Spoiler I know this as I have my very own "rhododendron thermometer" right outside my bedroom window. Every morning when I wake up in the winter, I can get an idea of how cold it is outside just by looking out the window at my rhododendron. If the leaves are curled tight, I know it's really cold out, at least 20 degrees F or even colder. If they are curled just a little, I know it's not super cold out, maybe around 30 degrees or so. At around 40 degrees F, rhododendron leaves begin to droop but they don't start to curl until the temperature drops close to freezing. So a rhododendron will tell you how cold it is outside by how much the leaves curl.
I was going to make a guess... and if the bright ladies who have already answered are correct... I would have been correct also.
Spoiler Often on winter mornings I look outside, before I look at a thermometer, at the rhododendrons to give me a ball park idea of the outside temperature. So, yes, a person can get a good idea what is happening. At 35°F the leaves start to curl. As the temperature continues to drop the leaves curl tighter and tighter and eventually droop. An interesting natural thermometer. Jerry
Spoiler I have lots of rhodies, since it is our state flower. I know that sometimes the leaves droop, will need to see if that occurs when it is cold.
