Developed by a Louisiana nurseryman in the late 20th century, Encore azaleas are the result of crossing spring-blooming with summer-blooming species, creating an evergreen plant that blooms at least twice a year. Typically, a large spring blossoming occurs, followed by a brief hiatus, and then another significant but smaller display in late summer or early fall. Encore azaleas are not overly winter hardy and so are suited only for the milder winter regions of the United States.
The best areas to grow the two dozen cultivars of Encore azaleas are in U.S. Department of Agriculture plant hardiness zones 7 through 9a, where winter temperatures drop into the range of 0 to 25 degrees Fahrenheit. They may be successfully grown in zone 6b in more sheltered garden settings, and a few select cultivars demonstrate winter hardiness down to minus 10 F, which is zone 6a. If the winter is too cold, dieback of twigs and leaves occurs, followed by death of the roots with prolonged, excessive cold.
Factors Affecting Damage
Encore azaleas are more prone to freeze damage if they are stressed by drought or disease heading into fall and winter. Bone-dry soils in winter, coupled with exposure to warm sunshine and cold, drying winds cause plants to desiccate and drop leaves or die back. Younger, newly planted Encore azaleas with less extensive root systems also may not endure cold weather as well as older, established shrubs. Freeze damage may become profound if cold weather returns after a mild winter thaw that causes flower buds to swell and sap to flow in the branches. Frozen sap causes the bark to split and dry out the vascular tissue.
Do not prune away freeze-damaged tissues on Encore azaleas until a time in spring when frosts are no longer expected. Pruning in late winter or too early in spring encourages new growth that is tender to cold. If another frost or freeze occurs, this new growth will be killed. Although the freeze damage may look unattractive, do not prune until early to midspring. Delaying the pruning also gives the azaleas time to reveal which tissues are dead. You may still get some flower buds to open and leaves to sprout after the freeze damage. If the flowering is nice, delay the pruning until late spring. New growth will then following the trimming across summer.
Root System Insight
Azalea roots are more susceptible to cold damage than leaves and twigs, according to the Azalea Society of America. The roots are fibrous and thin, mainly in the top 6 to 10 inches of soil. In fall, allow the first frosts and freezes to occur on the shrubs so the plants become dormant. Then, in late fall, create or maintain a 3- to 4-inch-deep layer of mulch over the roots to insulate the soil from cold and moisture loss. Healthy, nonfrozen roots survive winter best and provide excellent energy and water to fuel regrowth in spring following any winter freeze damage.