From 100 Flowers and How They Got Their Names by Diana Wells:
“Garden hydrangeas will turn from pink to blue if the soil is acid and if aluminum is available to them, but it still seems rather magical, and when they were first introduced it was inexplicable. It was initially thought that they might take their color from their surroundings, especially as cuttings from a plant of one color might well turn out the other color when propagated.”
From Hydrangeas by Naomi Slade:
“When the first Asian hydrangea reached maturity in The Royal Botanic Gardens, Kew, in the late 1700s, the flowers caused a sensation when their original colour gradually changed — as if by magic.
“Widely believed to be unique to hydrangeas, this chromatic instability is one of the plant’s most memorable characteristics, and it comes down to the availability of aluminium ions in the soil.
“Acid soils … contain aluminium ions in a form that the plant can absorb and then use to produce the pigment for blue blooms. Alkaline soils … and those that are high in phosphate ‘lock up’ the aluminium so the ions are unavailable to the plant. As a result, the same variety will produce flowers that are pink or mauve. The more acid your soil is, therefore, the more clear and intense the blue colour will be.
“Hydrangeas can take several years to settle into a site and assume their ultimate colour, but giving them a dose of ‘free’ aluminium in the form of potassium aluminium sulphate (potash alum) can speed up the process — but take care not to overdo it, as the plant may suffer.”
I’ve never actually tried the color-change trick on my hydrangeas, being content instead to let nature do its thing and produce whatever colors the plant thinks most appropriate. I’m actually not even sure if you can change the colors of hydrangeas of the lacecap variety; though I would say that the color variations I see even in these baby bluebirds suggest it might work.
The photos in this post and the previous one are all from three shrubs of the same kind, planted near each other four springs ago — yet some of the blooms show blue colors only, while others include bits of color in the pink/purple ranges. The bluer ones are planted farther back in my garden, where the soil is thicker (I had to build it up quite a bit to work around the ground-level roots of some pine trees) and that extra soil tends to absorb and store more water. Rainwater — we’ve had tons of rain this year — is slightly acidic, so I pretend to know what I’m talking about and say that lots of rain with its acidic content soaking into thicker soil causes the plant to produce more blue blooms, whereas water running off those in the front of the garden reduces the acidity (producing more pink and purple).
I could be wrong about that… or not!
Thanks for taking a look!