Human Flower Project

Wednesday, May 24, 2006

All I Know About Chemistry I Learned from Hydrangeas

A Japanese native teaches pH 101.

Hydrangeas in bloom

Columbia, South Carolina, July 2005

Photo: Human Flower Project

Through the southeastern U.S., hydrangeas are plentiful. We were fortunate once to inherit a garden in Lexington, KY, with so many white oakleaf hydrangea bushes that local florists would come “shopping” in wedding season. Starting about now.

Much as we love white flowers, when it comes to hydrangeas we prefer the colorful and more bosomy varieties—like H. macrophylla, also known as “mophead.” These pedagogues of spring taught us all we know about soil chemistry. The flowers announce how alkaline (high pH) or acidic (low pH) the soil is, tingeing toward blue in acid conditions and pink where it’s limier. Growing up in Louisville, we were enchanted to see how gardeners had steered their hydrangeas toward violet or rose, or sometimes managed to mottle blossoms by more complicated tinkering (or was it chance?). Neither being well versed nor terribly interested in science, this sort of gardening intervention seemed quite aggressive and exciting—a yard experiment conceived by Granny Frankenstein.

If you are fortunate to live where these beautiful plants do well, you might try playing around with soil chemistry to achieve a Utopian shade somewhere between pink and blue. This site contains excellent information on hydrangea selection, coloration and pruning.

“A wide range of depth of color exists in the pink and blue cultivars ranging from blush pink and robin’s-egg blue to brick red and cobalt blue. The depth of color is entirely dependant on the cultivar. Otaksa, for example, will never reach a rich deep color, no matter how much you may pile on the chemicals. And some hydrangeas refuse to be changed to clear blue, like Geoffrey Chadbund, making a royal purple flower at best, when treated.” Many of the white varieties won’t take on color at all.

Once you have a mutable species in the ground,  you may turn them bluer “by adding sphagnum peat moss, sulfur or aluminum sulfate to the soil. A pH range of 5.5 to 6.0 is needed, and a soil test is a good first step.” Aluminum is the key and usually present in the ground already, but “if the soil is alkaline, plants are unable to absorb the existing aluminum and the flowers will not blue.”  It’s possible to burn the roots of your hydrangea with too much aluminum, though. Water before you add chemicals. Allen Boger recommends, “Start with 2-4 applications of two tablespoons of aluminum sulfate per plant, at two-week intervals in the spring. Don’t overdo it.” Achieving Utopian Blue may take a couple of seasons.

Why does it seem effort always is extended in blue’s direction? Because “it is more difficult to acidify soils than to make them more alkaline.” You’re not kidding! Here in Austin we struggle along on a shelf of chalk. Azaleas and camellias have been known to perish as they near the curb—so we’re not going to attempt hydrangeas. We leave that to our gardening friends back in Kentucky and South Carolina.

Folks, we’d love to see how your pinks, blues and whites are faring this spring. Have your mopheads reached Utopia?