The Rhododendron: Nature's Winter Thermometer | Almanac.com

The Rhododendron: Nature's Winter Thermometer

Photo Credit
Robin Sweetser

Their leaves can help show just how cold the weather is!

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If you grow rhododendrons, you must have noticed how the leaves curl up in response to the cold. Our weather has yo-yoed back and forth so far this winter and the leaves on my rhodies have curled and uncurled to match the temperature swings. What’s that all about?

The way plants can “predict” the weather is fascinating and I’ve written about weather-predicting plants before. This winter, I’ve been watching the rhododendron leaves furl and unfurl as the temperatures swing.

A Living Thermometer

Their leaves are thermonastic, which means that they move in response to temperature changes. Many desert plants have this quality, as do flowers such as tulips or crocus, which close up at night or on cold, rainy days. When the temps are above 40°F (4°C), rhodie leaves are flat and horizontal. Like little solar panels, they are collecting sunlight. The colder it gets, the more they droop and curl up.

  • Drooping occurs the closer the leaves get to 32°F (0°C). It’s thought that lack of water in the stem causes this.
  • Curling starts to be noticeable at 25°F (-4°C).
  • They are very tightly curled by the time the temps have dropped below 20°F (-7°C). Since the leaves actually freeze at 18°F (-8°C), it’s thought that they curl to protect frozen cells from damage due to rapid thawing. With less surface area exposed to the sun, the ice crystals in the leaves thaw more slowly and cell membranes remain intact.
  • If there is a layer of snow on the leaves, it will insulate them and they will curl less or not at all.

Reading the Leaves

I had to test this on my own plants to see how they responded to temperature drops. I have a large hedge of Roseum elegans and several yakushimanums and wondered if the different species would act differently.

At 38°F (3°C) the leaves on my Roseum elegans are pretty flat and horizontal.

The temperature has dropped below 30°F (-1°C) and the Roseum elegans’s leaves have drooped down a lot and are starting to curl.

At 8°F (-13°C) the Roseum elegans’s leaves are tightly curled on sunny day.

The yakushimanum leaves are also rolled up tight on the 8°F (-13°C) day.

There are some species of rhododendrons that are not thermonastic, but all of the ones I have reacted to the cold in the same way.

Interestingly, curling happens whether or not the leaf is attached to the plant! Try bringing a curled leaf inside and see how fast it opens up.

Take some time to observe your own rhododendrons and see what they have to tell you about the temperature. I’d love to hear if it works for you!

See more winter weather lore and how to observe nature’s signs.

About The Author

Robin Sweetser

Robin has been a contributor to The Old Farmer’s Almanac and the All-Seasons Garden Guide for many years. Read More from Robin Sweetser