Rhododendron Adaptations
- Most cold climate trees and shrubs shed their leaves in winter. Freezing temperatures damage the delicate leaf cells. During winter, deciduous trees go into a dormancy state. Broad leaf evergreen rhododendrons keep their leaves all year. In winter, when other trees are bare and dormant, evergreen rhododendrons remain actively growing and continue to perform photosynthesise, giving this shrub an edge over its dormant competition, according to the Western Pennsylvania Conservancy web page.
- Rhododendron leaves adjust to cold temperatures rapidly by curling up and drooping. When tightly curled, rhododendron leaves lose less moisture. The curled, drooping leaves are less susceptible to damage from freezing temperatures and wind. Fine hairs on the underside of the rhododendron leaves provide further insulation and protection. The leaves respond directly to heat and light. Rhododendron leaves in full winter sun may be partially or fully unfurled while leaves in deep shade remain closed.
- Rhododendrons adjust rapidly to changing light and temperatures. As an understory shrub, rhododendrons grow in filtered to full shade all summer while the deciduous trees are in leaf. In winter, rhododendrons are more often exposed to winter light and shifting shade and sun. The ability to curl and uncurl the leaves in direct response to conditions allows rhododendrons to take full advantage of shifting sunlight without undergoing damage when the winter sun shifts.
- Rhododendron leaves can survive temperatures as low as -50 F, according to the Iowa State University Horticulture Department. Ideally, the leaves adjust during early winter as temperatures gradually drop and then warm again. Sudden low temperatures can damage rhododendrons that have not had time to adjust. The leaves begin to curl and droop as soon as winter temperatures begin to drop below freezing. The range of cold tolerance varies depending on the rhododendron variety.
