A Story of Acids and Bases: The Interesting Science Behind Hydrangeas and Their Pink or Blue Blossoms and How This Corresponds to Our Green Bean Lab

After experimenting with the effects of vinegar and baking soda in water that is being used to boil green beans, I was interested in researching what other effects acidity could have on plants. I can recall being little and playing in the gardens at home amongst large snowball hydrangeas of rosy pink and powder blue. Fascinated by the variety of shades of the hydrangeas, I asked my mother how they could be both pink and blue if they were the same flower? I can remember her attempting to explain to a very young Cassie that you could add certain substances, acids or bases, to the dirt that the flowers grew in to make them bloom either pink or blue. I didn’t quite fully grasp the idea at the time, yet the concept stayed with me, and was one of the first examples of the effects of acidity on plant life that I could think of after our green bean experiment.

The science behind why the presence of acid or base affects the color of blossom of a hydrangea is relatively simple. The flower color is blue when soil conditions are acid and then converts to pink when soil conditions become slightly acidic to alkaline. During this conversion the plant may have flowers of various shades of blue and pink, sometimes appearing to be purple.

The blue and pink color of hydrangea flower is due to an anthocyanin called delphinidin 3-monoglucoside. In the presence of aluminum, a blue color will form due to the aluminum binding with the anthocyanin. The reason for the blue color under acid conditions is due in part to an increase in availability and uptake of aluminum from the soil.

As soil pH decreases, availability of aluminum increases. Conversely, as soil pH increases, the availability of aluminum decreases. The pH of the soil should ideally be around 5.0 to create a bluish blossoming hydrangea.

Fascinatingly, aluminum is not considered to be an essential element for plant growth. In fact, for most plants, high levels of available soil aluminum are toxic.

Gardeners in the Eastern United States have a difficult time maintaining pink flowers because their native soils tend to be acidic and contain sufficient levels of available aluminum to maintain a blue flower color in hydrangea. In contrast, soils in the upper Midwest and South tend to be more alkaline, making it difficult to maintain blue flowers.

In much the same way that the presence of acid altered the vibrant color of green beans, the presence of acidic or basic soil can alter the blossom of a hydrangea. This concept fascinated me and I believe that parallels can be drawn between the effects of pH on the anthocyanin of a hydrangea and the chloroplast of a green bean:

As the green bean is cooked in acidic water, the hydrogen ions move into the chlorophyll and replace the magnesium ion. This alteration in chemical composition turns the chlorophyll into a compound called pheophytin. Pheophytin has a distinctly yellowish-brown color causing the green beans to lose their vibrant green and turn an earthy yellow, in much the same way that the presence of aluminum and acid yielded a blue appearance in hydrangeas due to the bondage of the delphinidin 3-monoglucoside with the aluminum. When baking soda, or other basic substances such as zinc, are added to the water, the zinc or bicarbonate ions replace the missing magnesium ion, allowing the green bean to retain its original vibrant green. When basic substances are present in the soil of a hydrangea, there is a lack of aluminum so no bondage occurs between the aluminum ion and the delphinidin 3-monoglucoside, therefore allowing for a pink flower.

While the science behind the color of a cooked green bean and the blossom of a hydrangea is not exactly the same, it is fascination to study the parallels between the two organisms and their high sensitivity to pH.