Garden Column for the Salisbury Post
Darrell Blackwelder
June 19, 2003

    Southern landscapes would not be complete without hydrangeas. The large, colorful blooms make it a colorful focal point in any landscape. Large puffy blue flowers are a vivid memory of both mine and my wife's childhood.
Used a standard shrub for decades, new and different cultivars are available displaying a variety textures and colors. Flower colors range from deep blues through white to deep pink to cherry red. Flower colors may be consistent or can vary depending upon the cultivars and under differing growing conditions. Some cultivars have variegated foliage; others are dwarf cultivars perfect for confined space or containers. Hydrangea petiolais is a climber with white lacy blooms in the spring.
    The bloom shape can also vary. Some blooms form perfectly round balls while others has a flat surface of florets. Oakleaf hydrangea blooms are regal and conical shaped. The most common hydrangea shrubs grown in our area is the bigleaf hydrangeas (Hydrangea macropylla or serrata). These shrubs have the potential to produce color shades everywhere within the spectrum. Hydrangea arborescens, paniculata and quercifolia are also common shrub hydrangeas grown in NC. However, they all have white flowers, some tinged in pink, which differ in how the flower color fades to pink or red but they do not turn blue.
    The oakleaf hydrangea is becoming a popular landscape shrub because of its multiple landscaping attributes. The showy shrub has large panicles of white blooms, brilliant red foliage in the fall and showy red exfoliating bark in the winter. This shrub automatically catches your eye in the landscape, especially at this time of year. This regal shrub is truly a must for any landscape.
    Those that have the bigleaf hydrangeas have a curiosity about changing the bloom color. Bigleaf hydrangea cultivars must possess certain color pigments to turn blue or pink. Cultivars without color pigments have white blooms.
    Pigmented shrubs turn a shade of blue if there is aluminum present in the soil. Without aluminum the shrub will be a shade of pink. The intensity and shade of the color depends upon how much pigment exists in that cultivar and how much aluminum is available. Some shrubs have more pigments than others creating a more intense blue colors.
    Soil pH and the presence of other nutrients often determine whether aluminum is available to the plant. Aluminum is more available in lower pH soils and less available in higher pH soils. There is usually abundant aluminum in our soils, particularly those with a reddish color.
    Most of the bigleaf hydrangeas in our area are blue. There is no reasonable way to turn them back to pink because of the abundance of aluminum in our naturally acidic soils. Those that want their pink hydrangeas to stay pink should plant them in soil less media in raised beds and lime to a pH of about 6.8. Supplemental iron may be needed to offset chlorosis caused by excessive lime.
    Those that want to flower blue but they are flowering pink and they are already in the garden should apply aluminum. The standard recommendation is an ounce of aluminum sulfate in a gallon of water and water the plant with it. Larger plants may need two ounces of aluminum sulfate in two gallons of water. The process may take a year to two to complete, so be patient.

Darrell Blackwelder is an agricultural agent in charge of horticulture with the North Carolina Cooperative Extension Service in Rowan County. For archived garden columns or other information, visit the Rowan County Master Gardener web site at www.rowanmastergardener.com , e-mail Darrell_Blackwelder@ncsu.edu or phone at 704-633-0571.