Their leaves can help show just how cold the weather is!
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.
Read Next
- 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.
Robin Sweetser
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I have loved and grown rhodies for decades... both in the NE and the Pacific NW.
Fanciers and gardeners have told me that rhodies curl their leaves in the cold partly to stop transpiration from the under surfaces of the leaves.... because cold air doesn't carry as much moisture and they cannot replace moisture lost without some warmth. So yes. They are very accurate thermometers as their lives depend on maintaining their moisture.
I've used my rhododendrens for years to deside how many layers of clothes I need to put on if I have to go outside. As a matter of fact, when my last outdoor thermometer gave up the ghost I didn't even bother to replace it.
Right now according to my ph app the temp is 16° and looking out the window even though it's a bright sunny day, the leaves on my rhododendrens are very tightly curled.
I don't know which species they are. They were already planted here when I bought my home. They get massive amounts of clusters of pink blossoms in the spring and the bumble bees adore them. The bushes come to life with all the buzzing. I always feel good when I walk by one and hear that. The bush on the side of the house has turned into more like a tree. It's now taller than the roof line. I love my rhododendrens.
I live in Canada and love the farmers almanac. I find the the articles so interesting and informative.
I regret not being able to purchase the GardenAnswers book as I was told they don’t mail to Canada. So sad. I do need garden help thanks Lou
