The hydrangea has long ranked as a popular garden staple for its range of color, its sizable “snowball” shape, and abundance of blossoms that are long-lasting. Landscapers cultivate them almost ubiquitously to make gardens look bountiful, and flower markets include hydrangea in many bouquet arrangements. Moreover, Pinterest features hydrangea in all manner of picturesque pins because of their lovely effect on the eye.
Interestingly, the hydrangea is not considered a true flower. Instead, the hydrangea is termed an inflorescence — because modified leaves, called sepals, comprise most of the bloom while overshadowing the smaller, fertile floral center portions.
The variety of color hydrangeas can be found in has led to their high demand. They exist in numerous shades of intensity in cream-whites, fuchsia-pinks, greens, blues, lavender-violets, pastels, even peach. It is this kaleidoscopic offering of color that makes hydrangeas even more fascinating — because their color is not due to pigmentation
Rather, hydrangea color occurs, in simple terms, as a consequence of the soil’s pH. That is, the colors seen on a hydrangea act in similar fashion to that of litmus paper.
Recall from basic chemistry that acids are substances that donate protons (or a hydrogen ion), while bases are substances that accept protons in chemical reactions. An acidic solution has a pH value of less than 7, whereas a basic solution has a pH value higher than 7. Thus, blue litmus paper dipped into a solution that is acid results in the paper becoming red. Meanwhile, red litmus paper dipped into a basic solution turns blue.
So, how does this relate to hydrangeas? Hydrangea, science has discovered, acts like a pH indicator of the soil upon which the plant grows. Essentially, what is unique about the hydrangea is how the color of its sepals can typically hint at the soil’s level of acidity.
Of course, there are limitations. If the hydrangea plant grows in a single container, then it is easier for the gardener or cultivator to control the container’s soil pH, and thereby control the hydrangea’s color. But that is not consistently the case if the hydrangea grows on the ground. After all, it is not unusual for folks to see varying colors on a shrub growing on the same ground, simply because one section of soil is different in pH level than the section of soil directly next to it.
Still, for gardening connoisseurs, having different bloom colors on the same bush can be appealing, in which case manipulating the soil with additives helps produce the hydrangea bush of varied hue. It’s not unheard of, for instance, for gardeners to pour vinegar or lemon juice in one section of soil, while mulching with coffee grounds in another. There are even instances of hydrangea bushes having citrus fruit rinds, pine tree needles, rusty nails, old tin cans, even copper pennies being buried beneath them as a strategy to alter the soil’s chemistry. There are further cases of hydrangea bushes being transplanted from one section of the garden to another — just to produce differing colors, because the plant adjusts to its environment change. Then, too, because the soil’s chemistry changes throughout the year and between years, it is commonplace to find that hydrangeas one year are of a different hue or intensity than from just the previous year.
However, science has revealed that another key factor affects hydrangea color: the presence of aluminum ions within the soil. Interestingly, aluminum ions are more mobile in acidic soil, so they can be more readily be taken up by the hydrangea plant. This explains why horticulturists know that to “blue” a plant requires both acidic soil AND aluminum additives. Conversely, to “red” a hydrangea plant requires liming the soil so that it becomes more basic in pH. The mechanism of color cultivation in hydrangeas therefore stems from soil pH being a necessary facilitator of aluminum’s availability and mobility.
The gardening of hydrangeas, nonetheless, requires a balanced touch — because hydrangeas can, for instance, die of cellular damage from extreme basicity of the soil. In other words, it is advisable that the soil not be put to excessive pH extremes, lest the plant undergo shock and eventual demise.
But the fascinating chemistry of hydrangea cultivation does not rest there! Scientists have recently discovered yet another key mechanism in the process of hydrangea coloration. Apparently, experiments have revealed that the hydrangea plant’s roots exude a citric acid complex at the base of the plant. This citric acid complex has within it citrate ions that bond with the aluminum ions in the soil to help transport the aluminum ions into and throughout the plant. Thus, the availability of citrate ions affects the amount of aluminum ions being absorbed into the plant. Scientists have found that the roots’ citric acid complex created is affected by the level of the soil’s pH.
In short, soil pH affects two critical things: (1) the availability of aluminum mobility in the soil, and (2) the availability of citrate ions in the hydrangea roots’ citric acid complex that are necessary to bond with the soil’s aluminum ions for transfer within the hydrangea plant. Indeed, without this citrate complex in the hydrangea plant’s roots, circulation of aluminum ions throughout the plants would be adversely affected, and sepal coloration affected accordingly.
The recent discovery of the roots’ citric acid complex being crucial to color cultivation opens new possibilities for hydrangea cultivation. That is, there can now be approaches wherein soil chemistry need not be affected to achieve color change in sepals. Indeed, scientists are proposing to introduce aluminum ion uptake directly at the sepals through an aluminum ion spray. Lab results show that dissolving aluminum ions in a buffered citrate-citric acid solution, when sprayed directly onto sepals, have changed reds to blue in as short a timespan as several days. The immediate results mean cultivators and gardeners no longer need to wait for an entire growing season to adjust color in hydrangea. Nonetheless, further tests still have to be administered, because the reverse process of turning blue sepals into red has proven to be more challenging.
In any case, the recent discoveries on the biochemical mechanisms behind hydrangea color change has made scientists hopeful for future innovations in hydrangea cultivation. Scientists believe they can create new colors in hydrangea that have not been previously seen before.
For example, a current challenge is that weather conditions (cold or hot, humid or dry) affects the wood of the hydrangea plant. It’s been known that old wood tends to lose their hydrangea cultivars for an entire season because of a cold winter or a late frost that freezes buds. “Endless Summer” cultivars, meanwhile, bloom on new wood and flower each year, unaffected by the weather. The only drawback is that the colors of the “Endless Summer” cultivars are more subdued than the year before. It’s as yet unknown why. Finding out how to keep the intensity of color consistent in the “Endless Summer” cultivar will go a long way in generating future hydrangea that don’t have to be at the mercy of old wood nor of color loss.
Another current hydrangea study being undertaken involves infusion of magnesium ions. Previous research has shown that infusion of magnesium ions in grapes brightens grape color. But the opposite is true with hydrangea, since magnesium ion infusion causes hydrangea sepals to become red, white, and blue simultaneously. While it might be great for patriotic enthusiasts, scientists are still puzzled as to why this result occurs. Studying the mechanism of how this happens might prove fruitful for future hydrangea cultivation.
Still another hydrangea study in the works revolves around the fact that hydrangea do not normally occur in yellow or orange. But a lab study involving molybdate ion infusion on red blooms had the surprising effect of producing yellowed sepals. And, when the scientists then changed their molybdate ion infusion target to the white blooms, the result was a bona fide yellow hydrangea. What struck the scientists as fascinating was that the yellowing occurred at the bottom of the sepals (unlike how bluing of sepals occurs at the top surface of the sepals). In other words, the molybdate ion infusion unveiled a new biochemical mechanism in sepal coloration that had never before been encountered. The only drawback is that introducing molybdate ions to the soil has proven toxic to the plant, therefore only the direct spray onto the sepals so far can produce the yellow blooms. Studying this mechanism’s process is likely to provide new insight into future color manipulation strategies for hydrangea cultivation. It is hoped that introducing the molybdate ions through the soil in some other manner can be performed in such way as to produce a viable plant in the future.
And still another hydrangea study also in the works centers around the unusual capacity for certain sepals — near the end of the growing season — flipping over to change colors from blue to red, while retaining the same amount of aluminum. This behavior is still a mystery to scientists and warrants further study.
Finally, another current study underway specifically involves the oak-leaf hydrangea, because it has the distinct ability of transforming its green leaves into red, just like its aging blooms transform from white to red with higher intensity over time. What’s more, the oak-leaf hydrangea does not turn blue in the presence of aluminum ions. Discovering why all these take place with the oak-leaf hydrangea species are challenges that scientists are looking forward to.
As shown in the preceding paragraphs, the biochemical study of hydrangea has proven to be a fertile pursuit for scientists. With so many mysteries associated with the hydrangea, even for the learned, more mystique seems to be added to this already alluring bloom. So the next time you see a hydrangea, don’t just think it is like Shakespeare’s proverbial rose “is a rose by any other name.” Rather, see the hydrangea as the object of new exploration — opening new realms of wonder, fascination, and discovery with its every bloom.