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Clethra comprises 64 species of evergreen or deciduous trees or shrubs, found in a disjunct distribution very similar to that of Persea, with one centre of diversity in eastern and southeastern Asia (Japan to New Guinea), another in southeastern North America, Central America and eastern South America, and one species on Madeira. The plant is generally hairy, the hairs varying in type from simple to stellate or fascicled. The leaves are usually entire or dentate, the toothing being most pronounced in juvenile plants; the secondary veins are decurrent on the midrib. The inflorescence is terminal, usually a solitary simple raceme, but sometimes with racemes being produced from the upper leaf axils on the shoot to give a more compound inflorescence, whose arrangement is an important specific character. Also necessary for identification are the bracts, which usually fall quickly, and the proportions of the pedicels. The flower is usually white (occasionally cream or yellowish) and fragrant (occasionally unscented). The calyx is five-lobed, the lobes becoming lignified in fruit. The corolla is also five-lobed, the petals usually being free but sometimes united at the base or for up to one-third of their length, obovate to spathulate and either entire or variously toothed or fringed, sometimes with a ciliate margin at their apex. The stamens are placed in two whorls, and the style has three lobes leading to a tricarpellate ovary. The fruit is a hairy subglobular or slightly three-sided capsule (Sleumer 1967).
Clethra is a useful genus for the garden where ericaceous plants predominate as it enjoys the same conditions – a high canopy, acidic soil – as many rhododendrons, but has the advantage of flowering later in summer. The most familiar species, C. alni folia from eastern North America, is justifiably popular, and there has been a flurry of cultivar selections in recent years (see Dirr 1998). Others are less well known, but the bark of some species can be extremely attractive.
From the closely related heath family the clethras are distinguished by having the five parts of the corolla so deeply divided that they appear to be separate petals. They are small trees or shrubs, all the hardy ones deciduous, but the tender ones all or mostly evergreen. Leaves alternate. Flowers white, fragrant, usually produced in racemes or panicles near the end of the shoot; stamens ten. Seed-vessel a capsule enclosed by the persistent calyx, and carrying many seeds.
Of the hardy species, three come from America, four from Eastern Asia. They all like a peaty soil, and are useful for flowering late in the season. Propagated by seeds, cuttings, and layers. The cuttings are best made in August, of side shoots 3 or 4 in. long, with a heel of older wood, and placed in gentle bottom heat.