Blue Is a Cool Color
This 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.
Summer 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.
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.]