- The flower color of hydrangeas varies from pink to blue based on soil pH. The amount of aluminum in the soil is the determining factor in flower coloration. In acidic soils with a pH of 5 to 5.5 aluminum content is high and influences pigments in mophead hydrangeas that produce blue flowers. In soils with a pH of 6 to 6.5 aluminum is not available in the soil and mophead hydrangeas produce pink flowers. Soils with pH values between 5.6 and 5.9 produce flowers in varying shade of blue and pink, the intensity of the flower color varies by cultivar. Some white cultivars do not change color at all, regardless of soil pH.
- You can control the flower color of mophead hydrangeas on your property by manipulating the soil chemistry. Apply wettable sulfur early in the spring before new growth appears to raise the pH of the soil and turn blue flowers pink. You can also apply aluminum sulfate around the base of hydrangeas to lower the soil pH and turn pink flowers blue. Always follow the instructions of the chemical additive you apply to hydrangeas. Excess aluminum in the soil can damage hydrangea roots and prevent the shrub from blooming, according to Washington State University Extension.
- Mophead hydrangeas are hardy shrubs that grow well in a variety of soil conditions. Hydrangeas grow best in moist, well-drained soils and are shade tolerant shrubs. Hydrangeas will continue to bloom through the summer and add color to shady areas on your property where few other flowering plants can grow. Too much sun will stress hydrangeas and may lead to leaf scorch, according to WSU Extension.
- Mophead hydrangeas develop flowers on the previous season's growth. New cultivars are being developed that bloom on old wood from the previous growing season as well as on new growth from the active growing season. These new cultivars of mophead hydrangeas that have two periods of blooming each season are called remontant cultivars, according to the Virginia Cooperative Extension. Ensure that you choose a remontant cultivar when selecting mophead hydrangeas to enjoy their colorful flowers throughout the summer and into the fall.