Overwintering Tender Plants

Bougainvillea, jasmine, allamanda, hibiscus, mandevilla, plectranthus, agapanthus, and taro. Now we can grow these tropical beauties we’ve lusted after in picture books or on vacations. It’s fun getting to know and admire these exotic plants up close. And the creative foliage and flower combinations are endless. But what happens when fall comes? The easiest solution would be to bid a sad adieu, let the frost claim its victim, and send the plants to the compost pile. But most gardeners are too tender hearted and thrifty to let a good plant go to waste. So we haul them indoors and do our best to keep them alive until next spring. Here are some suggestions to help you succeed:

First consider what sort of indoor growing conditions you can offer. There are two ways to get sun and heat-loving plants through the winter: They can be kept actively growing (which means providing plenty of light, warmth and humidity), or they can be coaxed into dormancy and kept cool and asleep until early spring. If you have a sunny spot and some extra room, most tender exotics can be kept growing and possibly even blooming right through the winter. Bougainvillea, jasmine, citrus trees and geraniums will bloom in a sunny window or on a glassed-in porch that doesn’t drop below 40 degrees. A grow light that provides 12 hours of light each day will also work well.

It’s important to keep these plants well watered and to fertilize regularly when they’re in active growth. Avoid crowding (a small fan improves air circulation) and keep the humidity level between 30 and 45 percent by misting or leaving pans of water to evaporate in among the plants. In cold climates keep the humidity just over 30 percent, any higher will cause condensation on your windows.

Keep in mind that the ideal winter environment for most of these plants would be a cool greenhouse: 50 degrees at night and 65 degrees during the day. Warmer air temperatures lead to weak, leggy growth and bug problems. Whiteflies, spidermites and scale are the most common indoor pests. As long as you keep on top of the situation, they can usually be controlled quite easily with several doses of insecticidal soap. For serious infestations, you can dunk the foliage in a dishpan filled with water and a teaspoon of liquid dish detergent.

Most plants that go through the winter in fairly active growth should be pruned back at least once or twice. This is good for the plant and gives you the opportunity to root some new cuttings. Coleus, plectranthus and geraniums respond well to this treatment. I find it keeps the plants bushy and reduces pest problems.

If you don’t have the sun or the space to keep your tender beauties in active growth, you can put them to sleep. Bring them into a cool, dark place and they’ll get the message. Their leaves will gradually yellow and drop. Herbaceous plants can be cut back to 6”. Woody tropical shrubs should not be cut back until early spring (unless you need to do so to get them into the house!). Keep these plants in a cool (40-45 degrees F), dark (or very low light) place such as the basement. Water sparingly. Revive them with water, sun and fertilizer in early spring, allowing for about a month of indoor growing time before the weather is warm and settled.

Tropicals that grow from bulbs or tubers such as elephant’s ears, caladium, and canna lily, can be overwintered right in their pots. Let them get nipped by frost to send the message that the end of the season has come. Put the pots in a cool, dark place and keep the soil moist, but not wet. You could also dig up the bulbs, wrap them in newspaper, and put them into a plastic bag. Check the bulbs monthly to make sure they are not drying out or molding. It will take some experience to find the ideal technique for your storage conditions.

Cuttings from soft-stemmed plants – such as plectranthus, coleus and geraniums – can be rooted in water on a bright windowsill. Once several roots have emerged, plant the cuttings in 3- or 4-inch pots and keep them on a windowsill or under lights. You’ll have plenty of plants for your own garden, and will be a popular guest at next year’s plant swaps!

When it comes to reintroducing your plants to the outdoors, do it slowly. Shield them from full sun and wind for at least a week or two. If they’re acclimated slowly, they’ll slide right into active growth. Too much early spring stress and they may sulk along all season.

The truth is, overwintering tropical plants requires a bit of trial and error. A technique that works for one person may not work for another. Indoor growing conditions vary widely, and specific varieties also have their own idiosyncrasies. For detailed information about growing and overwintering more than 100 tropical plants, I recommend “Hot Plants for Cool Climates” by Susan A. Roth and Dennis Schrader.