Have you heard it said that you can gauge the temperature on a winter morning by how tightly the cold, droopy leaves of the rhododendron are curled? I’m a regular science fair judge at the Sewanee Elementary School, and often I see a project for which some little kid has bundled up, morning after morning, to run outside, check the temperature, measure the width of a curled rhododendron leaf, and take its picture. They record those data and draw graphs showing a nice correlation between the width of a curled-up leaf and the temperature: tight curls go with low temperatures.
It’s a neat idea: “Leaves rolled tight as a cigar? The temperature’s in the 20’s. . . Pencil-width? It can’t be over 15 degrees.” But why? Does it prevent water loss? Decrease the potential load of ice and snow?
It wasn’t until I ran across a good paper on the subject that I finally understood a little about what was happening with those leaves. First, curling and drooping of rhododendron leaves are examples of – here’s a new word for me – thermotrophic movements, or movements in response to temperature. Second, the drooping and curling have completely different functions.
Drooping reduces the amount of sunlight that hits the leaf. We see the same thing in the tender young deciduous leaves when they first emerge in the spring, drooping so as not to catch the harsh sunlight straight on. Too much light when it’s too cold can actually cause photosynthetic damage to leaves.
But what about the leaf curling? Rhododendron leaves freeze at about 18°F. In the Southern Appalachians, where there are “balds” and “slicks” covered with Rhododendron catawbiense Michx. and impenetrable “hells” composed of R maximum L, it is common for the leaves to freeze at night and thaw during the day, time after time, throughout the winter.
If a plant is going to be frozen and thawed, it’s best for it to freeze rapidly and thaw slowly, because those conditions lower the chance of damage caused by ice crystals piercing the cell membranes from inside. And here’s the thing: a curled leaf thaws more slowly that a flat one! This probably has a lot to do with the fact that Rhododendron is one of the few evergreen genera of eastern North America to tolerate frequent freezing/thawing cycles.
We have only one small population of native Rhododendron (R. maximum L.) growing on the Domain, but it is a popular landscape plant, quite common on campus and in town. Now when I pass my neighbor’s house on the way to campus and see the tightly curled rhododendron leaves hanging from the branches, I can comment to myself on these remarkable adaptations. And when Science Fair time comes around, I might have something intelligent to say when those budding scientists ask me what’s up with Mother Nature’s thermometer.