Japanese walnut

Photo: Qwert1234 — CC BY-SA 3.0 · source

Survives zone 4 through 8

Harvest calendar

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

The Japanese walnut is grown as much for its huge, bold, downy pinnate leaves and ornamental spring catkins as for its nuts, which form in clusters of 4–10 spherical fruits inside a green husk. Its heart-shaped-nut cultivar, the heartnut (var. cordiformis), cracks cleanly into two halves and is prized for eating fresh; the species is also notable as a low-juglone, canker-resistant genetic donor used to breed disease-tolerant hybrids with the endangered butternut.

Hardiness

Zone 4–8

Habit

Tree

Edible part

Nut

Ripens

Sept–Oct

Mature size

Tree 10–20 m, broad-crowned — not containerable

Light

Full sun

Soil pH

6.0–7.5 (near-neutral; lime-tolerant)

Environment

Temperate

Native range

Japan — Hokkaido, Honshu, Shikoku and Kyushu — and the Kuril Islands, growing along streams and in moist lowland-to-mid-elevation forests. Long reported as native to Sakhalin as well, but a 2022 study found no evidence of natural occurrence there and concluded the island's trees were introduced from Japan in the early twentieth century.

Status

Both wild and cultivated

Years to fruit

5–10 yrs

Growth speed

Medium — one-year seedlings grow about 30–70 cm, and mature growth is broadly comparable to the 30–60 cm/year typical of other walnuts once established.

Spacing

1200 cm (472 in) apart — about 144 m² (1550 sq ft) per plant

Leaf shape

Large pinnately compound leaves 50–90 cm long with 11–17 leaflets, each leaflet 7–16 cm long by 3–5 cm wide, finely toothed, and downy-pubescent, giving the foliage a soft, bright yellow-green cast.

Root system

Deep taproot — seedlings are intolerant of root disturbance and should be planted in their permanent position while young.

Fruit description

Nuts are spherical, 3–5 cm across, borne in clusters of 4–10 inside a green, non-splitting husk; the shell is thick and deeply sculptured. Kernels have a very good, rich flavor and yield an edible oil that turns rancid quickly. The heartnut cultivar produces a flattened, heart-shaped nut with a thinner shell that separates into two intact halves.

Propagation

Grown from seed — sow fresh nuts in autumn in deep pots in a cold frame, since dry-stored seed needs several months of cold stratification before it will germinate; one-year seedlings typically reach 30–70 cm tall. Named heartnut selections (var. cordiformis), prized for their heart-shaped, easy-cracking nuts, are propagated by whip-and-tongue grafting in late winter onto heartnut, butternut, black walnut, or English walnut seedling rootstock, since seedling-grown heartnuts vary widely and unpredictably in nut quality.

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.