Curious chemistry guides Hydrangea colors

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Date: November-December 2014
From: American Scientist(Vol. 102, Issue 6)
Publisher: Sigma Xi, The Scientific Research Society
Document Type: Article
Length: 4,776 words
Lexile Measure: 1400L

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The blooms' varied hues trace back to both soil pH and the right additives. Could different combinations produce new tints never seen in nature?

One of the world's most popular ornamental flowers conceals a bouquet of biological and biochemical surprises. The iconic "snowball" shaped blooms of Hydrangea macrophylla (big-leafed hydrangea) are a common staple of backyard gardens. Many other, closely related hydrangea species are likewise known for their bountiful, showy, long-lasting blossoms, making them popular for both landscaping and the cut flower market. And their popularity continues to grow: Every year, gardening catalogs add new cultivars of these attractive plants. Hydrangeas are ubiquitous--but they are not what they seem.

For starters, the bloom of the hydrangea is not a true flower, but an inflorescence: Sepals, or modified leaves, make up most of the bloom and overshadow the small, almost unnoticeable fertile floral portions at the center.

The bloom colors are what really make the hydrangea stand out: They range from pink to blue, including all shades of lavender to violet to purple, as well as green and white. Color intensities run the gamut from vibrant to pastel. Noticeably absent from the kaleidoscope of possible hydrangea colors are yellows and oranges.

Hydrangea colors are not what they seem, either; they are not the result of a variety of different pigments, as is the case for flowers such as roses or tulips. They are more akin to the colors seen in litmus paper, the chemically treated strips classically used to determine whether solutions are acidic or basic. At the molecular level, acids are proton (or hydrogen ion) donators and bases are proton acceptors in chemical reactions. When one dips blue litmus paper into an acidic solution (pH < 7, where pH is a measure of the concentration of hydrogen ions), the paper turns red, whereas red litmus paper changes to blue in the presence of a basic solution (pH > 7).

In a similar fashion, the color of many hydrangea blooms acts as a natural pH indicator for the soil in which the plant grows. Such blooms have blue sepals when the shrub grows in acidic soil, but develop red or pink sepals when grown in neutral to basic soils. The hydrangea's bloom color reveals the pH of the soil, but with its distinguishing colors being the reverse of those for litmus paper. The hydrangea is unique among plants in this ability to indicate soil acidity.

Because of this trait, gardeners can chemically manipulate hydrangea bloom colors using soil additives. In fact, a hydrangea can have different bloom colors on the same bush if the roots of the plant sample soils of differing pH. Homespun recipes abound for changing the pink blooms of a hydrangea to blue: pouring vinegar or lemon juice on the soil; mulching the plant with coffee grounds, citrus fruit rinds, or pine tree needles; or burying rusty nails, old tin cans, or copper pennies next to the bush. All these strategies tend to turn soil more acidic, and...

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Gale Document Number: GALE|A389260651