Though hydrangeas are known for their ability to change color according to the type of soil they’re in, only bigleaf hydrangea can alter its flower color depending on soil acidity levels. Other types of hydrangeas will be white, greenish, pink or blue regardless of soil acidity. Although gardeners can encourage a different bloom color in bigleaf hydrangeas by judicious use of special fertilizers or amendments including aluminum, overfertilization can easily injure the plant.
Hydrangea Growth Requirements
Bigleaf hydrangeas (Hydrangea macrophylla) are popular for their large leaves and lush displays of showy flowers, as well the ease with which they are grown over a large swath of the country. Hardy from U.S. Department of Agriculture zones 6 through 9, depending on cultivar, bigleaf hydrangeas like deep, well-draining but moist soils, morning sun and afternoon shade. Pruning is usually optional, and most hydrangeas only require moderate soil fertility.
Hydrangea flower color is determined by the presence of certain pigments in the flower tissues as well as by the availability of aluminum in the soil. In soils with high acidity, or a pH below 6.5, aluminum is readily available to the plant, allowing for a blue flower color. In alkaline soils of low acidity, or a pH above 7.5, aluminum is unavailable, making flowers pink. Gardeners in areas with alkaline, limestone soils can use a pH-lowering product, such as aluminum sulfate or a sulfur compound. A soil test is the best method for determining the mineral makeup of a particular site and is a good aid for figuring out how much of a given amendment will be needed.
Strictly speaking, hydrangeas generally don’t require fertilizer to look nice, but a good rule of thumb is to feed lightly and infrequently. Weekly applications of fertilizer are likely to promote abundant leaves at the expense of flowers or even damage the plant. Most hydrangeas respond well to an all-purpose fertilizer with even ratios of nitrogen, potassium and phosphorous, labeled with numbers to indicate the ratio of each respective nutrient, such as a 10-10-10 or 8-8-8. In areas where the pH needs to be lowered to adjust the flower color, use an acid-based fertilizer such as one formulated for azaleas, roses or blueberries. Use a lime-containing product to raise the pH.
Application Rates and Frequency
Adjusting a soil’s pH is a slow process that takes several seasons. Avoid weekly applications of any amendment, be it fertilizer or aluminum sulfate, that would almost certainly injure the plant by burning its roots. Application rates for fertilizers and aluminum sulfate will vary depending on the local soil. In general, the recommendation is to feed hydrangeas three times a year, scattering 2/3 cup of granular fertilizer per 100 square feet in March, May and July for a total of 1 pound, or 2 cups, per year. Water thoroughly after fertilizing to help it dissolve and penetrate into the root zone.
Applications of aluminum sulfate are much smaller and less frequent. Plants only need 1 ounce of aluminum sulfate, or 2 tablespoons of dry material, dissolved in 1 gallon of water. Drench the root zone in spring as the flower buds are forming. Wait a year between applications to monitor the effect on that season's flower color.