I think most gardeners know that the color of their hydrangeas is related to the acidity of the soil. That's why some folks pour over a little vinegar if they want blue flowers. But, as it turns out, the color of hydrangea blooms is a little more complicated, and interesting, than just the local pH. It is an intricate balance of pH, aluminum ion concentration, and pigment chemistry. But the key to it all is the aluminum ion. As with most metals, the availability and mobility of aluminum depends on its chemical form, which, in turn, depends on the soil's pH. It's complicated, but the more acidic the soil is, the more mobile and available free aluminum ions are, and that’s bad for the plant. What is perhaps most interesting, though, is that in the presence of aluminum ions, the roots of the hydrangea exude citric acid, thereby converting the free aluminum ions (toxic) into an aluminum citrate complex that gets transported from the roots into the rest of the plant, including the sepals. Here, the citrate complex transfers the aluminum ion to the pigment molecules in the flower, changing the color from red/pink to blue in the process. So, while the pH effect is super cool, what the hydrangea is actually doing is a local detox of its environment, which is even more impressive.
Any way, something to think about the next time you admire the wonderful hydrangeas in your garden.
