‘Winter leaf curl’ is protection

Q. During the cold snap we had in late December I noticed my rhododendron leaves had all rolled up. The leaves looked like little cigars. When the weather warmed up the leaves straightened themselves out. Did the cold damage the leaves? Will my rhododendron be OK?

Beverly from Youngstown

A. Beverly, your rhododendron should be fine. What you observed is the rhododendron plant’s response to freezing temperatures called “winter leaf curl.” Rhododendron leaves are thermotropic, which means the leaves move, curl, in response to air temperature. It is believed that the curling helps protect the leaves from cold temperatures. In fact, rhododendrons act as living thermometers. When the temperatures are above freezing, the leaves are flat and extend out from the stems at right angles. When the temperature drops just below freezing, the leaves droop downward. When the temperature drops more, to around 25 degrees or lower, the leaves curl up stems again.

There is nothing you need to do. This is a natural response of rhododendrons to cold. Simply marvel in the plant’s ability to keep its green leaves through these cold spells.

There is another, separate issue to look for on these plants as winter progresses. If you find it, you’re usually too late.

Rhododendrons, like all evergreens, can be damaged by winter desiccation. This is different from winter leaf curl, although some symptoms overlap. Winter desiccation happens when the soil freezes in the root zone of an evergreen, shutting off normal water uptake from the roots to the rest of the plant. When the ground stays frozen for extended periods, the leaves and sometimes stems will begin to die, due to lack of water. In rhododendrons, winter desiccation can cause the leaves to curl, but when the weather warms the leaves turn brown instead of returning to normal.

To prevent winter desiccation in rhododendrons and other evergreens, water them thoroughly in the late fall and check soil moisture around their root zones during winter thaws, adding water when necessary. When we have a really warm day like last week, make a habit of checking the moisture around your rhododendrons. Checking ones that are close to your house or partially under overhangs is especially important. Watering in the winter on a warm day can go a long way in helping these plants come out of winter in near perfect shape.

For more information on Rhododendrons leaf curl and desiccation, visit http://go.osu.edu/drooping.

Today’s answer is provided by David Sprague, OSU Extension master gardener volunteer. Extension’s Winter hours for the Plant and Pest Diagnostic Clinic vary. Submit questions to the clinic at 330-533-5538 or drop samples off to the Extension Office in Canfield. Hours vary throughout the winter season.