Do you have Rhododendrons at home? If so, you have a remarkably reliable thermometer right there in your back yard that you may not have ever noticed. The ubiquitous Rhody is not just a pretty face- in the winter, its leaves can tell you a lot about the temperature outside.
At the first sign of freezing temperatures, the leaves will start to droop. On an ordinary, above-freezing day, the leaves will be almost horizontal to the ground, but as temperatures drop below freezing, they will start to point downwards, resembling shuttlecocks. As temperatures continue to fall, they will start to curl, until they are eventually rolled up tight, just as we all would like to be on a bitterly cold day.
The horticultural reason for this “thermotropic” movement is that when broad-leaved evergreens, like the Rhodendron, are exposed to cold, they protect themselves by rolling up. In this way, the underside of the leaf, where most moisture loss occurs, is out of the drying wind. Imagine that you are outside on a cold windy day – what do you do? You wrap your arms around yourself and huddle up to keep sensitive parts from being exposed to the cold. It’s not exactly the same thing as the Rhody, but the goal of self preservation is the same.
Regardless of the reason, looking at the leaves of a Rhododendron an excellent tool for us to gauge the temperature before we go out (or don’t.) So delete your thermometer app and plant a Rhododendron! Not only will it tell you when to bundle up, but it will bring you flowers in the spring. Is there an app that can say the same? (Don’t answer that.)