Hydrangeas change color (except for the white ones) based on the pH level of their soil. The more alkaline the soil, the pinker the flowers. To make pink hydrangeas turn blue (or to keep your blue ones from turning pink), increase the acidity of soil.

Your soil pH likely will need regular adjustment to produce consistently blue blooms. Gently work acidifying soil additives into the top layer of the soil around the root zone whenever you'd normally fertilize your hydrangeas, usually in early spring.