At 25F, the leaves have curled so tight that half the leaf surface has disappeared and the leaves droop. When temperatures hit the teens, leaves shrivel even tighter, turn brownish-green and dangle like stiff string beans. This response to temperature changes is a rhododendron's method of preventing loss of moisture through the leaves. The upper side of a rhododendron leaf is leathery. The bottom side is dappled with tiny air valves that control the flow of air in and out of a leaf.
Cold air contains less moisture than warm air.
So when low temperatures and high winds arrive, the leaf valves close. By looking out a window on a winter day, one can determine roughly how cold it is by the degree the rhododendron leaves have curled and drooped. When temperatures rise, the leaves open again. Rhododendrons are also subject to leaf scorch in winter months, particularly in January, February, and March when the sun marches northward. Winter protection in severe climates can be given a rhododendron by means of temporary windbreaks or spraying the leaves with anti-transpirants.