Hydrangea Drift — Print

Hydrangea — Chemical colour shifts

The dramatic colours of hydrangea inflorescences are governed by their pigment chemistry. Anthocyanidins are a family of pigments that control the red-orange pigments in flora, but also include delphinidins, yielding the purple-blue colour. Normally, flower colour is determined by combinations of different pigments, but with hydrangeas, it is limited to only delphinidins. How can one chemical cause colour shifts?

Horticulturists know that when the soil pH is acidic (low) and contains aluminium, the bloom is blue, and in alkaline pH (high) it is reddish. However, it is the cellular pH which changes delphinidin structure, and therefore colour (low pH: red, high pH: blue). Hydrangea exudes citric acid from its roots which detoxifies soil aluminium, then absorbing and distributing it throughout the plant. The cells in sepals with low pH use aluminium and delphinidin to shift from red to blue colouration, with their ratio controlling blueability (observe the shades).

Scientists have now developed sprays which contain different metals that can cause colour shifts and patterns in hydrangeas, obviating the need for aluminium in soil. In this species, the colourful “mopheads” are sterile, hiding the small, fertile flowers. So humans, not pollinators, are satisfied with this colourful addition to gardens.