How does the hydrangea change color?

By Chelsie Vandaveer

January 17, 2002

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The purified pigment from lichen (litmus) is used by gardeners to determine the pH (acidity or alkalinity) of soils. The litmus paper strip changes color according to the types of ions present. A predominance of hydrogen ions [H+] is acidic and results in red litmus; hydroxyl ions [OH-] are basic (alkaline) and result in blue. When plants fail to thrive, the reason is often the soil has the incorrect pH for the particular plant. But there is a common plant that behaves like the litmus test.

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Nikko Blue (H. macrophylla)
Sister Therese (H. macrophylla 'Sister Therese')
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The florist's hydrangea (Hydrangea macrophylla Thunberg) produces large rounded corymbs (inflorescences) of showy sepals. The sepals will appear pink if the soil pH is above 5.5 and blue if the soil pH is 5.5 or lower. The pigment that determines the color is anthocyanin and a colorless chemical called a co-pigment.

The anthocyanin/co-pigment complex is found in the vacuole (fluid-filled sac in a cell) of each cell of the sepals. (In 'normal' flowers, these pigments would be found in the petals and the sepals would be green.) The pH of the vacuole usually determines whether the anthocyanin/co-pigment will be red as in a rose or blue as in a cornflower. But soil pH does not affect the vacuole's pH.

So why does the hydrangea behave differently? The hydrangea takes up aluminum ions which are toxic to most other plants (and animals). When the soil is neutral to alkaline, the aluminum ions are bound to other ions like oxygen or phosphates. The hydrangea cannot take up these complex compounds. When the soil is acidic, the aluminum ions are freed and readily absorbed by the roots.

The aluminum goes to many tissues in the hydrangea including the cells of the sepals where it is stored in the vacuoles. The aluminum binds to the anthocyanin/co-pigment complex altering which wavelengths of light are reflected and making the 'flowers' appear blue.

The Missouri Botanical Garden has a beautiful photograph of Hydrangea macrophylla 'Nikko Blue', one of the big leaf hydrangeas which changes color dependent on the soil pH. To view the photograph and learn more about growing hydrangeas, click on the link:

http://www.mobot.org/gardeninghelp/plantfinder/Plant.asp?code=K550

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