Hydrangea

The hydrangea, Hydrangea, is a shrub of showy rounded flower-clusters, many species native to East Asia and the Americas; its name comes from the Greek for water-vessel, given for the cup-shaped seed capsule and reflecting the plant's great thirst for water. In Japan the hydrangea, ajisai, is the flower of the rainy season of early summer, its blue and violet heads famously associated with temple gardens in the rain, and it features in the Buddhist festival of the Buddha's birth, when a sweet tea, amacha, brewed from a related hydrangea is poured over the image of the infant Buddha.

A plain caution belongs beside its beauty: hydrangea leaves and flower buds contain cyanogenic compounds and are toxic if eaten in quantity, capable of causing vomiting and, rarely, more serious illness in people and pets, a genuine hazard worth stating plainly despite the plant's cheerful and devotional associations. Some Asian hydrangea species have long been distinguished by regional folk practice from the more ornamental garden varieties grown in the West, though the ordinary garden hydrangea sold in nurseries worldwide should not be treated as a food or tea plant on the strength of these separate and specific regional traditions.

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.