Not all hibiscus varieties survive winter cold. Hardy hibiscus types are better suited to cold weather survival, though the plants won't grow as evergreens in areas that experience frost. Two varieties suited for outdoor overwintering are the althaea and rose mallow types. These varieties produce the oversized blooms typical of hibiscus while providing winter hardiness.
Climate and Dormancy
Hardy hibiscus plants grow outdoors into U.S. Department of Agriculture hardiness zone 4. The plants go dormant in winter in locations that have freezing temperatures, but they remain evergreen in warmer climates. These hibiscus types produce new wood each spring so it doesn't affect flowering if all the previous season branches die back from winter cold. The plant forms a low-growing shrub in areas where the plant experiences die back, but reaches a height of 5 feet or more if the plant doesn't die back in winter.
A thick layer of mulch protects the roots of the hibiscus from winter frost and temperature fluctuations. Freeze and thaw cycles in winter cause the ground to shift, which can lift and damage the hibiscus roots. Providing insulation to the soil surface prevents heave damage and helps maintain a constant temperature around the roots. A 3- to 6-inch layer of organic mulch, such as bark, pine needles or straw, provides optimum protection. Only the roots of the hibiscus must survive winter for the plants to bloom the following summer.
The leaves begin dropping from the hibiscus after the first freeze in fall. Leaf drop indicates that it's time to apply winter mulch. During severe winters, the branches may die back completely to the ground. Warmer winters may still result in leaf drop but the branches may survive for a second year. Winter killed branches are dry and brittle. They also don't form buds in spring. Complete branch die back doesn't affect the health of the hibiscus as long as the roots remain alive.
Pruning occurs in early spring before the new branches begin growing from the base of the hydrangea. The method used depends on the severity of die back. Hibiscus that experiences complete die back requires total pruning to remove all the branches to within 3 inches of the ground. Cutting back the dead tips of the branches suffices on a hibiscus that only experiences slight winter damage. If in doubt, cut back the entire hibiscus. The plant still grows back from the base regardless if it suffered total or partial die back.