The hydrangea that was a gorgeous shade of blue when I bought it last year bloomed pink this year. How can I get it back to blue?
The flower color of certain types of hydrangeas depends on the amount of aluminum they draw from the soil, which in turn depends on soil pH. In general, they bloom pink when soils are alkaline or neutral, or toward the blue/violet end of the spectrum when soils are acidic.
Many of the beloved mophead and lacecap varieties of Hydrangea macrophylla respond to changes in soil pH by shifting their flower color. Gardeners have learned to manipulate the soil to take advantage. It’s too late to change the color of your hydrangea this summer, but here’s how to alter it by next summer:
To obtain blue flowers, increase soil acidity by adding sulfur, with your goal being pH 5.5 or lower. Sprinkle about ¼ cup (if the soil is sandy) or ½ cup (for clay or loamy soil) of garden sulfur at the base of the hydrangea and water it in.
For pink flowers, use dolomitic lime to decrease acidity; aim for pH 6.5 or higher. Start with about a pound of lime, scattered around the root zone of the shrub and scratched into the surface. Use half that amount if your soil is sandy.
It’s not an exact science. Some soils (and some hydrangea cultivars) are stubbornly resistant to change. You may need to repeat the application, and even then you may never get the color you want. And don’t expect immediate results; lime or sulfur applied in fall will affect the following summer’s blossoms. Some soils can maintain the color shift for several growing seasons while others will revert after a year.
I used to live in Kansas City, where naturally limey soils tend to keep hydrangeas pink, and gardeners go out of their way to achieve blueness. Now I’m in Pennsylvania, where blue hydrangeas are commonplace and discerning gardeners strive for pink. To my eye, they’re all beautiful. —Doug Hall