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How To Tell Outside Temperature by Observing Rhododendron Leaves

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My Neighbor's Rhododendron

A mild afternoon in late February. The view I have of my neighbor's rhododendron from my kitchen window.

A mild afternoon in late February. The view I have of my neighbor's rhododendron from my kitchen window.

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.

Technically Speaking

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

"No way! It's friggin' cold out there, and you ain't goin' nowhere." This aspect of rhododendron leaves makes sure I never leave the house. These are the skinny cigarette aspects. They say to me that the temperature is well below freezing.

"No way! It's friggin' cold out there, and you ain't goin' nowhere." This aspect of rhododendron leaves makes sure I never leave the house. These are the skinny cigarette aspects. They say to me that the temperature is well below freezing.

"It's a nice day, leave the scarf behind." The leaves at the base of the plant are almost completely opened by warmth from the ground, while the leaves above are slightly curled and mildly pendent. This is a Fahrenheit 45- to 50-degree day.

"It's a nice day, leave the scarf behind." The leaves at the base of the plant are almost completely opened by warmth from the ground, while the leaves above are slightly curled and mildly pendent. This is a Fahrenheit 45- to 50-degree day.

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.

This is what I'm waiting for.

This is what I'm waiting for.

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:

Use Your Right Brain To Convert between Fahrenheit and Celsius


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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© 2012 Sally's Trove. All rights reserved.


Dianna Mendez on September 06, 2014:

I used to have these lovely bushes when I lived in the cool mountains. If I knew then what you have told me today, it would have made my outings much more exact in dressing for the weather.

Sherri (author) from Southeastern Pennsylvania on April 07, 2012:

RealHousewife, it sounds like you have had a wonderful day out of doors. The weather patterns have been odd, here in the northeast, too. Wonder how they will affect the rhodies. But I also think the rhodies will be a barometer, a way of telling us what is happening. Thanks so much for your awesome comment.

Kelly Umphenour from St. Louis, MO on April 07, 2012:

This is so neat! I am meeting a whole group of friends and family at the Zoo today here in St. Louis - they have a goregous natural preserved area "The River's Edge" we love to visit. I will be checking out the blooms now! lol

I wonder - with all these weird weather patterns we have had in the midwest - what we will see this year!

Up and very interesting!

Sherri (author) from Southeastern Pennsylvania on April 04, 2012:

Kwett, thanks so much for your awesome comment. I did go to school for landscape design, but never finished the program. I applaud your efforts and commitment in hort and ag and hope that you will be a voice that is heard as you find your place in this wonderful field. Life on earth depends on new research and young minds.

KnWettstein from United States on April 04, 2012:

I really like what you shared I am just finishing up a 4 year degeree in hort and ag plant sciences and love all the new things I am learning about plants. Thanks for helping a college girl expand her knowledge.

Sherri (author) from Southeastern Pennsylvania on March 28, 2012:, I love my rhodie friend. :) I am privileged in this way. Thanks so much for your good words.

Sherri (author) from Southeastern Pennsylvania on March 28, 2012:

2patricias, I wish you good luck with rhodies in the UK. That's not their natural environment. What is 'chalk' soil? on March 28, 2012:

Loved this! The rhodies are great plants to have around you and the one that has become your friend is certainly amazing! Thanks


2patricias from Sussex by the Sea on March 26, 2012:

Neither of us knew that! When Pat lived in London there was a rhododendron under her bedroom window, but that was a long time ago.

Both of us live in an area with a chalk soil, so a rhododenron planted in the ground will die within a few years (at most). However, we both have small specimans in pots. Now we will be looking out to see if they exhibit the same properties.

Thanks for an interesting and unusual hub.

Sherri (author) from Southeastern Pennsylvania on March 12, 2012:

Trish, thank you for your beautiful and rich comment. I am very lucky indeed to have you for a friend.

About your own statements not turning on that lightbulb, think of it this way: I get to see your granddaughter seldom, but when I do the first thing I notice is how tall she's become; although you know she's taller, her height doesn't stand out to you because you live with her and see only subtle changes day to day. We may fail to notice things about us that others do simply because we're so used to those things. So happy to tickle your brain. :)

trish1048 on March 11, 2012:

Love this! I am so fortunate to be privy to this first hand. In all the years I've had the pleasure of sitting on your porch, I don't recall the bit of information of its former grand size. Or, perhaps I've just forgotten it :)

It is truly amazing how you can look at or interact with something every day of your life, and not realize that therein lies a story. Maybe this will be the key for me to unlock my writing. I also noticed, after reading your hummus hub, that my thoughts were led to two words, which I won't reveal at the moment, because it may become my next hub :)

Actually, now that I think about it, all I really need is you! You have a way of tickling my brain. Many times I've been amazed, while conversing with you, how often you'll interject with a hey, there's your next hub! Why don't my own statements turn on that lightbulb and speak to me the way they do to you? I'll have to think on this :)

Sherri (author) from Southeastern Pennsylvania on March 07, 2012:

Eddy, not all rhododendron respond to the same degree to temperature changes. All of the research isn't in on this topic yet, so I do wonder what your mini will tell you. Whatever it does tell you, please share here or in a new hub. :)

Eiddwen from Wales on March 06, 2012:

Thanks for this gem and I will have to study my Mini Rhododendron;a great hub and I vote up.

Take Care And have a wonderful day.


Sherri (author) from Southeastern Pennsylvania on March 02, 2012:

FP, I thought that would get your attention. :) :) and puurrrrr.

Sherri (author) from Southeastern Pennsylvania on March 02, 2012:

RTalloni, I really appreciate your words about a slice of life piece with encouragement to write. Marcy's words and Anne's thoughtful patience were the kicks in the you-know-what I needed to get back in action. It seemed so appropriate to include my thanks to them in this hub. When inspiration and help come from someone else, it's good to say so, whether you're writing an article or a book.

You are in rhodie country, and any nursery there will have rock solid advice for planting and nurturing your first rhodie. Give it a little love from me. :)

Sherri (author) from Southeastern Pennsylvania on March 02, 2012:

robie2, in that wonderful micro environment you have there, I can imagine any rhodie would be thrilled and thrilling! And best is when it's doing well and it's your neighbor's responsibility. Thank you so much for the good words and the share. :)

RTalloni on March 02, 2012:

Well how about rhododendrons! Being from the sometimes unbelievably hot region of Central Florida I was not familiar with these plants, and still do not own one of my own. However, you may have just made a sale for one of our local Carolina nurseries.

There really is an endless supply of topics to write about, and the different approaches we could take for each topic can make one's head dizzy. The real problem is time to do them justice! You've done a neat job of wrapping up a two for one here. :)

Thanks for a nice slice of life piece with encouragement to write!

Feline Prophet on March 01, 2012:

Rhododendron wine?!! Now you're talking! :D

Roberta Kyle from Central New Jersey on March 01, 2012:

Now, this is so fascinating it makes me want to plant a Rhododendron bush er- on second thought -- it makes me want to watch my neighbor's Rhododendron very closely. Who knew? This is amazing and I love the pix too. Kudos ST, you have another winner. Voted up across the board and am off to share on Facebook

jenubouka on March 01, 2012:

"stop the world, I want to get off!" Now that is a worthy statement I am instilling in my mind for sure!!

Sherri (author) from Southeastern Pennsylvania on March 01, 2012:

@Kathleen Cochran, you are so fortunate to have these magnificent plants in your woods and gardens there in Georgia. Although I can't speak for the rhodies in Georgia, I know about them here where I am but also in North Carolina, where they live wild in the woods. My daughter taught summer camp there and took me on a magnificent trail ride through the NC woods around the camp. The rhodies were not in bloom, but their stature, their dominance under the canopy of large trees, was impressive. And thanks for sharing your experience in WA, much aligned with Steph's comment.

@AliciaC, I think you should plant one, too!

Sherri (author) from Southeastern Pennsylvania on March 01, 2012:

@jenubouka, I so share your thoughts about things to do away from the screen undermining the things you can do or could be doing on the screen. I get scattered easily, and when expansive (and very exciting) ideas fill my head, I just want to yell, "Stop the world, I want to get off!" Then it's time for a snack and a dive under the covers. Uhh, some call this procrastination. :)

@Gloshei, I'm glad you have rhodies in the garden. They are such a delight when they flower, aren't they? Thank you for reading and commenting.

Sherri (author) from Southeastern Pennsylvania on March 01, 2012:

@annemaeve, there's no one else on earth I'd like to tinkle ideas around with than you. This hub would never have come together without your expert editorial assistance and your saintly patience with your mother's ramblings all over the place. :)

@Feline Prophet, here's one for you about rhodies, from Wikipedia: "In the Indian state of Himachal Pradesh, Rhododendron flowers, have been used for some time to make popular fruit and flower wines." Who knew! Thank you as always, my friend, for your thoughtful comments. :)

@Dolores, exactly! Nice rhodie leaves=warm or at least warmer temps. Enjoy! Spring really is on its way. :)

Sherri (author) from Southeastern Pennsylvania on March 01, 2012:

Marcy, your hub was quite the inspiration for me. Have you ever read Lewis Thomas's "The Lives of a Cell?" Your hub and his book remind me of William Blake's "To see a world in a grain of sand..." The things underneath our noses reveal the greatest truths. Thank you so much for that reminder and for your good words.

Sherri (author) from Southeastern Pennsylvania on March 01, 2012:

Steph, thanks so much for the map and other info you shared with me on fb. And I didn't know the rhodie was Washington's state flower. You are right, it's quite telling, temperature-wise, that there are no rhodies for you to observe where you are in Oregon! Thanks for the awesome comment. :)

Linda Crampton from British Columbia, Canada on March 01, 2012:

Thanks for the very interesting - and useful - information. Describing how rhododendron leaves can tell us the temperature is a great idea for a hub! There are lots of rhododendrons in my area, but unfortunately not one that I can see from the windows of my home. I think I should plant one!

Kathleen Cochran from Atlanta, Georgia on March 01, 2012:

Here in Georgia we take our Rhododendrons for granted. After reading your hub, I will pay more attention to these beauties. I had a fabulous one at my quarters at Ft. Lewis, WA but that is a fairly mild climate compared to PA thanks to the Peuget (It's been too long to remember the spelling!) Sound factor.

Great info! Thanks

Gloria from France on March 01, 2012:

This is a great hub Sally thanks for sharing it. I too have a couple of Rhododendron bushes in the garden, not as big as your neighbours though. They are absolutely beautiful when they flower, I have bookmarked this to finish reading later.

jenubouka on March 01, 2012:

This is so awesome! I never knew that Rhododendron leaves responded so to the climate. I use my Lilac bushes to determine when spring is approaching as well as the tulip leaves pushing through the frozen earth. I think nature is the best at showing actual proof that the seasons are changing. I pay no mind to the crazy weather, we just got about 3 inches yesterday.

The bit for needing inspiration for writing was just up my ally as well, I can think of a million things to write, however when I come face to face to the screen, I then think of a million things I can do away from it.

Thanks so much for a wonderful and informative hub!

Dolores Monet from East Coast, United States on March 01, 2012:

Hi, Sherry - whoah, this was so cool! Just looking at my own rhododenron I was noticing the leaves looked so nice, but it was one of those warm winter days. Now I know what it all means! Voted up and awesome!

Feline Prophet on February 29, 2012:

There's something to learn from all your hubs, ST...and this one taught me what a rhododendron looks like! Makes me realise how ignorant I am about many things, and how much I take for granted! :)

annemaeve from Philly Burbs on February 29, 2012:

It was my pleasure to tinkle ideas around with you. Great job on this hub!

Marcy Goodfleisch from Planet Earth on February 29, 2012:

What an amazing thing to learn! We always hear how Indians and others who are much smarter about living as one with nature were able to tell things through observation, but I'd not heard about this telltale sign of temperatures. It makes sense, though, doesn't it? Thank you for publishing this, and thank you for the kind acknowledgment! You inspire so many of us here!

Sherri (author) from Southeastern Pennsylvania on February 29, 2012:

drbj, yes, rhododendrons talk to me, but I am not a skilled interviewer. I'd love for you to interview them. Just say the word, and I'll make the connection. Love your comment. ;)

Stephanie Marshall from Bend, Oregon on February 29, 2012:

I really love rhododendrons (the state flower of Washington, where I grew up). Here in Central Oregon, the temps are a little too cold for them to thrive, so maybe that in itself tells me something about the weather...LOL! Anyway, great hub and I especially like how you drew inspiration from your surroundings. Awesome! Best, Steph

drbj and sherry from south Florida on February 29, 2012:

Fascinating, Sherri. So rhododendron leaves talk to you? We're not so different after all. Undead celebrities and weird animals talk to me!

Thanks for this interesting info and excellent photos.

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