This is a story of a displaced New Englander, soil chemistry and turning flowers blue.
Where I grew up, summers are full of pink beach roses and blue hydrangea flowers. Along the shore of Lake Erie, where I live now, hydrangeas tend to be pink, or purple.
Hydrangea flowers act as plant litmus paper. Litmus paper is one of the oldest forms of pH testing. Dye is extracted from lichens, and absorbed onto filter paper. At neutral the dye is purple. It changes color if the surrounding solution is basic or acidic. If the soil they are grown in is more basic, as it is here in Western New York, the flowers are pink. New England soils are often more acidic and here, hydrangea flowers are blue.
Hydrangea is a genus of about 70 species, five of which are cultivated in the US. Only Hydrangea macrophyll has flowers that range in color from blush pink all the way to dark blue. Also called ‘big leaf hydrangea’ and ‘florists hydrangea’ it’s what you might find around Easter time in over-saturated shades at the grocery store.
The great mop of hydrangea flowers is called a cymose corymb - quite a mouthful of c’s and y’s to describe a whole bunch of flowers all together. Corymb means the individual flowers attach at different points along the stem. Cymose indicates that the group of flowers is flat topped and the flowers at the center of the bunch bloom first and those at the edge bloom last. The showy parts of each little hydrangea flower are sepals, not petals. Sepals are part of a flower that protect the flower bud, or support the petals once they’ve opened. Hydrangea sepals are what change color based on soil pH.
Soil pH, the measure of the acidity or alkalinity, affects nutrient availably to plants. It also affects the mobility of aluminum in the soil solution. If soil pH decreases, a greater abundance of aluminum ions start floating around in the soil solution, ready to be absorbed by plant roots. pH levels lower than 5 can cause aluminum toxicity to many plants. Too much aluminum entering plants through the root tip can cause cells to stop dividing, leading to a loss of nutrient and water uptake. Some plants, including hydrangeas, buckwheat and rye, are tolerant of high aluminum levels. The roots of these plants exude citric acid, which forms a stable molecule with aluminum ions that can be absorbed into the plant.
However, aluminum ions don’t change hydrangea sepal color. They merely initiate color change from pink to blue, called bluing. The other player in the equation is the pigment molecule delphinidin-3-glucoside. It’s the same pigment molecule in toxic delphinium flowers and is named for the dolphin-like shape of the flower spurs.
Delphinidin-3-glucoside is an anthocyanin, similar to the molecules that make the red color of changing leaves in the fall and the blue of blueberries. Anthocyanin pigments appear red, purple and blue. Yellow and orange are not part of the spectrum, which is why there aren’t any hydrangea flowers with these colors.
Back to the physiology of a hydrangea plant. If soil pH is low, there aren’t many aluminum ions available for roots to absorb. Therefore, the amount of aluminum ions within a plant is low and the delphinidin molecule shows pink in flower sepals. When aluminum ions outnumber pigment molecules, the chemical structure of the pigment changes. Hydrogen ions are lost, and what was once a positively charged molecule becomes negatively charged. Oppositely charged delphinidin molecules stack one on top of the other forming a stable complex. These molecular changes show up in hydrangea flowers as shades of blue.
Can I make all these chemical reactions happen in my garden?
The way to do it is by adding acidifier, like sulfur, to the soil. Decreasing the soil pH unlocks aluminum ions so they are available for uptake by plants. An increased amount of aluminum inside hydrangea plants starts the bluing process of flowers. As easy as this sounds, it’s not guaranteed to work. Acidifier has to be applied regularly for a few seasons (years) before any results are visible. Even then, acidifier has to be continually added to keep the soil pH lower than the surrounding area. My best bet for true New England blue hydrangeas? Take a summer vacation.
Curious Chemistry Guides Hydrangea Colors in American Scientist