iron walnut

Photo: Kristof Zyskowski & Yulia Bereshpolova — CC BY 2.0 · source

Survives zone 6 through 9

Harvest calendar

Best planted in early spring after your last frost, or in fall before the ground freezes, depending on your climate.

Iron walnut is a Chinese endemic centered on Yunnan Province, cultivated for many centuries and named for its extremely hard, deeply pitted shell — it is closely related to, and possibly a landrace of, the Persian walnut, distinguished mainly by its higher leaflet count and far tougher shell.

Hardiness

Zone 6–9

Habit

Tree

Edible part

Nut

Ripens

Sept–Oct

Mature size

Tree 10–20 m — not containerable

Light

Full sun

Soil pH

6.0–7.5 (near-neutral; lime-tolerant)

Environment

Montane

Native range

Endemic to southwestern China — chiefly Yunnan, which is the center of its diversity and cultivation, with populations also in Guizhou, Sichuan and southeastern Xizang (Tibet) — and extending into Bhutan and northeastern India (Arunachal Pradesh, Sikkim). It grows in forests in valleys and on mountain slopes at 1300–3300 m elevation.

Status

Cultivated

Years to fruit

5–10 yrs

Growth speed

Initial growth is rapid — seedlings often behave as low, shrubby specimens for their first year or so before developing into a standard tree under good growing conditions — though specific annual growth increments are not documented in the sources available.

Spacing

900 cm (354 in) apart — about 81 m² (872 sq ft) per plant

Leaf shape

Large pinnately compound leaves 15–50 cm long with a 7–12.5 cm petiole, bearing 9–15 (usually 9 or 11) leaflets that are ovate- to elliptic-lanceolate, 6–18 × 3–8 cm, with entire to obscurely serrulate margins and acuminate tips; the terminal petiolule is 2–3 cm long.

Root system

Cultivated trees are typically grafted onto wild J. sigillata rootstock in Yunnan's orchards; like other walnuts it is presumed to develop a deep taproot, though dedicated root-system studies specific to this species are limited.

Fruit description

Fruits are globose to subglobose, 3.4–6 × 3–5 cm, with a shell that is extremely hard and covered with numerous deep pits — the feature that gives iron walnut its name and distinguishes it from the smoother-shelled J. regia. It is cultivated in southwestern China specifically for its nuts, despite the shell's toughness making cracking difficult.

Propagation

Wild populations reproduce sexually from seed, while cultivated trees are propagated clonally by grafting onto wild rootstock — a practice so widespread across Yunnan's walnut-growing regions that grafted cultivars are now replacing many natural stands. Seed introduced to Britain in 2005 from the China National Tree Seed Corporation showed good germination.

Pollination

Monoecious but HETERODICHOGAMOUS — a 2nd cultivar of the opposite type greatly improves set

The edible parts listed above are a starting point, not a field guide. Identify a plant with certainty — ideally against a local authority — before eating any part of it, and read the full disclaimer. Spotted something wrong? Report it.