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Rhododendron, Nature's Winter Thermometer | Almanac.com

Rhododendron, Nature's Winter Thermometer

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Robin Sweetser
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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 this month from 40 degrees above to teens below zero 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 and move according to temperature change. Many desert plants have this quality as do flowers including tulips or crocus that close up at night or on cold rainy days. When the temps are above 40, 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 degrees. It is thought that lack of water in the stem causes this.
  • Curling starts to be noticeable at 25 degrees.
  • They are very tightly curled by the time the temps have dropped below 20. Since the leaves actually freeze at 18 degrees, it is 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.

Read 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 degrees the leaves on my Roseum elegans are pretty flat and horizontal.

The temperature has dropped into the 20s and the Roseum elegans leaves have drooped down a lot and are starting to curl.

At 8 degrees the Roseum elegans leaves are tightly curled on sunny day.

The yakushimanum leaves are also rolled up tight on the 8 degree 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.