Why Do Rhododendron Leaves Curl in Cold Weather?
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.
What Is Thermonasty in Plants?
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 crocuses, 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 leaves occur the closer the temperature gets 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 Rhododendron Leaves
I had to test this on my own plants to see how they responded to temperature drops. I have a large hedge of Rhododendron catawbiense ‘Roseum Elegans’ and several R. yakushimanum plants, and I wondered if the different species would act differently.
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.
Frequently Asked Questions
Why do rhododendron leaves curl in cold weather?
Rhododendron leaves curl to reduce surface area and protect frozen cells from rapid thawing and sun damage during cold temperatures.
At what temperature do rhododendron leaves curl?
Leaf curling usually begins around 25°F (-4°C) and becomes tight below 20°F (-7°C).
Is leaf curling a sign of damage or disease?
No. Curling is a normal, protective response to cold and does not indicate disease or stress.
Do all rhododendrons curl their leaves?
Most do, but some species are not thermonastic and remain flat even in cold weather.
Should I protect curled rhododendron leaves?
Generally no. Snow cover actually helps insulate leaves and reduces curling naturally.
Have you noticed your rhododendron leaves curling and uncurling this winter? Watch closely—and tell us what your plants are predicting in the comments!
See more winter weather lore and how to observe nature’s signs.
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