January 7, 2004
Woodbury Bulletin
Betsy Halden

Why Rhododendron leaves curl in winter

While watching the cardinals at the bird feeder recently I noticed the leaves on the nearby rhododendrons were curled and hanging down. I thought this called for a little research. Here is what I found.

When the temperature drops below 35 F rhododendron leaves begin to cup and curl at the edges. At 24 F the leaves have curled so tightly that half the leaf surface has disappeared and the leaves drop. 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 wintry day, you MAY be able to determine roughly how cold it is by the degree the rhododendron leaves have curled and dropped. When temperatures rise the leaves open again.

The rhododendron makes the same response in summer when temperatures become excessive. When temperatures rise 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 comes northward. The plants prefer broken shade or the comfort of a nearby hedge or building. Temporary windbreaks may also offer winter protection to the plants. Our rhododendrons grow on the southwest corner of our house in broken shade. Although there are no leaves now on the deciduous trees, the branches of nearby evergreens provide some protection from direct sun.

Rhododendrons also thrive better when mulched with loose material after the ground has frozen.

If you are looking for a rhododendron that should grow well in our climate you might consider Rhododendron mucronulatum (Korean rhododendron), recommended by Mike Zins, University of Minnesota Extension Service Horticulturist. This is a deciduous rhododendron with excellent cold hardiness. Rhododendron mucronulatum is native to Korea, China, and Japan. It blooms in early May and can sometimes be hit by a late frost. The flowers are magenta colored and appear before the leaves, giving a bright floral show. A pink flowered form of this species, equally hardy, is a cultivar called "Cornell Pink."

Betsy Halden is a resident of Woodbury. She consulted Cornell Cooperative Extension and the University of Minnesota Extension Service materials when preparing this column. You can reach Betsy Halden at eahalden@comcast.net.