Jennifer Schultz Nelson
Extension Educator, Horticulture
Tropical plants and tender bulbs have grown in popularity for use in outdoor landscaping. Some have been a garden staple for years, like cannas, elephant ears, and dahlias. Others are less familiar, like banana, mandevilla, and bougainvillea. Most will not survive the cold temperatures of an Illinois winter without being moved indoors.
Though killing frosts have not made their way into our area yet, temperatures have started to dip, reminding us that winter is on its way. Soon those of us with tropical plants and tender bulbs in our landscapes will have to decide whether to let them succumb to the cold winter winds or try and overwinter them indoors. It's not an impossible task, but will require some planning and preparation.
Overwintering tropicals like cannas, elephant ears, and dahlias is best done by allowing the tops to turn brown after the first frost, and then dig the underground tubers or rhizomes. There are many different ways to store the tubers and rhizomes.
In order to successfully survive the winter, tubers and rhizomes need to be in a cool 40 to 50 degree area, with good air circulation. An unheated garage or shed is a good choice. Some sources recommend removing all of the soil, and packing the tubers and rhizomes in ventilated milk or bread crates loosely with sawdust, peat moss or vermiculite which will keep them from shriveling. I have had good luck using standard plastic storage totes, leaving just a little bit of soil on the tubers and rhizomes, and leaving the lid cracked open for ventilation.
Last year I overwintered a couple of container grown callas and tropical bulbs in my garage, still in their container. The foliage died back, and in the spring new shoots began to emerge, even before I watered then thoroughly. While it wasn't the perfect textbook way to store these bulbs, it still worked. I risked losing everything to pests or disease overwintered in the soil, but given that I only had time to either put the pot in the garage or let it freeze, I took my chances.
But what about plants that don't have a tuber or rhizome to dig? It's natural to want to save your tropical plants, as many of them are quite costly to purchase. But remember overwintering plants indoors requires space and time to care for them. Even if plants are brought indoors and allowed to go dormant, they still need to be checked on from time to time.
Many tropicals, including banana, hibiscus, mandevilla, bougainvillea, and angel's trumpet will go dormant if brought indoors and the soil is allowed to dry out. The leaves will turn yellow and drop, and banana trees should be cut down to the soil level. The plant should then be moved to a location that is 40 to 45 degrees. Throughout the winter, keep the soil barely moist, watering sparingly when the soil is dry two to three inches deep in the pot.
If you have good light and space, you may want to attempt to keep your tropicals actively growing through the winter. I do keep my tropical hibiscuses growing through the winter with supplemental lighting. The plants produce a good flush of flowers in the fall and again in the early spring. But they do tend to get leggy and spindly even though they are under lights. When they move back outdoors in the spring, they each get a good pruning to encourage more bushy growth.
One major factor to consider in overwintering tropicals is involuntarily overwintering pests as well. Bringing in containers of tropicals typically also brings along insects that may not be welcome in your home. But it may be entertaining as well.
I once accidentally brought in a very large praying mantis that blended in quite well with on of my hibiscus plants. Once my cat found him, it was quite entertaining. I did manage to safely escort him back outside.
Some pests don't show themselves until a few days or weeks after being brought indoors. If you are trying to keep your tropicals actively growing, and conditions aren't perfect (they rarely are indoors, even in a greenhouse) you will likely have problems with common houseplant pests such as mealybugs, scale, and spider mites.
All of these pests can be controlled indoors, but the question is whether you really have time to dedicate to controlling them. Left unchecked, these pests will spread to other plants and create quite an infestation.
If you have houseplants with a lot of sentimental value, you may want to be selective about tropicals you bring indoors. You need to honestly assess whether it's worth the risk to bring every tropical indoors, none of them, or a select few.
Another option to consider is taking cuttings of your favorite plants to start indoors so you have a small plant to start with next spring. This may be a way to overwinter more of your favorites in less space.
Overwintering tropicals can be a way to extend your gardening pleasure into the winter months. Many people look at it as a challenge and get great satisfaction when they are successful in producing plants from year to year. In these tough economic times, it can also be a way to stretch your gardening dollar since you are not replacing plants each year.