The simplicity of winter has a deep moral. The return of nature, after such a career of splendor and prodigality, to habits so simple and austere, is not lost either upon the head or the heart. It is the philosopher coming back from the banquet and the wine to a cup of water and a crust of bread.

~John Burroughs, 1866

After a strange couple of days of unseasonably warm temperatures, winter has returned. A reminder that I am one of those odd folk that enjoys cold weather. In fact, the colder the temperatures, the better. I remember fondly the coldest day I ever experienced – a frigid -33 degrees Fahrenheit morning in my favorite place, Yellowstone. But it was spectacular! The air was clear and crisp, no wind, and the world was twinkling with tiny crystals of ice, called diamond dust, suspended in the air. Magical, indeed. Back here in the Piedmont of North Carolina, we have had some unusually cold temperatures this winter, dipping down to 9 degrees a week or so ago, and supposedly headed that way again this week. It makes for very active bird feeding stations, brisk walks under clear blue skies, and a better-than-usual seat bu a roaring fire as you read a good book. It has also has an interesting impact on one of my favorite plants – rhododendron.

Rhododendron leaves at 32 degrees (click photos to enlarge)

It turns out that rhododendron leaves can be used as a biological thermometer. This is a phenomenon that is well-known, though the cause is not so well understood. I have made a resolution to get outside at least once a day at work, an easy thing to convince myself to do since I work in such a beautiful setting. Our mountain habitat has several large rhododendron shrubs and I noticed the leaves had started to droop as the weather got colder. I remember seeing them tightly curled in the true mountains on a few freezing occasions, so, with the predicted cold spell last week, I decided to photograph the tip of a single rhododendron branch at different temperatures. The first photo shows the branch at 32 degrees, the temperature at which the leaves are known to start to droop.

Rhododendron leaves at 26 degrees

As it got colder, the leaves drooped even more, and began to curl.

Rhododendron leaves at 15 degrees

On the coldest morning we had recently, the leaves were tightly curled, resembling green cigars. This curling is called thermonasty. That’s right, thermonasty. This odd-sounding name comes from the two root words- thermo (temperature), and nastic, which are non-directional plant movements that occur in response to environmental stimuli, in this case, temperature. The nastic movements of rhododendron leaves follow a fairly predictable pattern – when temperatures fall below freezing, the leaves start to droop but remain flat. At 25 degrees F the leaves start to curl and by 20 degrees F they are as tightly curled as they can get. Many people believe the curling is to protect the leaves from desiccation by shielding the stomata (the openings in the bottom of the leaf which allow the leaf to “breathe” or transpire). But recent research shows that the stomata are already closed when it is cold, and one researcher suggests a different theory for the change in leaf position.

According to Dr. Erik Nilsen (Why Do Rhododendron Leaves Curl?), the stomata are always closed in cold weather — they have nothing to do with drooping or curling of the leaf. The drooping is more likely a way to protect from the thawing that can occur on a sunny winter day. When the leaf is held in its normal flat and horizontal position, it will absorb sunlight and heat up and thaw, then could refreeze at night. Experiments have shown that flat leaves thaw faster than curled leaves. This is because a curled leaf exposes far less surface area to the sun than does a flat one. By thawing more slowly, cured leaves are better able to avoid the damaging effects of daily freeze-thaw cycles which can rupture cell membranes and eventually kill the leaf. By drooping and curling the leaf may be protecting itself from too much sun — opposite of what you might think it would try to do.

Rhododendron leaves at 58 degrees following the cold spell

My observations last week agree with the overall temperature response of these evergreen leaves. The thing I don’t yet have a good feel for is how quickly these changes occur. I need to watch the leaves this coming week to see how rapidly the rhodo-thermometer can track temperature changes. Always something to ponder and discover at the Garden.