Q: I bought several beautiful tropical hibiscus plants in the spring. They are thriving. However, they were outside all summer, and they are now too tall for my kitchen garden window. I don’t have room for plants that large, so I wonder if and how I can cut them back—and will they still live and bloom?
A: Congratulations on growing your plants so well. Sometimes it is just easier to find someone else that has room for the larger plants. Many tropical houseplants are large trees or shrubs in their original habitat, and they can grow rapidly in one summer outdoors. The following information will apply to many tropical houseplants with woody branches that have been outdoors all summer.
If you want to keep them, pruning is quite easy. The main pruning should be done after they finish blooming. That will probably be in the fall. After a plant finishes blooming is the proper time to prune just about everything, unless you want to eat the fruit produced from the flowers. Cutting off old flowers prevents the plant from wasting energy on seeds and sends it back into producing more flowers.
Hibiscus in particular bloom on the ends of the new growth. The plant will go somewhat dormant over the winter if it gets less light, cooler temperatures and less water. It probably won’t grow many new branches over the winter unless it is in a sunroom. It will begin to grow again in the spring, but don’t keep pruning it back or you will remove all the flower buds, since they develop on the ends of the branch.
The pruning you do now can remove as much as one-third or one-half of a branch. All small interior branches can be removed, and any branches that angle toward the middle of the plant should be cut off. Make the pruning cuts just above a leaf or bud. The bud should be facing the outside of the plant, not aimed toward the middle. Branches that are completely removed should be cut to leave a small bump and not a long stub.
There should be a live bud wherever there is a green leaf. Farther back down the branches where the leaves have already fallen off, the buds may or may not be alive. Don’t just cut back a branch into the middle of nowhere; look for buds and cut back to them.
A lot of hibiscus are sold in the spring at the big box stores. They make beautiful patio plants and can be grown in the house if given the right conditions. They are tropical and will need to be brought in for the winter. You shouldn’t wait until the day before a freeze to do it. Indoors, the best temperature range is between 55 and 70 degrees, so bring it in while the outdoor temperatures are in that range.
After you prune the plant, you may want to wash it off with a little bit of insecticidal soapy water to kill off insects. Be careful to follow label directions as too much soap can kill the leaves. Don’t forget to wash behind the ears—I mean, under the bottom of the leaves—where many insects hide. You can run small plants under the sink and large ones in the shower. It is helpful to occasionally dust the leaves, too.
A west- or south-facing window will help give the hibiscus the warmth and sunlight it needs. Keep it out of dry drafts from the furnace and cold drafts from open doors.
In the spring as new leaves appear, you can repot the plant into a pot an inch or two larger in width. Use a lightweight potting mix, or add peat moss to a normal potting soil. Adding slow-release flowering plant food will help promote more flowers. Liquid fertilizers can be used every few weeks in the active growth periods of spring and summer.
