A number of year ago I planted a few hydrangea bushes. I like look of the bushes and the large, mop-head flowers that come in shades of pink, blue and white.
The interesting thing about hydrangea is that the color of the flowers depends on the pH of the soil. Soil pH is simply a scale that indicates whether the soil is acid, neutral or alkaline. The number 7 is neutral and anything less than 7 is acid and anything greater than 7 is alkaline.
If you remember back to chemistry class, litmus paper is used to tell if a solution is acid or alkaline. If the solution is acid, the strip of litmus paper turns pink and if it’s alkaline, the paper turns blue. You could say that hydrangea are the litmus paper of the plant world, the only difference being that the colors are reversed. If the soil is acid, the hydrangea will be blue and if it’s alkaline, the flowers will be pink. (White hydrangeas will always be white.) The cause of this difference has to do with aluminum. When the plant takes up aluminum (which is present to some degree in all soils), the flowers are blue. The reason pH makes a difference is because in very acid soils, aluminum is available for the plant to absorb and the flowers are blue; in more alkaline soils, the aluminum is bound in a form that the plants can’t absorb so the flowers are pink.
I wanted the hydrangeas I had planted to be blue so I add some aluminum sulfate to the soil. Aluminum sulfate is a soil additive that lowers the pH; various sulfur compounds can also be used to lower the soil’s pH. I was expecting that all of the hydrangea flowers this summer would be blue but instead I’m finding that the plants are primarily blue but there are still a lot of bright pink flowers. I’m not sure why this happened – some suggest it results from the plant adjusting to a change in pH – but I like the mixture of pink and blue. Who knows what color they’ll be next year!
Below are some pictures of hydrangeas that came from the same bush – one pink and one blue. Clicking on the photos will let you see them full-sized.