How well your hibiscus plants handle winter cold depends on their variety. The hardy hibiscus common rose mallow (Hibiscus moscheutos) and althea (Hibiscus syriacus), for example, survive winters outdoors in U.S. Department of Agriculture plant hardiness zones 5 through 8a and 5 through 9a, respectively. Tropical hibiscus, also called Chinese hibiscus (Hibiscus rosa-sinensis), however, is hardy outdoors all year in only USDA zones 9 through 11. For it, even the rare frosty night that sometimes surprises Mediterranean-climate gardeners is too much. Store tropical hibiscus plants indoors through their winter dormancy to enjoy their summer blooms for years.
Grow hibiscus in pots if you intend to overwinter the plants indoors. A hibiscus can live for years in a 10- to 14-inch-diameter container, according to The Tropical Hibiscus website. If you love the idea of inground hibiscus shrubs, then sink potted ones up to the pots' rims in the soil for the summer. When you’re ready to move the plants indoors in fall, lift and rinse their containers. Hibiscus planted directly in the ground often succumb to root rot after they are lifted and potted for winter.
Preparing Your Plant
Prepare your hibiscus plants for the move two or three days before the fall nighttime temperature is expected to hit 40 degrees Fahrenheit. Using clean, sharp tools, prune back the plants to within 4 inches of their main stems, disinfecting your tools with rubbing alcohol between cuts to avoid spreading plant diseases. Also remove dead leaves, flowers and other debris from the plants and their containers. Preparing helps eliminate insects that otherwise might move indoors with the plants. Set a garden hose’s spray nozzle on its finest spray, and hose down the plants until all their surfaces drip water.
Making the Move
When your hibiscus are dry, place them indoors near sunny, south- or west-facing windows. If your home is short on sunlight, set the plants under timed fluorescent lighting for 16 hours each day. The ideal indoor location has a temperature between 55 and 70 F; the cooler it is, the less likely hibiscus plants are to host insects as winter progresses.
Waiting and Watering
Tropical hibiscus normally drop their leaves after a move indoors. So don't panic if yours lose leaves. They’ve simply become dormant for winter. During their dormancy, wait for their potting medium to dry almost completely before putting them in a sink or shower for a long, slow soaking. Let them drain completely before returning them to their usual spots. Misting the plants daily with a fine spray of water compensates the low humidity of warm indoor air. Alternatively, set the hibiscus' pots on shallow trays of water-covered pebbles, and top off the trays' water as it evaporates.
Waking Them from Winter
In February or March, remove and replace the top 2 inches of your hibiscus' growing medium. Boost the effects of the fresh medium with a dose of slow-release, 19-6-12 indoor plant food. For each 10-inch pot, sprinkle 1 tablespoon of fertilizer, or the fertilizer label's recommended amount, evenly over the growing medium, and water the medium. Plants in larger pots require more fertilizer, as the fertilizer label directions indicate. Cut each branch back to a leaf node, wait for glossy, green leaves to emerge and move your hibiscus plants back outdoors when nighttime temperatures are consistently above 55 F. Set them in a shaded location, and move them a little closer to the sunlight each day for about 10 days, or until they acclimate to outdoor life.
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