Hydrangea macrophylla, commonly known as French hydrangea blooms in shades of pink and blue. The color depends upon the pH levels of the soil in which the shrub grows. It is possible to add nutrients to the soil to transform the colors of hydrangeas. The process may take a year or two to complete. Hydrangeas do best in a partially shaded area of the garden. Too much shade may result in a lack of blooms during the growing season.
Test the pH level of the soil according to the package directions on the soil test kit. Local county extension offices also sometimes do soil testing for home gardeners for a fee. Hydrangea shrubs need a pH level of 5.2 to 5.5 to absorb aluminum from the soil.
Apply aluminum sulfate to the soil, before blooming, around the hydrangea, according to the package directions. Avoid contact with the plant as the aluminum could burn the leaves. Always wear protective gloves when working with chemicals.
Apply the aluminum sulfate to the hydrangea two to three times during the growing season. Although the flowers may not turn blue the first season, regular applications will generate a change in color over the next one or two years.
Fertilize the hydrangea shrub at the beginning of the growing season with a low-phosphorus fertilizer. High doses of phosphorus keep the hydrangea from absorbing the aluminum.
Prune the hydrangea in the fall after all blooms are spent. This allows the shrub to grow new buds for the following spring.