06/12/2025
The color of these flowers isn't just genetics — it's the soil they grow in.
Specifically, soil pH affects the availability of aluminum, which in turn influences flower color.
In acidic soil (pH 5.2–5.5), aluminum becomes more accessible to the plant, producing blue or lavender blooms. As the soil becomes less acidic or neutral (pH 5.6–6.5), hydrangeas may take on a purple or violet hue.
In alkaline soils (pH 6.6–7.5+), aluminum is less available, and blooms tend to shift to pink or red. This fascinating color shift is most common in Hydrangea macrophylla and Hydrangea serrata varieties.
Gardeners can even intentionally change hydrangea colors by adjusting soil pH. To encourage blue flowers, lower the pH by adding aluminum sulfate or other acidifiers to the soil, aiming for a pH between 5.0 and 5.5. For pink blooms, increase the pH using agricultural lime, ideally reaching 6.5 to 7.0. It’s essential to test the soil first and make changes gradually—results can take time to appear. With patience and the right adjustments, you can literally paint your garden with pH.
Source: Bailey, D. A., & Dole, J. M. (2001). The Physiology of Flower Color in Hydrangea macrophylla. North Carolina State University.