Juglans sigillata Seeds | Iron Walnut

Deeply pitted, sealed shells of iron walnut, thick and dark against a weathered hand — Iron Walnut (*Juglans sigillata*): Juglandaceae, warm hill country of Yunnan and the south-west.

LATIN NAME: Juglans sigillata

COMMON NAME: Iron Walnut

Plant Profile

Iron walnut is the walnut of Yunnan, and its shell is a signature: deeply pitted, sealed with ridges and too thick for a nutcracker. The tree reaches thirty metres in the warm hill country of the south-west, with a long, clean trunk and a broad crown of pinnate leaves that turn a soft gold before they fall. The nuts are small, hard to open and extraordinarily rich in oil, which is the point of the species in its home range. It grows slowly, fruits late and lives long, and the timber is heavy, dark and handsome. Old trees in village groves are often the oldest living things in the valley.

Science & Biotechnology

Juglans sigillata is now generally accepted as a species in its own right rather than a form of the common walnut, separated by the sculptured shell, the small nut and a chromosome complement that has been re-examined more than once. Molecular work places it close to J. regia but distinct, and the two hybridise where they meet in cultivation, which complicates both orchard planting and the seed trade. The oil is unusually high in linoleic and linolenic acids, and the press cake is fed to livestock. Its genome has been assembled in Chinese breeding programmes, where the species is used as a rootstock and as a source of oil quality and hardiness.

Uses & Benefits

In Yunnan the species is an oil crop before it is a nut crop: the kernels are pressed for a dark, strongly flavoured oil used for cooking and for oiling wooden utensils, and the residue goes to animals. The wood is hard, dark and takes a fine polish, though the tree is rarely cut now that the orchards are valued. As a garden tree it is a slow, dignified specimen for a large site, grown for the crown and the golden autumn rather than for a crop. The shell has a second life in handicraft, carved and polished, and pairs are sold as ornaments.

Growing from Seed

Iron walnut needs a winter: sow fresh nuts in autumn where they are to stand, or stratify them in damp sand at two to five degrees for three months and sow in spring. Plant about twice the nut’s thickness deep, and guard against rodents with a wire cage, since they find the nuts before you do. Germination is hypogeous, and the shoot emerges a month or more after the root has gone down. The seedling makes a deep taproot in its first season, which is why it should be planted young and never moved. Give it full sun, deep soil and shelter from late frost.

Our Take

A walnut for oil rather than for cracking, and a tree that rewards a long view of the garden. Plant it where it can stand for a century, and the shell will still be too hard when your grandchildren try it.

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