Answer: It is difficult to diagnose a leaf curl problem without actually inspecting the plant, but the most common cause is winter damage. When the temperature drops below 35F, rhododendron leaves begin to cup and curl at the edges. At 25F, the leaves will curl so tight that half the leaf surface disappears 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 rhododendron makes the same response in summer when temperatures become excessive; only the leaves curl upward to prevent undue moisture loss.
Rhododendrons are also subject to leaf scorch in winter months, particularly in January, February, and March when the sun marches northward. That is why these plants prefer broken shade -- from a tall tree, a tall building, or a nearby hedge.
The other problem associated with leaf curl is root rot from poor drainage or overly wet soils. I hope it's only winter damage on your rhodies!
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