How To Tell Outside Temperature by Observing Rhododendron Leaves
My Neighbor's Rhododendron
I hate cold weather. The sight of snow chills my bones, the short days of winter make me want to hide under covers, and when I hear winter's freezing temperatures reported on the Weather Channel, I want to move to southern California. I certainly don't want to go outside, not even to read the outdoor thermometer mounted near my kitchen door. Fortunately for me, a few years ago I learned that I didn't have to turn the TV or radio on to know the outside temperature and I didn't have to step outside to watch my outdoor thermometer's red indicator dip below freezing. Instead, I learned to read the rhododendron leaves I could see outside my kitchen window.
Now, every morning throughout the winter and into spring, after waking up, pulling on a fleece bathrobe, stepping into my slippers, and shuffling my way to the electric coffee pot on the kitchen counter, I look outside my kitchen window toward the huge rhododendron plant growing against my neighbor’s porch to read what it has to tell me about the cold. I’ve learned its language of changing leaf shapes and angles, a language that tells me how dreadfully cold it is or isn’t outside.
The Rhododendron Outside My Kitchen Window
The impressive rhododendron outside my kitchen window is at least 35 years old, having been planted as part of the original landscaping plan when my town home community was built. Up until five years ago, it stood more than 25 feet high and its branches spanned perhaps 20 feet at its base. After its pruning five years ago, it is still impressive, although now it is about two-thirds its original mass. Rhododendrons this large are uncommon where I live, so I must assume that the original landscaper knew what he was doing when he chose the site for this magnificent specimen. The plant is protected from prevailing winds and receives only a few short hours of direct sun in the late afternoon. As far as I know, it hasn't been fertilized in the 15 years I’ve lived here, although it does get a new coat of mulch each spring. Up until this past autumn, it had been completely disease free but is now showing signs of stem dieback. I am hoping the current owners will address the problem. Not only is this evergreen shrub stunningly beautiful, whether in flower or not, it is also my gauge for estimating the outside temperature. I’d hate to see it die.
Rhododendron leaves curl in response to their own temperature, not in response to ambient air temperature. Of course, when the air is cold, the leaves are cold; however, snow covered leaves may show less curl than bare leaves because of the insulating property of snow. And, just to add another layer of complexity, the drooping aspects of leaves are responses to water availability in the soil and air as well as to ambient air temperature.
Rhododendron Leaves Respond to Temperature Changes
Rhododendron leaves curl up into tight, former aspects of their fully-fleshed spring and summer forms at cold temperatures. The colder the temperature, the tighter the curl. This process is called thermotropism. It is believed that this process gives a plant certain survival advantages under harsh conditions.
For an informative discussion of thermotropism in rhododendron, including the history of thermotropic theory and the physiology of rhododendron leaves, read Why Do Rhododendron Leaves Curl by Erik Tallak Nilsen. His article poses six theories about why thermotropism exists in rhododendron and is beautifully written for anyone, scientist or layman.
Reading Temperature from Rhododendron Leaves
The key to reading temperature from rhododendron leaves is observation. I've been observing rhododendron leaf changes for years, and I’m getting better and better at interpreting what they mean.
In the early years of observing the rhododendron outside my kitchen window, I noticed that leaves curled up into tight, skinny cigarette shapes when the temperature was well below freezing. As time went on, I saw that sometimes the leaves took on cigar shapes, a bit fatter than skinny cigarette shapes. As you can guess, the cigar shapes indicated temperatures warmer than well below freezing. Much later, I realized that leaf shapes were not the same across the entire plant. For example, at the base of the plant, closer to the ground, leaves were more expanded than their relatives higher up who were exposed to colder air than that emanating from the ground. Little by little, I began to understand these differences in leaf shapes and relate them to outside temperatures.
Keep in mind that my goal through these years of observation was to have the rhododendron tell me whether I should go out into the cold or not, or how many layers of clothing I'd have to don if I had to go out. The rhododendron came to be a friend, a much more intimate and interesting friend than the Weather Channel.
Rhododendron Leaves Speak to Me
This Is What I'm Waiting For
The rhododendron will be telling me, "Now you can wear the shorts and tank top!"
The leaves in full sun will be fully open and the rhododendron will be bursting in bloom. It will be late May and about 70 degrees Fahrenheit.
You Might Be Asking...
"Why don’t you just turn your outdoor thermometer around so you can see it from inside the kitchen window?" I suppose my temperature readings would be more accurate, but they wouldn’t be nearly as fascinating to take.
Need Help Converting Fahrenheit and Celsius?
Some have no trouble converting Fahrenheit to Celsius and the other way around, and some do. If you are a right-brain learner, and the conversion formulas haven't stuck in your head yet, you might want to try this visual approach:
Lately I'd been feeling a bit blocked about my writing; coming up with ideas had become problematic. Just a few short days ago I read Marcy Goodfleisch's hub, Need Inspiration to Write? There are tons of ideas, right under your nose!, and realized that my problem was a simple lack of paying attention to my surroundings. Thank you, Marcy, for your inspiration. Your suggestions led me directly to and through the rhododendron, a writing journey I really enjoyed.
While being in that temporarily idea-barren state, I was also unable to focus clearly on the scope, logic, and purpose of any piece of writing I had already in progress. No matter how I tried to limit a topic to something manageable, too many ancillary thoughts kept filtering through, essentially creating a mountain out of what should have remained a molehill. Thank you, Annemaeve, for helping me narrow the scope of this hub about rhododendrons to focus on what really mattered.
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