By Phran Novelli

PHILADELPHIA (CBS) – How cold has it been? Well, if you live in a virtual cave with no weather reports or thermometers, but happen to have a rhododendron just outside your door or cave opening…look at the leaves.

Evergreen rhododendrons are often thermotropic, which means their leaves are temperature sensitive. While you’ve probably noticed other plants, like hydrangeas, wilting in the summer sun, many rhododendrons lower their leaves in cold winter temperatures.

Around 40 degrees rhododendron leaves are fairly flat and horizontal, below freezing, they droop to become vertical, and down around 20 degrees Fahrenheit or so, they’re curled up tight so they look like green pencils hanging on the shrub. Below that you need a real thermometer, but honestly, below 20 degrees, never mind the exact number, put on a hat and coat already – it’s COLD!

If you don’t have a rhododendron nearby, you can see what I’m talking about in the photo.