When I was in the tenth grade I learned how to change the flower color of the common garden hydrangea from pink to blue or vice versa in a high school horticulture class.
Mrs. Davis, a neighbor that lived across the street from my childhood home, found out I was taking a course in horticulture. One old hydrangea bush grew beside the brick steps at the front porch door of her home. She could see the plant from her chair on the porch. Mrs. Davis had heard that the flower color on hydrangeas could be changed. She wanted me to change hers from pink to blue. Her hydrangea had always produced pink flowers.
The following March I started the procedure. I mixed one tablespoon of aluminum sulfate in one gallon of water in a watering bucket and secretly drenched the soil around the plant. I did this several times over the next couple of months, without getting caught.
When the flowers began to form but before they showed any color, I walked across the street with a handful of flour in full view of Mrs. Davis. I sprinkled the flour around the plant and said some sort of chant. I then informed Mrs. Davis that her hydrangea would produce blue flowers. And it did.
I don’t remember ever telling Mrs. Davis what I actually did to cause her hydrangea flowers to change color. But I’ll tell you. The flour had nothing to do with it.
The presence or absence of aluminum compounds in the flowers influences the flower color of the French hydrangea, Hydrangea macrophylla. The availability of aluminum is influenced by the soil pH. When the soil is acid, aluminum is generally more available to the roots and the flowers will be blue. When the soil is alkaline, availability of aluminum is decreased and the flowers will be pink.
Soil pH in close proximity to foundations of many homes is usually alkaline as a result of mortar and/or cement – the reason why Mrs. Davis’ hydrangea produced pink flowers.
To change pink hydrangea flowers to blue, or perhaps to make blue flowers a deeper blue, dissolve one tablespoon of alum (aluminum sulfate) in a gallon of water and drench the soil around the plant. This should be done in February, again in March and again in April. To make the flowers pink, dissolve one tablespoon of hydrated lime in a gallon of water and drench the soil around the plant in February, March and April. Avoid getting the solution on the leaves because foliar damage may result. And, do not get the hydrated lime in your eyes because it can result in permanent eye injury.
You may be disappointed in the results if you wait too late in the flowering cycle to apply the treatments. Waiting too late or not applying the proper amount of lime or alum may produce little to no color change or an “in between” purplish color.
With this knowledge, you may now practice this horticultural wizardry and impress your friends and neighbors.
Larry Williams, UF/IFAS Extension Agent, Okaloosa County, August 20, 2015