One of the most fascinating, albeit frustrating, characteristics of hibiscus is the way the flower colors change. A few hibiscus produce the same colors at all times of year in every kind of weather. But the vast majority of hibiscus change their colors with changes in temperature, hours of daylight, and who-knows-how-many other variables! In this article we will explain at least some of the color changes that hibiscus go through. Hibiscus flower colors, like the colors of most flowers and fruits, are made up of three basic pigment groups: carotenoids and two types of flavanoids - anthocyanins and flavonols. While the study of flower pigmentation can be very complex (and confusing!), some fairly simple principles emerge that we can apply to growing our own hibiscus.
Carotenoids ~ The Yellow, Orange, Red Spectrum
Carotenoids - think carrots, pumpkins, corn, yellow squash. Beta-carotene, lycopene, zeaxanthin, and violaxanthin are examples of the carotenoid pigments, and yes, these are the same carotenes we take in our daily vitamins! This is one of the reasons why hibiscus are being studied for their human health benefits.
Carotenoids Hold their Color
Carotenoids are the most stable of the flower and fruit pigments. They are enclosed in their own little compartments, called "plastids," nestled inside the cytoplasm of individual plant cells, out of reach of many of the substances that a plant absorbs. Pesticides don't reach them, nor do most of the nutrients and toxins plants absorb from the soil or air. When damaging substances do manage to get to them, these carotenoids have a second line of defense in their anti-oxidant actions that protect the plant even further. So the carotenoid pigments last and last. Whatever color the flower opens with, it maintains that color until well after it folds back up. We see this in hibiscus. Our bright yellow and orange flowers tend to hold their color, even through the hottest summer days.
Hot Weather Increases Carotenoids
Anthocyanins: The Blue, Purple, Pink, Red, Black Spectrum
Anthocyanins are best known as the red pigment in fall leaves. We all know that fall leaves change color in response to weather conditions, and this is the hallmark of anthocyanin pigments. Anthocyanins are plentiful in hibiscus, giving the flowers their many showy bands of color, as well as their tendency to bloom with different colors depending on many different variables.
Anthocyanins are Unstable
Anthocyanins are a much different kind of pigment. They are much less stable than carotenoids. Anthocyanins aren't safely enclosed in plant cells. They are created in the roots, dissolved in the water that makes up plant sap, and move all the way up the plant to where the flowers develop. The least little change in the environment or health of the plant affects the sap, which means it also affects the anthocyanins. This is why blues and purples tend to be so changeable in hibiscus flowers - they are produced by the notoriously unstable anthocyanin pigments. A single anthocyanin pigment can be deep blue or deep red or anything in between, depending on many different variables. One variable we understand to some extent is pH. The levels of alkalinity and acidity in flowers change the color of anthocyanins from the blue ranges to the red ranges. Gardeners use this knowledge to change the colors of hydrangeas between pinks and blues by changing the soil pH. This means we should be able to predictably change the color of a hibiscus flower by changing the plant's pH then, right? Don't we wish! Many of us have tried this, but unfortunately, pH in hibiscus is dictated almost entirely by genes. Within a single flower, there will be pockets of pigment with different pH levels, creating different colors from the same anthocyanin pigment.
Anthocyanins Increase and Turn Red in Response to Cold
Nature does do some things that alter pH and other anthocyanin characteristics of plants though. Anthocyanins function as a sort of anti-freeze chemical in plant sap. In response to dropping temperatures, plants produce more anthocyanins, and the anthocyanins become redder. For some reason, redder anthocyanins seem to have a more protective anti-freeze effect in plants. This happens in maple leaves, and it's the same mechanism in hibiscus flowers. Dry weather increases this darkening and reddening effect even more, as we see sometimes when mid-summer drought causes tree leaves to develop fall colors early. But it does take a certain amount of sunlight to create anthocyanins, so fall weather, with bright sunny days, less rain, and cold nights, is the perfect weather to maximize anothocyanins, to turn leaves their brightest reds and oranges, and to increase and darken the reds and pinks in hibiscus flowers.
'High Voltage' is a good example of this effect. In the hottest summer, the flower is almost pure white, with just a hint of pink blush from anthocyanins. As fall weather cools the nights, anthocyanins increase, and the pink blush starts to deepen and spread. In the coldest weather, the whole flower fills with anthocyanins and turns almost completely pink. In mid-winter here in Southern California where hibiscus keep blooming until Christmas, the December flowers can all be so red and pink that at times we have difficulty identifying them.
Anthocyanins Degrade and Disappear with Heat
Some anthocyanins are very sensitive to heat. Enzymes in plant sap completely destroy several of the anthocyanins in hot weather and bright sunlight, and this creates the fading effect we see in so many hibiscus flowers in summer heat. These are the hibiscus that we recommend keeping in partial shade during the heat of the summer, like 'Sleeping Beauty.' Its anthocyanin pigments are beautiful in the cooler, cloudier times of year, showing a rainbow array of 5 different bands of color, and holding the color well. In summer heat though, the flowers have only 2-3 colors, and in bright sunlight, these colors fade very quickly. But a 'Sleeping Beauty' growing inside a house in a window that gets an hour or two of sun every day will bloom with a full array of colors that will last for 2-3 days. Inside a house, or in partial shade, the anthocyanins are protected from too much heat and sunlight.
Anthocyanins Increase Production and Show a Wider Range of Colors with Maximum Nutrition
Anthocyanins are produced in sap by a reaction between sugar, or brix, and protein. Higher sugar or brix content in the sap and an ability to produce proteins plentifully are both requirements for a plant to produce maximum levels of anthocyanins. Although horticulturists keep looking for tricks to increase brix production in food plants, the most sensible and easiest way is to maximize your plant's health in every possible way. Extra good soil, extra good nutrition, extra good care will, over time, increase both sugar and protein production, and you will see the difference in your flower colors. Some hibiscus are very, very sensitive to this nutritional effect, and will even stop producing flowers all together if the sugar and protein levels in the sap get too low. Others will produce flowers, but the colors will be pale, or some colors will be missing. The health of the plant really does matter when the colors depend on anthocyanin pigments!
Flavonols ~ Pale Yellow, White Spectrum
Flavonols are in the flavanoid family with anthocyanins, and have all the same characteristics of anthocyanins - they degrade in heat and lots of bright light, and they increase in a healthy plant and in cold weather. But flavonols have their own distinctive pale yellow color veering into white. The yellow edge of 'Sleeping Beauty' is produced by flavanols, and increases and decreases in response to the same conditions that increase and decrease it anthocyanin blues and pinks.
'Acadian Spring' is a good example of a hibiscus flower that uses flavanols exclusively. It opens with a soft yellow flower that fades in sunlight and heat to almost pure white. In cooler weather, 'Acadian Spring' flowers open almost completely yellow and they don't turn white during the day.
Now you know everything there is to know about hibiscus pigments. Well, not exactly.... It's a pretty complex field of study, and you probably know more now than you ever wanted to know. But as you watch your blue hibiscus flowers turn pink, your pink flowers turn red, and your flowers turn yellow this fall, you'll at least have some idea of and why this happens.
Reference: Hidden Valley Hibiscus