Rhododendrons may need help to survive the winter in regions where temperatures drop below zero and the soil freezes. These plants continue to transpire through the winter months and lose moisture through their leaves, especially on warm, windy days. When the soil is frozen, however, rhododendrons’ roots are unable to take up and replace this moisture, resulting in an irreversible break in the water supply. While this injury may not be immediately apparent, ultimately it becomes clear that branches or whole plants are dead.
1. CONTINUE WATERING
As the weather cools in fall, it can be easy to forget about watering. However, continuing to irrigate your plants will ensure that they enter winter with moisture in their tissues. Until the soil freezes, use a soaker hose or sprinkler to water every couple of weeks, soaking the soil to a depth of about two feet.
Rhododendrons grow best with a year-round mulch, which helps to control weeds, conserve moisture, and provide nourishment as it breaks down. Renewing the mulch every year in late fall by applying it in a layer thick enough to reach the lower leaves will help limit the penetration of frost into the ground and allow the deepest roots to continue drawing moisture. A thick layer of winter mulch will also protect the surface roots against alternate freezing and thawing. Oak leaves, pine needles, wood chips, and ground bark are all excellent choices for mulch. It is best to avoid using peat moss, however, since it prevents water from penetrating the soil once it becomes dry.
3. SPRAYING WITH AN ANTIDESICCANT
When sprayed on leaves and branches, antidesiccants or antitranspirants such as Cloud Cover and Wiltpruf will form a thin film that seals in moisture and helps to prevent water loss during winter. While best applied before the first frost of the season, they can also be applied as late as two hours before a predicted frost. Most applications will last for about three months, but check the labels of specific brands to determine if further applications will be needed.
4. CONSTRUCTING A WINDBREAK
Rhododendrons in especially exposed locations may need further protection from bitter, drying winds. A simple windbreak can be made by driving three or four stakes into the ground around the plant and stapling or nailing burlap to the stakes. For very large plants, a two-sided, V-shaped windbreak, positioned so that the corner points in the direction of the prevailing wind, is easier to construct than one with three or four sides. Do not use plastic film for the windbreak, since it cuts off needed air circulation and can cause the plants to overheat on warm days.