Baby, it’s cold outside!
How cold? you ask.
So cold that my rhododendrons have fainted.
As I was looking outside the window of my home office on a particularly frigid January day, I noticed how the leaves of my rhododendrons were all droopy, like a collection of small umbrellas. Over the next couple of hours, the leaves started curling inwards; and by late afternoon, they were hanging like weird large green beans.
I’m not particularly good at reading nature signs, but I knew this meant it was very very cold outside. It might just be the winter equivalent of that summer temperature gauge when you break an egg on the sidewalk during a heatwave just to see if you can fry it.
It wasn’t the first time I noticed this happening to my rhododendrons, so I knew exactly what was going on. But it was the first time I thought, hmm, maybe I should tell others, just to avoid the “help, my plant is dead” panic, especially for our readers who are new to growing rhododendrons.
Why are rhododendron leaves curling inwards in the winter?
This reaction to below zero temperatures is called thermonasty.
Thermo obviously indicates temperature, while “-nasty” stands for “nastic movement, aka a plant movement in response to a stimulus. So thermonasty is a movement in response to temperature.
But why does it happen in rhododendrons?
One of the best features of rhododendrons – and why they’re an excellent shrub choice for any garden – is the fact that they’re evergreens. Their foliage will stay the same color whether it’s the middle of winter or the height of summer.
However, there’s a catch:
In the cold season, rhododendrons don’t need as much light for photosynthesis. And this process slows to a halt when the temperature dips below freezing.
But rhododendrons still have their canopy of leaves on, which will still act like solar panels, even when the plant has no use for solar energy. If the plant absorbs this solar energy, it will heat up and thaw. Then it will freeze again at night when the sun is gone. Then heat up and thaw again the following day. And so on. This diurnal freeze-thaw cycle would be very damaging to the cellular structure of the leaves.
So in order to protect themselves from absorbing too much light when the temperature is too low, the leaves curl in on themselves. The less surface exposed to light, the higher the chances of surviving the freeze unscathed.
In addition to this, let’s remember that rhododendrons, in their native habitats, grow as an understory – one layer below the tall crowns of trees. They don’t need as much sunlight. In the cold season, there is no tree canopy to protect them from the sun. So they need to protect themselves.
In other words, they don’t curl up to stay warm – like people do – but to stay shaded.
The curling of the rhododendron leaves is getting worse!
It may look like it’s going from bad to worse, but it’s just a natural adaptation to how cold it is. The nastic movement in rhododendrons is not an immediate flop, but a gradual movement that starts with the umbels lowering (like the closing of an umbrella) at about freezing temperatures (32F or 0C).
When the temperature reaches about 25F (about -4C), the individual leaves will start to curl inwards, like little tacos. Then at around 20F (around -7C), the leaves will be fully rolled up.
A positive side-effect of this response is that it also makes it harder for ice and snow to accumulate on the leaves. Since the leaves are both pendant and convex (not to mention naturally glossy), the snow slides off faster than it would off a flat horizontal surface.
Will my rhododendrons recover after winter curling?
Good news! Yes, they will recover. As soon as the temperatures climb again, the rhododendron leaves will uncurl. That’s because the thermonasty is a reversible leaf movement. Once the stimulus is gone, the leaves stop responding and go back to their usual shape.
Will rhododendron leaf curling affect flowering?
Another piece of good news! No, the curling of the leaves due to low temperature will not have a noticeable effect on future blooms.
However, there may be other reasons why your rhododendrons aren’t blooming. Have a look at this article, in which I explain in more detail the most common reasons.
Are there any other plants that curl in cold weather?
Have a look at how the leaves of my camellia shrub were curling when the thermometer dipped to below freezing. Just like rhododendrons, camellias are evergreens and grow as an understory in their natural habitat.
It’s the same mechanism at play, although there are more studies about rhododendrons. (Seriously, just type “thermonasty in rhododendrons” in Google Scholar and you’ll be amazed at the number of studies done.)
And just like with rhododendrons, the curling of the leaves didn’t affect the subsequent blooming of the camellia.
Which is even more surprising, since camellia shrubs bloom much earlier in the year than rhododendrons. Have a look at the beautiful flowers opening up less than a month later on the same shrub. Isn’t nature amazing?
So to sum up, the rhododendron leaves curling in the winter due to cold temperatures is just nature’s way to adapt. Just think of it as one more proof that the garden is still alive and transforming during the cold months.