Thermonasty: Why Rhododendrons' Leaves Curl in the Winter
Photography by Joy Yagid, unless otherwise noted. Let’s go check the rhododendrons. Which means, you can use them as a thermometer, sort of. Rhododendron leaves curling tighter as the temperature nears 20 degrees Fahrenheit. “Thermo” means temperature and “nastic” means movement—thermonasty is the movement of plant parts in response to changes in temperature. You’re likely already familiar with plant movement in the form of heliotropism, in which flowers, like sunflowers, turn to follow the sun. Why do rhododendrons curl their leaves? Photograph by Marie Viljoen, from Fresh Brews: Two Unusual Shrubs to Grow for Tea Leaves. Flat leaves on a Rhododendron tomentosum in the spring. Scientists aren’t completely sure, but they have an idea. During the summer, rhododendrons are happiest in the shade or at least in partial shade. But don’t all plants like some sun? Normal flat rhododendron leaves during the winter—must mean it’s above freezing. Now, you may think that more sun is a good thing in the middle of winter. However, scientists conjecture that since rhododendrons are understory plants, their leaves don’t have the built-in protection to handle direct sunlight during the winter, when they’re in their dormant phase. How can you tell how cold it is by looking at your rhododendron? Rhododendron leaves starting to droop and curl as the temperature dips below freezing. On a warmish winter day, note the temperature and go out and look at the leaves on your rhododendron. The warmer the winter day, the more normal the leaves look.