Hydrangea macrophylla 'Nigra', is large perennial bush that is fairly new to the United States, with a long history in England and Japan. When the stems are young, they are an unusual dark purplish-black color, giving rise to this plant’s common name of black hydrangea. The name itself may cause some to expect a plant with very dark or black flowers, in the black hydrangea it is only the young stems that are dark. The flowers themselves are in pastel shades of blue and pink.
In addition to the unique stem color, the flowers of the black hydrangea bloom in different colors depending on the soil’s pH level, a measure of how acidic or alkaline the soil is. Acidic soil has a low pH, and when the black hydrangea is grown in such soil, the result will be blue flowers. Alkaline soil has a high pH, and when ‘Nigra’ is grown in this type of soil the blooms will be pink. Growing in mildly acidic soil will result in light purple or lavender blooms, a mix of the two extremes.
Black hydrangea cannot be propagated from seed, because the blooms in this cultivar, or variety, of hydrangea are sterile, meaning they will not produce viable seeds. Cuttings from a healthy plant are taken and placed in water or a special rooting medium, and after a few weeks they will develop roots and can be planted. A completely new plant can grow from each cutting.
A representative of the United States National Arboretum encountered the black hydrangea in Japan in the mid-1980s. He brought some plants back to the U.S. with him where they were evaluated for almost a decade before being released for sale. ‘Nigra’ has long been grown in England, with a history in that country dating back at least a hundred years before the black hydrangea came to the United States.
The black hydrangea typically grows anywhere from 3 to 6 feet (1 to 2 meters) in height, and covers an area about the same size in diameter. Harsh winters will kill it, though if it is well protected it can survive quite a bit of cold before succumbing to it. In areas where winters are severe, it is best grown as a potted plant that is placed outside for the spring and summer and brought inside when hard frosts begin in the fall. With careful treatment, this plant will provide an eye-catching accent for many years.