Of the myriad of flowers blooming in my neighbourhood, among the most impressive are the bushes covered in pompoms. You may know these botanical cheerleaders as hydrangeas.
They come in white, pink and blue. I assumed these were just varieties of the same species, until I saw a plant with both pink and blue flowers, which made me wonder how that worked.
The first part of this colourful story unfolded when I found out that the flowers are technically sepals, or modified leaves. Many of them do not have actual flowering bits left. So, I guess I can't ask whether one colour has a reproductive advantage over the others.
The main pigment in the sepals is myrtillin. Scientifically, myrtillin is known as 3-glucoside of delphinidin, a member of the anthocyanin family, which includes pigments that turn leaves red in autumn and give berries their colour. On its own, myrtillin is red, but when it forms a complex with aluminum in acidic soil, it turns blue. The aluminum forms a bridge between the myrtillin and other pigments.
In acidic soils, aluminum reacts with the citric acid that the hydrangea roots produce. Aluminum combined with citrate ions gets absorbed through the roots and eventually carried up to the sepals, producing the blue colour.
Aluminum seems to be fairly common in soil, but its chemical availability is affected by pH. In more basic soils (higher pH), aluminum combines with hydroxide ions, to produce aluminum hydroxide. This solid does not get absorbed by the hydrangea. At extremely high pH levels that are potentially lethal to the plant, the aluminum forms different ions that can result in the plant going from blue to red.
So in general, hydrangeas are reddish in more basic soil and bluish in acidic soil (which is opposite to the colour code of litmus paper if you have ever used that to test pH before).
White hydrangeas simply have no pigment.
If you are the sort of person who needs to be in control of everything, including the colour of your hydrangeas, you can get your soil tested and then fiddle with it. If want them blue, you could add aluminum sulfate, for example, which lowers the pH and adds aluminum. If you prefer red, you might add calcium hydroxide to raise the pH. But it may take some time and these compounds could affect your other plants.
I'm still not sure what was happening with the multicoloured bush I saw, but one possibility is that it was recently transplanted and was still adjusting to the new environment.