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Blue Is a Cool Color

July 26, 2012

garden blues2This phrase could mean a lot of things. It could mean that blue is a wonderful color to have in the garden (Cool!!). It could be referring to the color wheel, where blue is one of the three colors (with green and purple) on the “cool” side of the color spectrum. In fact, since blue is the only one of these three cool colors that is a primary color (both green and purple are created by mixing blue with a warm color), blue is the coolest color. But when I say that blue is a cool color, what I really want to focus on is air temperature and its relationship to the color blue in flowers.

zwanenburg blue 2011Summer in Maine can include quite a broad range of temperatures, sometimes in a single day. When we have “cool, dry Canadian air” coming from the northwest, overnight temperatures can get down into the forties (Fahrenheit), even in July. On the other hand, if a high pressure area off the coast circulates air from the southwest, we can have high humidity and temperatures in the 90s, or occasionally over 100. For a number of years now, I’ve noticed that the blue colors in my garden are particularly intense when the air is cool. I first noticed this phenomenon with the flowers of Tradescantia x ‘Zwanenburg Blue’. On cool mornings, I love to go look out at these as soon as I wake up because they are such an amazing electric blue. On a warmer morning, however, or later in the same day, the color often looks more violet. Once when I posted photos of this blue flower, a reader who gardens in the southeast United States (Alabama) commented that she had never seen blue flowers on Tradescantia in her area, that they mostly got purple ones. I now wonder if even this blue variety would not flower as blue in her warmer climate.

I found an even more dramatic example of this cool blue effect when I added Phlox paniculata ‘Blue Paradise’ to my garden. These flowers will start out blue in the morning (and the colder the temperatures, the more intense the blue) and turn a bright hot pink by mid-afternoon. The first time I noticed those hot pink flowers at the back of the garden on a summer afternoon, I was flabbergasted; I knew I hadn’t planted any pink flowers in my Blue and Yellow Border! The three photos below show the color of these flowers (1) on a recent morning, with temperatures in the low fifties, (2) by late morning of the same day (with the temperature about 70F; note that the flowers are now purple), and (3) in the mid-afternoon when the temperatures were about 80F and the sun was shining on the flowers.  The flowers are distinctly pink in the last image – and this was not a particularly hot day; the pink would have been much more intense on a day when temperatures soar up into the nineties.

blue paradise morning blue

blue paradise afternoon pink


So what is going on here? Have others also observed this phenomenon? And, more importantly, why does it happen?

My first attempts at research, in botany textbooks, turned up nothing on this color change. Some on-line research, however, quickly showed me that flower color variation in response to ambient air temperature is a well-documented garden phenomenon; one can find discussion of it in on-line garden forums, on nursery websites, and even in a few scientific papers. I found information on color variation in response to temperature in flowers of asters, chrysanthemums, orchids, hibiscus, plantago, plumeria, and roses.  Interestingly, while most of these sources focused on the ways that flower color is more intense in cold temperatures or on how the same varieties (e.g., of roses) bloom in darker shades in colder climates, some (e.g., the discussion of Plumeria on  The Garden Web) focused on the way colors become more intense in the heat. I can see both effects in my ‘Blue Paradise’ phlox; the morning blues are more intense at colder temperatures, but the afternoon pink becomes more intense as it gets hotter.

To make sense of this, we need to consider the chemicals that create color in flowers. According to Raven’s Biology of Plants (8th edition, W.H. Freeman and Company), the presence of chemicals called carotenoids are often responsible for red, orange, and yellow pigmentation in flowers, but another set of chemicals, the anthocyanins, are the most important determinants of blue and red flower color. The clearest explanation I found of how these chemicals interact with air temperature was at the web site of the California company, Hidden Valley Hibiscus (but I couldn’t find any information on the site about who wrote this page, so I don’t know how trustworthy the information is). Most sources agree that plants produce more anthocyanins at lower temperatures, making colors dependent on anthocyanins (like my blue flowers or like pink roses that bloom a deeper pink in colder climates) more intense when it is colder. According to Hidden Valley Hibiscus, however, the carotenoids behave very differently. These chemicals are more stable in flowers so they do not vary as much as the amount of anthocyanins; but to the extent that carotenoids do vary, plants produce more of them in hot weather.  This means that flower colors that depend on carotenoids will be more intense in the heat. This seems a likely explanation for the more intense pink color of my phlox on hot afternoons.

So it’s true: My blue flowers really are bluer when the air is colder, and there is a scientific explanation for why that is the case. Cool!!

[A disclaimer: This post is very much an amateur’s attempt to understand these phenomena. While I have made use of my professional skills in researching, synthesizing and summarizing information, I have no botanical or horticultural training. I hope those who are more knowledgeable than I am will leave comments correcting any errors in my understanding and interpretation.]

44 Comments leave one →
  1. July 26, 2012 9:37 pm

    I know that oranges and reds definitely seem to become more intense in the summertime heat here. They really do seem ‘hot’ in our wicked summer sunshine. It’s certainly an interesting topic. Your Phlox certainly seems bluer in that top photo.

    • July 28, 2012 9:38 pm

      Bernie, If my information is correct, the oranges and reds in your tropical flowers are pigments due primarily to carotenoids, and they are indeed more intense in your wicked summer sunshine. My blue/violet/pink phlox provides a particularly dramatic example of the air temperature effects.

  2. July 26, 2012 9:45 pm

    I enjoyed your post. My Blue Paradise is exactly the same. I had noticed that sometimes it is blue (it was quite blue this morning) and sometimes violet. I hadn’t noticed the pattern, but after reading your post, I realize it is what you describe, temperature related. I think my cranesbills are the same and now must pay better attention to Brookside. I wonder about Clematis Prince Charles and must make better observations.

    • July 28, 2012 9:41 pm

      Harriet, I’ve been wondering about Brookside, too. I have it planted in several places in my garden, but in one flower bed it’s near a closely related cultivar (sibling? first cousin?) called ‘Nimbus,’ which is more of a violet/magenta color. On some days, I have trouble telling Brookside and Nimbus apart; on other days, it is very clear to me that Brookside is a distinctly different (blue) color. Maybe next year, we can both make observations of this plant and see what we come up with.

  3. July 26, 2012 10:59 pm

    How very interesting, and a good explanation why true blue is not so common here, but rather always has a purple tint!

    • July 28, 2012 9:45 pm

      Deb, It had never occurred to me before I started looking into this that blue flowers were an advantage to gardening in a cold climate (but see Shirley’s counter-example below).

  4. July 27, 2012 1:52 am

    How fascinating, Jean! I think there is one other factor which enhances the ‘chemical’ explanation, and that is the quality (including colour) of the light itself. Mostly high temperatures are associated with stronger sunlight – and thus redder/warmer light, with the opposite being bluer/cooler light. I shall definitely be on the lookout myself now for more specific examples in my own garden. Jack

    • July 28, 2012 9:55 pm

      Hi Jack, Good point! There are so many variables in color perception, from the pigments in the object being perceived, to the characteristics of the light (and the medium through which it is passing), to variable characteristics of the eyes/brains perceiving the color. One of the issues I’ve been grappling with here is how relative humidity affects color perception. Colder mornings here correspond to lower humidity levels, and I know that the sky looks bluer when humidity is low. Could this be part of the reason the blues look bluer in flowers, too? I did test out the humidity hypothesis in relation to the phlox by watching the color transform from blue to pink on a very dry, but also quite warm day.

      All of this has me wishing I’d applied myself more in my high school and college science classes. 😉

  5. July 27, 2012 6:37 am

    I have lots of phlox but no blue ones. I’ve got to get one and hope for that beautiful array of color. Fascinating to watch how perennial plants adapt and thrive in various weather conditions. Great post.

    • July 28, 2012 9:57 pm

      Judy, Blue Paradise is definitely a phlox worth having (see Eileen’s comment below); and I would think your growing conditions in New Hampshire would be pretty similar to mine in Maine with the same range of color variation.

  6. July 27, 2012 12:45 pm

    Very interesting post. I have to admit I’d never really thought about this before but you are absolutely right. Photos I’ve taken of flowers on cool mornings do have a lot more “blue” in them than those in the bright afternoon (i.e. hotter) sun. Will have to try to find Blue Paradise phlox. Looks like a good one.

    • July 29, 2012 9:08 pm

      Karen, Blue Paradise is a beautiful phlox (see Eileen’s comment below about its distinguished provenance) — especially if you have summer mornings cold enough to get those true blue colors.

  7. July 27, 2012 8:11 pm

    This is quite amazing. I’ve never noticed this before but will certainly take a look at some of my flowers in future. I was really surprised at the obvious difference in your phlox, there’s no denying that’s a complete colour change.

    • July 29, 2012 9:11 pm

      Marguerite, The color change is particularly dramatic in Blue Paradise phlox; it got my attention and got me to look more closely at some of my other flowers. I think it helps here to be a morning person, because I’m likely to be out in the garden at the time of day when temperatures are at their coolest.

  8. July 27, 2012 10:27 pm

    Last year I noted that my Russian Sage bloomed bright, intense blue instead of the paler lavender that is typical of the plant. It was our hottest and driest summer on record and the nights did not cool as usual so in this case at least it was not cool air. Another local blogger commented that it was possibly a result of our clear, cloudless skies with even more sun. That’s another option to consider.

    • July 29, 2012 9:14 pm

      Shirley, It’s really interesting to have this counter-example. I’ve been wondering about the effects of humidity on color perception — but it’s hard to test here because our overnight lows typically get down to whatever the dew point is, so low temps are almost perfectly correlated with drier air. Was your hot summer dry in the sense of low humidity or just dry in the sense of drought/no rain?

  9. July 28, 2012 3:58 am

    Fascinating findings indeed. The colour changes in your Phlox are very obvious. I like blue flowers. Blue flowers dry away my blues!

    • July 29, 2012 9:15 pm

      Autumn Belle, I love blue flowers, too; it’s nice to garden in a climate where those blues show up so brilliantly.

  10. July 28, 2012 6:57 am

    Jean, this is a great phlox, introduced by Piet Oudolf in 1990, first and last to bloom in my garden and has a lovely fragrance. I have found to remain upright for the season it requires some support. It seems to be the heat of midday that promotes the color change.


    • July 29, 2012 9:19 pm

      Eileen, Thanks for the information about the provenance of this cultivar. Mine has taken a while to get established, and this is the first year when it’s had more than a couple of stems. I was noticing the lovely fragrance yesterday when I was lifting the flowers up off the ground after they were knocked down by overnight rain. Next year, I’ll give them some support.

      I agree that the pink coloration intensifies in the heat of the midday, but the degree of blue is different. For the past couple of days, we’ve had humid air and overnight lows in the sixties; when I get up in the morning, the flowers are a blue-violet color much closer to the second picture than to the first. Those really intense blues seem to require temperatures below 55.

  11. July 28, 2012 8:22 am

    Gosh, I don’t think I’ve ever noticed change in flower colour with temperature but I don’t have the range of plants you do such as the phlox and tradescantia. The photos of the flowers changing colour really show up – I thought they had to be different plants and re-read that part again and look closer!

    • July 29, 2012 9:21 pm

      Sunil, The color change in the phlox can be very dramatic. I know what you mean about thinking they must be different plants. You can imagine me the first time I spotted those hot pink flowers, thinking, “What’s that? I didn’t plant that!” But I did plant it :-).

  12. July 28, 2012 12:43 pm

    Jean I had thought my bluer blues were such in the morning due to light…but I can see the bluer blues in spring when the temps are coller. i see less deep blues in summer and I am sure this is why and why in tropical climates they do not have many blue flowers…fascinating and I will have to watch my phlox…

    • July 29, 2012 9:23 pm

      Donna, I’m happy to have the corroboration about the bluer blues in cool temps. None of the on-line discussions I found of this color change phenomenon focused on blue flowers. I’d also be interested in knowing if other blue phlox cultivars change color as dramatically as Blue Paradise.

  13. July 28, 2012 9:15 pm

    Jean, I certainly have noticed this with my roses, as mine here in London flower all year round and some of them have quite different colour depending on which season we are in. I have tried to think if I have any true blue flowers, but the only one I have are crocuses! My garden is more on the warmer colour chart – trying to make up for what we lack in air temperatures at times!

    • July 29, 2012 9:24 pm

      Helene, Several of the on-line sources I found focused on roses, and particularly pink roses that are darker in cooler seasons and/or cooler climates.

  14. patientgardener permalink
    July 29, 2012 12:59 pm

    How fascinating. Its not something I have noticed but I dont think we have such extremes of temperature in such a sort period in one day here in the UK. Thanks for sharing

    • August 1, 2012 7:00 pm

      Helen, I’m glad that other gardeners find this as fascinating as I do. I would think that the UK would tend toward a maritime climate with fewer temperature extremes. I live inland in Maine; the coastal areas tend to have somewhat more modulated temperatures.

  15. July 30, 2012 2:32 pm

    The minute I started reading your post before you even mentioned it, I thought of my Blue Paradise phlox. I had noticed this phenomena and thought it was the lighting but couldn’t understand how the change could be so dramatic. Thanks for explaining what happens so well. Blue Paradise is the only true blue phlox of which I am aware, but then my customers look at it at certain times of day and say: “But this is purple.”

    • August 1, 2012 7:04 pm

      Carolyn, I’m not sure I ever would have noticed this color phenomenon if I hadn’t planted Blue Paradise in my garden. As you say, the change in color is so dramatic, you know that you’re not imagining it and that it’s not just a matter of the quality of light. Having become aware of it, though, I’m now noticing temperature-related color changes in other blue flowers, including tradescantia and platycodon. And like Harriet (above) I’m going to try to observe Geranium ‘Brookside’ more systematically.

  16. July 30, 2012 4:27 pm

    Jean, we have wild pink phlox growing here in the mountains. Just from casual observation, I think the intensity of the color changes depending on the altitude. Also as a rose judge, I can tell you weather conditions can have a huge effect on color. You might have the same two roses side by side and they can look very different depending on the climate where they were grown.

    • August 1, 2012 7:07 pm

      Lynn, Thanks for adding more data to our informal set of observations here. I wonder if the altitude effect works primarily through the mechanism of temperature (I’m assuming that higher elevations are, on average, cooler) or whether there is a separate effect of altitude. Any thoughts on this?

  17. July 31, 2012 8:23 am

    Hi Jean, I have not noticed a shift in color in any of my flowers (and phlox in particular), but I will watch out for this interesting phenomena after reading your post.

    • August 1, 2012 7:08 pm

      Hi Jennifer, If you do notice changes in the future, do let me know. I’ve become quite fascinated by this whole topic.

  18. July 31, 2012 11:47 am

    Jean, this is fascinating. You’ve also just rehabilitated one of my favorite nurseries in my eyes. They’re based in Santa Fe and bill their West Texas grass sage (Salvia reptans) as having cobalt blue flowers, but in my garden they have regular old purple ones. I’d been rolling my eyes at the exaggeration in their catalog, but between the temperature difference from here to Santa Fe and what Lynn said above about the altitude, that would explain it.

    • August 1, 2012 7:11 pm

      Stacy, Your nursery example is very interesting. I can imagine how a nursery that sells to a clientele from different climates from their own could find itself having to explain why the color of the flowers is not what they advertised!

  19. August 3, 2012 8:53 pm

    There’s no cooler colour than blue! Love it. Sorry to be out of the blogging picture, but I’ll be back in the winter. Just not able to find time this summer. Happy gardening, Jean.


    • August 3, 2012 10:32 pm

      It’s good to hear from you, Diane. It’s much more important to spend the garden season in the garden than in the blogosphere.

      I remember that you also grow Blue Paradise phlox; I hope you are enjoying it. After I had taken these pictures, mine got hit with a deer/woodchuck double whammy. First the deer came through and nibbled off all the flowers; then the woodchuck came through during the night, broke all the stems by pulling them down and then stripped off all the foliage. Ah, the trials and tribulations of gardening.

  20. August 4, 2012 10:10 pm

    Jean – it would have never occurred to my that blooms would change color, like a “Mood Ring” (…I know – I’m dating myself again!). “Blue Paradise – what a cool phenomenon.

  21. July 22, 2016 10:20 am

    Thank you Jean for a very pertinent article! I live in Hampshire, UK, we have had a heat wave this week with temperatures in the 90s. I have a blue phlox in flower and realised it changing colour through the day, just as you describe. At first I thought the variation must be due to changing light levels but then realised that the only explanation was that it was the temperature that was causing this phenomenon. We Googled ‘Do phlox change colour?’ and found your very informative post. Thank you!
    Your yellow and blue border is beautiful.


    • July 22, 2016 8:39 pm

      Pam, I’m so glad you found this helpful. We are also having hot weather this week, and I’m watching my ‘Blue Paradise’ phlox’s dramatic color changes each day. I do think there is more to this color story than I have yet gotten a handle on. What role does humidity (that, at least here, tends to vary with temperature) play in color perception? Why do some blue flowers change color so dramatically while others seem to be much more stable in color? I’m currently reading Nature’s Palette: The Science of Plant Color by David Lee (University of Chicago Press, 2007) and hoping that it will give me a deeper understanding of the color change phenomenon.

  22. Jan permalink
    June 26, 2019 3:26 am

    Jean, you have saved my sanity today! I live in Wiltshire and planted ‘Blue Paradise’ in my garden about three weeks ago. The first buds opened yesterday and I was so disappointed because the flowers were pink. Lovely, but not the blue I wanted and expected. Then I went out in the cool of this morning (yesterday was warm) and, lo and behold, those same flowers were blue. I honestly thought I was going nuts or had an eyesight problem. Dashed to the computer and, thankfully, found your blog. There is a real case for changing the name of this plant to Phlox paniculata ‘Gaslight’. Or just Phlox panic, perhaps. 🙂

    • June 26, 2019 10:25 am

      Jan, I’m happy to be of help. I remember my own outraged reaction when I first noticed that hot pink flower blooming in my Blue and Yellow Border: “Who planted that there?!?” Turns out I did. 😉


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