Hills-of-snow, or wild hydrangea (H. arborescens), a shrub slightly more than 1 metre (4 feet) tall, has rounded clusters of white flowers. The French hydrangea, or hortensia (H. macrophylla), is widely cultivated in many varieties for its large globular flower clusters in colours of rose, lavender, blue, and, rarely, white. These cultivated varieties are the florist’s hydrangeas, forced for holiday use. The flower colour is variable, depending on the acidity of the growing medium: rose-pink under neutral to low soil acidity and blue under conditions of stronger acidity. Blue flowers can be produced by the early addition of chemicals to increase the acidity of the growing medium.