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Koelreuteria comprises three species: K. paniculata and K. bipinnata from China, and K. elegans from Taiwan and Fiji. They are single- or multistemmed trees with thick, sometimes fissured bark and prominent lenticels on the branchlets and petioles. The leaves are deciduous, alternate and imparipinnate; the leaflets elliptic to ovate, entire, serrate or crenate, glabrous to sparsely pubescent; the median leaflets are usually the largest. Inflorescences are terminal, paniculate and pyramidal. The flowers are unisexual or hermaphrodite; they have five calyx lobes, three long and two short, often glandular, four to five (to six) petals, strongly reflexed, yellow, turning orange-red, and eight (to nine) stamens with hairy filaments. The fruit is an inflated capsule with papery wing-like valves (Meyer 1976).
Koelreuteria paniculata is justly popular for its handsome pinnate leaves, broad inflorescences of yellow flowers, and inflated fruits. A number of recent selections have been named, with attributes ranging from later flowering (‘September’) to pinkflushed capsules (for example, ‘Rose Lantern’) and pink-flushed shoots (‘Coral Sun’). These are probably worth seeking out for planting instead of unnamed seedlings. In addition there is the older ‘Fastigiata’ which is perfect for small gardens, though it prefers summers warmer than those of the United Kingdom. Koelreuteria paniculata has no particular foibles for cultivation and it may be assumed that, where the climate is appropriate, the other species will be equally tolerant of soil conditions. They should all be planted in full sun.
A genus of a few species of deciduous trees, natives of eastern Asia and the Fiji Islands. They have alternate, pinnate or bipinnate leaves, and flowers in large terminal panicles; calyx unequally five-lobed; petals four; fruits bladder-like. Named after J. G. Koelreuter, a professor of botany at Karlsruhe, 1733-1806.
The genus is revised by F. G. Meyer in the Journal of the Arnold Arboretum, Vol. 57, pp. 129-66 (1976).