The same hydrangea can bloom blue or pink, depending on your soil.
Bigleaf hydrangeas are natural pH indicators. In acidic soil their flowers usually turn blue; in alkaline soil, pink or red. The switch comes from aluminium: in acid soil the plant can take it up, and it changes the colour of a single pigment in the petals.
Hydrangea macrophylla, the bigleaf hydrangea, is native only to Japan, where it grows wild in coastal habitats from Honshu southwards. It is now grown around the world and has even become invasive in the Azores and Madeira. The wild plant has flat 'lacecap' flower heads: a cluster of small fertile flowers ringed by a few large, showy, mostly sterile ones that act as signposts for pollinators.
Gardeners have transformed it. In more than 500 'mophead' or hortensia cultivars, the small central flowers have been replaced by large sterile ones, so the head becomes a dome or a whole sphere of colour. A smaller group of over 20 lacecap cultivars keeps the wild form but with more intense colours. Some hardier types come from crosses with the closely related mountain hydrangea, Hydrangea serrata.
The colour trick is chemistry. All the reds, mauves, purples, violets and blues come from one simple pigment, delphinidin 3-glucoside, which forms complexes with metal ions. In acidic soil, below pH 7, aluminium is available for the plant to absorb, and the pigment shifts towards blue. In alkaline soil, above pH 7, the flowers tend to be pink or red. The plant is what botanists call a hyperaccumulator, able to take up unusually large amounts of the metal. Why it evolved to do this, whether for defence or to attract pollinators, scientists don't yet know.
The practical upshot is that a hydrangea's colour tells you something about the ground it grows in, and the same cultivar may flower differently from one garden to the next. Beyond the border, the plant has other uses: in Japan, fermented leaves of one variety are used to make a drink called amacha.
Source: Wikipedia — Hydrangea macrophylla · Text summarised from Wikipedia (CC BY-SA 4.0)
