Last week while I was off, this question came in from Gloria Coldwell, who lives in Nova Scotia’s Annapolis Valley: “Have you ever checked how cold it is outside by checking the leaves of the rhododendron shrub?”
Gloria, I do have a lovely rhododendron shrub in my yard and I watch it closely for that very same information. You can indeed gauge the temperature by the curl of the rhododendron leaves. There is a correlation between the width of the curled-up leaf and the temperature: the lower the temperature, the tighter the curl.
The curling of the rhododendron leaves is an example of thermotropic movement, or movement in response to temperature. Researchers found that rhododendron leaves freeze completely at temperatures below – 9 Celsius.
When living tissues freeze, ice crystals build up to the point that they can rupture cell membranes. This is only exacerbated if the tissues thaw out quickly. If you’ve ever tried to freeze and then thaw leafy vegetables, you know the best way to do so is to freeze them quickly. That reduces the size of the ice crystals that can form. To bring them back you should then thaw slowly.
Experiments were able to demonstrate that flat leaves thaw much more rapidly than curled leaves. This is because a curled leaf exposes far less surface area to the warming sun than does a flat leaf. Curled leaves don’t thaw out as fast and therefore are able to avoid much of the damaging effects of daily freeze-thaw cycles.
Have a look:
If the leaves are rolled up like a cigar, the temperature is near – 4 C. Pencil width?
It’s closer to – 8 or – 10 C.
Gloria says she doesn’t have an outdoor thermometer but doesn’t need one because she has a rhododendron.
Nature truly is remarkable.