Clethra is a genus of small flowering shrubs or trees, part of the family Clethraceae, which contains only one other genus, the closely related Purdiaea. The species in this genus are clustered in east and southeast Asia, as well as eastern North America, the Carribbean, Central America, and northern South America, with one outlying species in Madeira. They are commonly known as summersweet.
In the eastern United States, there are two recognized species Clethra alnifolia and Clethra acuminata. Clethra alnifolia is found along the entire east coast, while Clethra acuminata is found in the Appalachian Mountains. Both shrubs have oblong leaves and will form stands through the growth of rhizomes. They grow naturally along streams and in wet areas, but they can tolerate drier sites in cultivation. They prefer part to full shade, producing more flowers in part shade. Clethra have long flower clusters that form at the ends of branches, and are almost always white, although there is a pink cultivar widely available (‘Ruby Spice’). The flowers are often fragrant, and attract bees and butterflies in profusion. They are also one of the few flowering shrubs that blooms in mid to late summer, hence the common name summersweet. Clethra are usually medium sized shrubs, to six feet, but some can grow up to 20 feet and have a tree-like form.
The most popular cultivars are ‘Sixteen Candles’ and ‘Ruby Spice’, both of which are widely available for purchase. The garden currently has 7 species of Clethra in cultivation, two from North America and five from east Asia, with Clethra alnifolia making up the bulk of the plants on display.