The hydrangea has a genuine botanical peculiarity that fed its symbolism: the colour of many varieties depends on the acidity of the soil, the same plant bearing blue flowers in acid ground and pink in alkaline, so that it seemed changeable and inconstant. From this the flower took, in the Victorian language of flowers, a double and contradictory set of meanings, gratitude and heartfelt emotion on one side, and boastfulness, frigidity or heartlessness on the other, the last from its abundant show of bloom but its lack of scent and nectar. In Japan it has carried associations both of apology and gratitude and, in some readings, of fickleness. A lush, thirsty, colour-shifting flower, the hydrangea gathers meanings as changeable as itself.
The chemistry behind the colour change is now well understood: in acid soil, free aluminium ions become available to the plant and bind to a blue pigment, an anthocyanin called delphinidin, in the petal cells, producing true blue flowers, while in neutral or alkaline soil the aluminium is locked up and unavailable, leaving the same pigment to show pink or red instead, so that gardeners wishing to blue a pink hydrangea add aluminium sulphate to the soil rather than any dye, a genuine and still commercially exploited piece of plant chemistry.