaf drop, especially on weakened plants. Check to see if your plants have become pot bound. Maybe some root pruning is in order, or a larger container. As we move into spring the plants should grow out of the problem. You may also use a fungicidal spray such as Bayer Advanced 3-in-1 to protect the new and undamaged foliage if you feel the need to spray something. One tablespoon of baking soda and one tablespoon of Murphy's Oil Soap or a dash of dish detergent in 1 gallon of water, sprayed weekly, also acts as a good protection against black spotting fungi on various plants.

How to evict oleander caterpillars

Q: What can I do about the worms on the oleander bushes I have? Picking them off and squashing them is getting a bit old. I tried spraying with Bug Be Gone from Ortho that I had on hand, but it damaged the ends of the leaves a little and after a rain the caterpillars were back. Heidi Willeford, Palm Harbor

A: It would be best to apply Bt, Bacillus thuringiensis, a bacterium that kills only caterpillars. As they munch, they get sick and die. It is sold as Bt, Dipel and Thuricide. You need to reapply after rain or irrigation hits the foliage. Follow label directions and your oleanders, Nerium oleander, should once again have a full coat of foliage.

What's a tough ornamental shrub?

Q: We have cordoned off a small area with brickwork at one side of our garage door. It measures about 2 feet by 2 feet. We are filling it with potting soil. We live here only seven months out of the year (fall to winter). What can we get established in the spot (which get sunlight half a day) that is an ornamental shrub-type plant that can withstand the heat in the summer without anyone tending it? It would have to be something that grows no higher than 3 feet (or which we could keep cut at that height — like a topiary shaped tree). Of course, we cannot have something that makes big roots that would crack our concrete driveway and sidewalk to the house that surrounds this area. John R. Bent, Bradenton

A: It sounds like a good spot for a red band or tricolor dracaena, Dracaena marginata or Dracaena marginata Tricolor. They have some character and after establishment are very drought tolerant. Another choice would be bamboo palm, Chamaedorea seifrizii, a somewhat delicate dwarf reed palm, also drought tolerant after establishment. And if it is topiary (pruning plants into art forms) that you would like, yew podocarpus, Podocarpus macrophyllus, which grows slowly, is drought tolerant and trainable. Thryallis glauca is a drought tolerant shrub that flowers yellow most of the year. Rosemary, Rosmarinus officinalis, is great if you're into herbs. Walters viburnum, Viburnum obovatum, is a good native choice.