Hydrangeas come in many varieties besides the commonly recognized Bigleaf variety with its snowball-shaped white, pink or blue flowers. All varieties can benefit from a fertilization program. Some require more than others, and some can make use of certain soil additives that manipulate flower color. Know which type of hydrangea you have before implementing a fertilization program. And remember that too much fertilizer can be harmful.
The Oakleaf varieties of hydrangeas are native to American soil and do well in drier climates. They are recognizable by their cone-shaped flower heads and brilliant leaf colors in the fall. A general-purpose fertilizer of 12-4-8, 16-4-8, or 10-10-10 is suitable for these hydrangeas. The best time to administer it is in April and again in June at a rate of 1 pound per 100 square feet.
Panicle hydrangeas are fast-growing, cold-hardy bushes with cone-shaped flowers of white or pink. They grow to a height and spread of between 6 and 10 feet. Panicle hydrangeas also appreciate an application of general-purpose fertilizer in April and in June. Use 12-4-8, 16-4-8, or 10-10-10 at a rate of 1 pound per 100 square feet.
Smooth hydrangeas are similar to Oakleaf varieties except for the shape of their leaves. Rather than the 5-pointed oak leaf-like appearance, these hydrangeas have oval-shaped leaves and smaller flower heads. A general-purpose fertilizer is acceptable here too although smooth hydrangeas require only one application late in the winter. Use 12-4-8, 16-4-8, or 10-10-10 at a rate of 1 pound per 100 square feet.
Bigleaf hydrangeas are rapid growers reaching only 3 to 4 feet in height and 4 to 6 feet in width. This rapid growth mandates more frequent applications of fertilizer. A general-purpose blend of 12-4-8, 16-4-8, or 10-10-10 at a rate of 1 pound per 100 square feet is adequate for the plant's overall health. Applications should be made in March, May and July for best results.
Only Bigleaf and Serrated varieties can have their color manipulated. Blooms are white, pink or blue. The color as well as the intensity is influenced by the cultivar, soil and weather conditions and overall plant health. With an unhealthy specimen or one lacking proper sunlight or water, the first step is to cure or properly care for the plant. If the plant is healthy, the gardener can influence the pH of the soil to change the color of the flowers. If the soil has low pH levels, the plant is able to absorb aluminum from the soil, allowing the flowers to bloom blue. If the pH is higher than 6.0 to 6.5, the absorption of aluminum is blocked and the flowers bloom pink. This can be manipulated by adding ½ cup of sulfur per 10 square feet and watering to make the flowers blue. One cup of dolomitic lime added to the soil in the fall and watered will produce pink flowers next season. Also, for blue flowers on Smooth hydrangeas, make sure your general-purpose fertilizer has a low pH (the middle number).