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The Garden Decoder: What Is ‘Thermonasty’? (Hint: Your Rhodendron’s Secret Winter Trick)

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The Garden Decoder: What Is ‘Thermonasty’? (Hint: Your Rhodendron’s Secret Winter Trick)

January 30, 2024

Baby, it’s cold outside! How cold? Let’s go check the rhododendrons. Wait…what? What do rhododendrons have to do with figuring out how cold it is? Some rhododendrons exhibit thermonasty. Which means, you can use them as a thermometer, sort of.

Photography by Joy Yagid, unless otherwise noted.

What is thermonasty?

Rhododendron leaves curling tighter as the temperature nears \20 degrees Fahrenheit.
Above: 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. Nastic movement is a plant’s non-directional response to a stimulus. For rhododendrons, this means the leaves react to the cold temperatures by drooping and curling. The tighter the curl, the lower the temperature.

Why do rhododendrons curl their leaves?

Above: Flat leaves on a Rhododendron tomentosum in the spring. Photograph by Marie Viljoen, from Fresh Brews: Two Unusual Shrubs to Grow for Tea Leaves.

Scientists aren’t completely sure, but they have an idea. The current hypothesis is that thermonasty protects leaves from sunburn. The native East Coast variety of rhododendrons are evergreen understory plants that have large flat oval shaped leaves that fan out like fingers on a hand. During the summer, rhododendrons are happiest in the shade or at least in partial shade. During the winter, however, they lose the protection of shade when the deciduous trees drop their leaves. Scientists believe the leaves curl to reduce the exposure to the sun. On a cold clear day, the sun’s radiation can be very strong; the trigger for thermonasty is the temperature and not the amount of sunlight.

But don’t all plants like some sun?

Normal flat rhododendron leaves during the winter—must mean it&#8\2\17;s above freezing.
Above: 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. The key word here is dormant. When the plant is dormant, it can’t turn that extra sunlight into food. The extra energy has nowhere to go and will result in sunscald.

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.
Above: 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. (Keep in mind your plant may be in a microclimate, if it’s, say, under a pine tree or close to a house.) The warmer the winter day, the more normal the leaves look. They will look normal even when it is just above freezing. However once the temperature drops to around 25°F, the leaves will start to droop and curl. At about 20 degrees Fahrenheit or below, they will be at their tightest.

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