In an interview with Dr. Michael Dirr, we asked "Why are Hydrangea macrophylla blue in some places and pink in others?"

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Hydrangea macrophylla is going to be blue or pink or in-between. The in-betweens are called “blurples.” Aluminum is the causal agent in the bluing of hydrangeas. The aluminum has to be absorbed, has to be taken up by the vascular system into the leaves and the flowers. The aluminum is complexed with a pigment called delphindin-3-glucoside. This pigment when it’s complexed with aluminum goes from pink to blue. The key is you need to saturate the pigment sites in sepals. It’s not the petals that are so important in hydrangeas, it’s the sepals. You have to get aluminum into that sepal. PH itself has no effect on bluing hydrangeas. PH is only an intermediate catalyst if you will. It frees up aluminum so it’s available to the system to be absorbed in the sepal.

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Growers use aluminum sulfate, and they have formulated some called Blue Max. There are others that they put on in a slow-release dry form. It’s encapsulated and gradually it bleeds out into the pine bark medium the hydrangea is planted in. You can do that if you want to “blue” your hydrangeas. However, in Georgia for the most part our soils are acid and most hydrangeas are blue. In the Midwest, they’re going to be pink. PH in that part of the country is usually around 6.5, 7, 7.5. And bark that we typically grow hydrangeas in, pine bark, has a pH of 5.5. So just by inference, they should be blue as can be. But they’re pink.

 

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