FLORIDA — 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 from blue to pink.
A neighbor who lived across the street from my childhood home told me that she wanted to change her hydrangea flower color from pink to blue. I told her that I could do it. Her hydrangea had always produced pink flowers.
The following February, I secretly started the procedure. I mixed about one tablespoon of aluminum sulfate in one gallon of water in a watering bucket and drenched the soil around the plant. I did this again for a total of three or four applications over the next several months without getting caught.
When the flowers began to form and before they showed any sign of color, I walked across the street with a handful of cooking flour in full view of my neighbor. I sprinkled the flour around the plant and said some sort of chant. I then informed my neighbor that her hydrangea would produce blue flowers, and it did.
Of course, the flour had nothing to do with it.
The presence or absence of aluminum compounds in the flowers influences the flower color of the common garden hydrangea, Hydrangea macrophylla. The availability of aluminum is influenced by the soil pH.
When the soil is acidic, aluminum is generally more available to the roots, and the flowers will be blue. When the soil is alkaline, the availability of aluminum is decreased, and the flowers will be pink.
The soil pH near the foundation of many homes is usually alkaline due to mortar and/or cement. This is the reason why my neighbor’s 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.
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.