A genus of 4 or 5 species of deciduous tree, one from the eastern United States and the others from east Asia. All share a low, spreading habit, and smooth, thin, grey or grey-brown bark patterned vertically with small rounded lenticels. Buds are enclosed through summer within the inflated base of the leaf-stalk, which is rounded. The leaves are pinnate, with 5–17 leaflets (one terminal); the leaflets are untoothed and tend to be carried alternately along the rachis, and are paler or greyer underneath. The showy, fragrant flowers open in late spring to summer in large (15–40 cm) terminal paniculate racemes, erect or drooping, and lacking bracts; the calyx is campanulate, with 5 teeth; the corolla is white (rarely pink), with more or less equal petals. There are 10 free stamens. The fruit-pod is 3–8 cm long, compressed but not conspicuously winged, containing 1–6 bean-like seeds. (Flora of China 2021; Spongberg & Ma 1997).
Cladrastis belongs in the Tribe Cladrasteae within the pea family. Its nearest allies are Maackia, Styphnolobium, Platyosprion, and Pickeringia (a genus with a single shrubby species). From Maackia, the species in Cladrastis are distinguished by their hidden summer buds, by their flowers which are usually paniculate rather than racemose, by the unusual, alternate arrangement of the leaflets, and by the thin grey bark which does not flake in horizontal layers. Styphnolobium differs in the opposite arrangement of the leaflets; the widely cultivated S. japonicum is also distinct in its taller habit and ruggedly fissured, brownish mature bark. The genus Platyosprion was resurrected in 2020 by Duan et al. (Duan et al. 2020), who used phylogenetic analysis to determine that those species of Cladrastis described here are more closely related to Styphnolobium and to Pickeringia than they are to one or more further species traditionally included within the same genus. Of these further species, which differ from Cladrastis in their winged seed-pods, Platyosprion platycarpum (formerly Cladrastis platycarpa) from China and Japan is occasionally cultivated in the west; see Platyosprion for its coverage on this site.
Within Cladrastis as now understood, C. kentukea (sometimes still known as C. lutea) is the most familiar, particularly in its native United States. The Chinese C. delavayi (often still known as C. sinensis) has also been introduced to cultivation on several occasions, but authentic examples of C. wilsonii, also Chinese, are very rare in the west. Species which may not yet be cultivated outside their natural ranges are C. shikokiana, from the subtropical south of Japan, and C. lichuanensis, described in 1988 from forests in central China, which is only questionably distinct from C. wilsonii (Duley & Vincent 2003).
The three species of Cladrastis (sensu stricto) which are cultivated in Europe and North America form highly attractive flowering trees, though they may not blossom every year and flowering is most reliable when summers are hot (Cambridge University Botanic Garden 2021). The remarkably smooth grey bark, the low, wide-branching habit, and the leaflets whose short stalks twist to turn them to the sun and which are set alternately along the rachis, like the climbing spikes on old telephone poles, combine to form a delightfully distinctive generic signature. In the wild, Cladrastis are forest trees, often forming an understorey (Hill 2007), but in the lower light levels of northern Europe all seem to prefer full sun. Unusually within the bean family, their roots lack nitrogen-fixing bacteria (Foster, Horner & Graves 1998). Consequently, they do not thrive poor, acidic soils, but C. delavayi, at least, has been grown successfully in England in very alkaline conditions. Plants are best grown from seed but may be raised from root-cuttings (Bean 1976) or by grafting.
The name Cladrastis appears to have been derived by C.S. Rafinesque from the Greek clados, a branch, and thraustos, brittle (Bean 1976); all species have a weak timber and seem prone to major break-out wounds. It is possible to eliminate weak forks by formative pruning; however, pruning should be undertaken when the tree is in full growth since, as in Prunus, cuts made in winter bleed, and heal poorly (Robertson 1977).
Leaflets broad, the terminal one almost as wide as long; pedicels slender, 15–20 mm long:
Leaflets narrow, generally 2–3 times as long as broad; pedicels 6–13 mm long:
Leaflets blunt, oblanceolate, narrowest near the base; flowerheads erect; legumes with a slender stipe 2–3 mm long:
Leaflets pointed, ovate-lanceolate, broadest halfway up; flowerheads pendulous; legumes with a stout stipe 4–6 mm long: