Winter chill
We’ve had a great winter season on the West Coast but winter did arrive in early February with temperatures below zero for a few weeks. It made regular landscape work difficult and it covered landscape plants in frost. And I love the look of frost-covered plants, which is why I recommend that people not rush perennial and ornamental grass cutback.
Then my crew mates started wondering out loud if the rhododendrons in our landscapes were alright. The leaves were curled up and drooping, as if they were wilting. Now what? Is it time to panic and start planning shrub replacements?
Normal response
It turns out that rhododendrons curl up their leaves into tubes every winter; the lower the temperatures, the tighter the leaf ‘tube’. It’s nothing to worry about. Your shrubs should be ok.
But this habit leads to questions: why do rhododendrons curl up their drooping leaves? I found the answer in Steven Vogel’s book “The life of a leaf“.
This is how I understood Vogel’s explanation. The temperatures near the leaf surface are colder than the surrounding air. So the shrub points its leaves downward as if they are wilting. Then, depending on actual temperatures, it curls up its leaves into tubes. The colder it is, the tighter the tube is.
Now, since colder air is denser, it moves downward through the leaf tube, and as it does, it draws new air in from the top of the leaf tube. This air is warmer than what’s near the leaf surface, thus warming up the leaf. Vogel refers to this mechanism as ‘anti-chimney‘.
When temperatures rise, the leaves perk up again and move to their usual horizontal positions. No harm done, every gardener hopes.
Leaves
Leaves are amazing factories. That’s why Steven Vogel was able to write a whole book on the life of a leaf. And this also reminds about how much there is to learn about plants.
If your rhododendrons are drooping with curled up leaves, don’t panic. It’s a response to cold temperatures. Anti-chimney response!
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