Many gardeners are gravely concerned when they wake up one morning and their rhododendrons’ leaves have all curled into tight spirals. It’s not pretty, but a healthy plant is not in danger.
Leaf curl in rhododendrons in winter is usually not a cause for concern.
As the temperature moves towards freezing (32F/0C), leaves on rhododendrons may begin to droop or curl…as temperatures plummet further the leaves curl tighter. Water evaporates from the undersides of rhododendron leaves, and the curling is a mechanism to slow down evaporation and keep moisture. This tendency is called thermotropism…a movement of the plant due to changes in temperature. Another example of thermotropism are plants that actually turn their leaves towards warmth (normally the sun).
It is not normally cold temperatures that harm rhododendrons in winter, but lack of enough water. Newly planted rhododendrons or plants with other problems (disease, infections) may suffer winter injury, but the leaf curling is a secondary issue. Healthy rhododendrons will sail unflinchingly through cold weather. Even in cases of dieback, the plant will often make a complete recovery…although the plant’s appearance may be disconcerting.