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Medium tree to 25 m, closely related to J. regia. Leaves 15–50 cm, petiole 7–12.5 cm, glabrescent, rachis glabrescent, terminal leaflet present. Leaflets 9 or 11(–15), ovate- or elliptic-lanceolate, 6–18 × 3–8 cm, glabrescent, margins entire or obscurely serrulate, apex acuminate; lateral petiolules absent or to 1 mm, terminal petiolule 2–3 cm. Male spikes 13.5–18 cm long; flowers with 24–27 stamens. Fruiting spikes with 1-3 fruits; fruits glabrescent, 3.4–6 × 3–5 cm, globose to subglobose, irregularly dehiscent; nuts valvate, valve forming a prominent, longitudinal ridge around the nut, the surface otherwise irregularly pitted; the shell forms deep ridges inside, the seed contorted and convoluted through filling the spaces between. Flowering March to April, fruiting September (China). (Lu et al.1999, Grimshaw & Bayton 2009).
Distribution Bhutan China Guizhou, Sichuan, SE Xizang, Yunnan. India Sikkim.
Habitat Forests in valleys and on mountain slopes; 1300-3300 m.
USDA Hardiness Zone 8-9
Taxonomic note Juglans sigillata is closely related to the Walnut, J. regia L., but can be distinguished by the number of leaflets (9–15, vs. 5–9 in J. regia) and by the shell of the nut (extremely hard and covered with numerous deep pits, while in J. regia the shell is wrinkled, not pitted).
In the course of researching Juglans it came to light that J. sigillata, a Chinese species, had apparently never been introduced to Europe or North America, although it was said to be in cultivation in New Zealand. It seemed odd that a Juglans species cultivated for its nuts should never have been collected, so I (JMG) asked my friend Michiel Zwaan of the seed company K. Sahin Zaden B.V. to look out for it when travelling on business in Yunnan. He was unable to locate it in person, but asked the China National Tree Seed Corporation if they could help. The result was that 5 kg of nuts arrived on my doorstep in December 2005. As shown in the illustration (Figure 46), they varied considerably in size and degree of pitting, but their extremely hard shells warranted the Chinese name Iron Walnut. The nuts were distributed to dendrologists throughout the United Kingdom and Europe and good germination resulted, so this species should appear in many arboreta in the near future. The seedlings have grown very fast (over a metre in height by July 2007 in one cultivated by Susan Grimshaw in Maidenhead, Berkshire); they have narrower, more acute leaflets than J. regia, but the relationship is clear. Juglans sigillata should make a tree very similar to J. regia and require the same conditions. The species was also collected in the wild in the autumn of 2005 by Keith Rushforth and Koen Camelbeke, and it has been growing at Hackfalls in New Zealand for some years (A. Coombes, pers. comm. 2004).