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A deciduous tree, 50 to 80 ft high, of rounded habit and branching low down when growing in the open, but capable of forming a tall clean trunk when close planted. Bark downy when young, glabrous later and dark greenish brown; on old trunks it is grey, and corrugated rather like an ash. Leaves rich green, pinnate, 6 to 10 in. long, composed of nine to fifteen leaflets, which are ovate or oval, 1 to 2 in. long, half as wide, covered with small appressed hairs beneath. Flowers in terminal panicles 6 to 10 in. long and wide, creamy white, each about 1⁄2 in. long; calyx 1⁄8 in. long, bell-shaped, green, shallowly toothed. Pods 2 to 31⁄2 in. long, glabrous, one- to six-seeded; rarely seen in Britain.
Native of China (not of Japan); introduced to France in 1747 and thence to England in 1753. It is one of the most beautiful of all leguminous trees, although it does not flower in a young state – not commencing until thirty to forty years of age. Old trees flower freely, especially after hot summers. The blossoms are not developed until September, and in wet cold summers do not develop at all. They do not fade on the tree, but drop off quite fresh, making the ground white beneath. On the continent of Europe, thanks to the warmer summers, it attains a larger size than with us, and frequently ripens seeds, which are the best means of increase. All parts of the plant, even the wood, contain a purgative principle said to be so potent that turners working on the green wood are immediately afflicted by colic, and that well-water becomes laxative if the leaves fall into it in autumn.
The largest specimen of S. japonica recorded recently grows at Syon Park, London; it measures 76 × 151⁄4 ft at 4 ft (1967). Others are: University Parks, Oxford, 59 × 91⁄4 ft (1975); Aldenham House, Herts, 60 × 61⁄4 ft (1976); Angelsey Abbey, Cambs., 48 × 53⁄4 ft (1973); Penrhyn Castle, Bangor, 44 × 61⁄2 ft (1974). The decrepit tree at Kew in G.15 was planted soon after the introduction of the species and is kept for its historical interest.
specimens: Syon House, London, in Wilderness near river, 88 × 161⁄4 ft at 4 ft (1982); University Parks, Oxford, 55 × 91⁄2 ft (1981); Angelsey Abbey, Cambs., pl. 1929, 60 × 71⁄4 ft (1984); Linton Park, Kent, 85 × 83⁄4 ft (1984); Nymans, Sussex, Magnolia Garden, 46 × 111⁄4 ft at 2 ft (1983); Kingsmere, Shawford, Hants, 75 × 91⁄4 ft (1977); Jephson Gardens, Leamington Spa, Warwicks., 45 × 9 ft (1981); Bath Botanic Garden, 38 × 4 ft (1978).
S. pubescens Tausch
S. korolkowii Dieck ex Koehne
S. japonica var. korolkowii Zab. ex Henry