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Article from New Trees by John Grimshaw & Ross Bayton
'Koelreuteria bipinnata' from the website Trees and Shrubs Online (treesandshrubsonline.
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Tree to 20 m, 0.6 m dbh. Bark brown, splitting into small square plates. Branchlets glabrous or slightly pubescent; lenticels circular, prominent and corky. Leaves bipinnate, 27–30 cm long; leaflets alternate to opposite, (3–)8–10(–12) on each major division, narrowly ovate to elliptic, 7.8–14 × 2–5 cm, upper surface dull dark green with short hairs on the midrib, lower surface with longer tufts of hair and some glandular trichomes, margins entire to uniformly serrate, apex acute to short-acuminate; leaflets sessile or with petiolules to 0.3 cm long; rachis with line of hairs on upper surface or glabrous. Inflorescences 35–70 × 20–40 cm, densely pubescent; pedicels 0.2–0.3 cm long. Flowers: calyx lobes ovate to deltoid, glandular, petals four, 0.5–1 cm long, stamens exserted. Capsules ellipsoidal, 3.7–6.6 × 3–5 cm, rose-purple when young, brown at maturity. Flowering July to September, fruiting August to October (China). Meyer 1976. Distribution CHINA: Anhui, Guangdong, Guangxi, Guizhou, Hubei, Hunan, Jiangsu, Sichuan, Yunnan, Zhejiang. Widely cultivated both in China and elsewhere. Habitat Open fields, hillside forests, thickets and similar, between 250 and 2600 m asl. USDA Hardiness Zone 7b. Conservation status Not evaluated. Illustration Meyer 1976; NT429. Cross-references B509, K199.
Koelreuteria bipinnata was introduced to the United States by Francesco Franceschi of Santa Barbara, California in 1911, and this material became established in cultivation, whereas that sent by Delavay from Yunnan to France in 1887 did not. It was offered by Simon-Louis Frères in 1908, which resulted in a tree at Birr Castle and another at La Mortola, according to Meyer (1976).
The species is now thoroughly established in American horticulture, and offers a useful variation on the Koelreuteria theme. Its particular value comes from its flowering in September, when there are few flowering trees and when, as Dirr (1998) says, it ‘shines like a yellow star’. The flowers are followed by inflated pink capsules that continue the display as the foliage turns its autumnal yellow. It can become quite a large tree, with heavy branches that have the potential to create an open canopy: Dirr (1998) recommends careful formative pruning. The tallest specimen seen in the research for New Trees was one planted in 1979 at the US National Arboretum, 15–16 m tall, and very elegant and shapely. It is accounted fully hardy on the East Coast, at least as far north as Philadelphia (Meyer 1976) and possibly further. Bean (1981a) says that the species is not hardy in London or Paris, but the passage of time suggests that it would now be worth planting K. bipinnata at least as far north as southern England. There is a 7 m tree at Kew, and other respectable specimens elsewhere from Cambridge to Bristol (Johnson 2007).