According to UC master gardener Carol Rogala, the colour of a hydrangea planted in the garden is affected by the pH in the soil. The blue colour is produced in soil strongly acidic (below pH 5.5) and pink is created in a neutral to alkaline soil (pH 7.0 or higher). The lower the pH number, the more acid the soil. Flower colour treatment is not effective unless started way ahead of bloom, typically in late autumn the previous year.
Exact amounts of any amendment to the soil to get the desired flower colour will vary depending on your current pH and soil type. White varieties of hydrangea stay white and cannot be changed.
According to hydrangea experts, it’s not uncommon for hydrangeas that are normally pink or blue to bloom green in some years. No one seems to know exactly what causes this, but often it occurs for only one or two years and then the blooms return to their normal colour. The green flowers also may be prompted by heat, particularly a very warm and dry spring.
