Colour of Hydrangea Flower and Soil pH

For certain types of hydrangeas—specifically Bigleaf (Hydrangea macrophylla) and Mountain (Hydrangea serrata)—the color of their blooms acts like a living, growing pH strip for your soil.

However, the pH itself isn’t what directly dyes the petals; it’s actually all about aluminum.

The Aluminum Connection

Hydrangeas need aluminum to produce blue pigment. While aluminum is naturally present in most garden soils, its availability to the plant’s root system changes entirely depending on the soil’s acidity.

• Acidic Soil (pH 5.5 and lower): At lower pH levels, aluminum becomes highly soluble. The hydrangea easily absorbs it, resulting in blue flowers.
• Neutral to Alkaline Soil (pH 6.5 and higher): As the soil becomes more alkaline, the aluminum gets locked up in the soil chemistry. Even if the aluminum is there, the plant can’t take it up, resulting in pink or red flowers.
• The Middle Ground (pH 5.5 to 6.5): In this transitional zone, you’ll generally see purple flowers. Often, you’ll even see a single bush sporting a mix of blue, purple, and pink blooms simultaneously as the root system navigates different pockets of soil.

The White Hydrangea Exception: If you have a white variety (like an ‘Annabelle’ or an Oakleaf hydrangea), their color is strictly dictated by genetics, not soil chemistry. No amount of soil tweaking will turn a white hydrangea blue or pink.