Japanese walnut

Family

Juglandaceae

Habit

Tree

Height

26 m

Spread

14 m

Growth

medium

Lifespan

150 years

First yield

after 8 years

Hardiness

zone 4–6

Shade tolerance

full sun

Soil

Sand, Silt, Loam

Soil moisture

semi-dry – very moist

pH

6 – 7.5

Times apply to the northern hemisphere in a temperate climate and shift with altitude and cultivar.

Edible: The nuts are sweet and good-tasting, distinctly milder than those of the black walnut. They are faintly ribbed and flattened ovoid to heart-shaped — hence the name heartnut for the variety cordiformis, whose shell splits cleanly into two halves with a single blow on the edge and releases the kernel whole. That is the real advantage over the common walnut: no laborious picking out of chambers. The price is size, for the kernels come out smaller than those of good walnut cultivars, in some cases smaller even than small-fruited walnuts. They are eaten raw, roasted, and in baking and desserts, and a fine edible oil can be pressed from them. The fruit ripens in October.

Other uses: The wood of the Japanese walnut is light and soft and does not reach the quality of common walnut; it is nonetheless worked into furniture and used in turnery. More relevant in the garden is its staining power: the sticky-haired green husk and the leaves contain juglone, which stains hands, clothing and stone paving permanently brown — the same substance has traditionally been used to dye wool and wood brown. The autumn leaves rot slowly and take about a year to become usable leaf mould; in the meantime they serve as a mulch protecting the soil from erosion and frost. At 12 to 18 metres tall with a wide-spreading, half-open crown the tree also makes an effective shade tree and windbreak.

Site: The Japanese walnut needs room — 12 to 18 m tall and about 8 m of crown width, often multi-stemmed and widely spreading. The position should be sunny to lightly part-shaded, the soil deep, free-draining to humus-rich, fertile and moist; dry sites and waterlogging are both ruled out. What matters most is its effect on its surroundings: like all walnuts it releases juglone into the soil, which checks the growth of sensitive neighbours such as tomatoes, potatoes, apples and pears — only juglone-tolerant plants do well beneath the crown. The sticky green husks stain terrace, paving and laundry brown, so the tree does not belong over a seating area, a path or a parking space. Deep-rooted, it stands firm and will not lift paving. Care: Young trees are staked for the first few years and protected from deer, which readily fray smooth-barked young stems. Prune as little as possible and only in late summer, never in winter or spring — walnuts bleed heavily from winter cuts and close them poorly. On good soil no feeding is needed. The species is hardy to about −34 °C and so suits harsh sites too; like all nut trees it remains vulnerable to late frost on the new growth, so frost pockets are best avoided. Butternut canker cannot touch it. Harvest in October as the fruit drops; remove the husk promptly wearing gloves, then dry the nuts. Squirrels and corvids harvest alongside you.

Seedlings are the simplest route. The nuts are husked and sown fresh in autumn into a sheltered bed or deep pots and cold-stratified outdoors over winter; they germinate reliably in spring. Because of the strong taproot, sow directly in the final position where possible or transplant very young — older seedlings establish badly. Mice readily dig out sown nuts, so a wire mesh over the seedbed helps. Seedlings come out variable and take many years to bear. Named cultivars such as 'Campbell CW1' or 'Imshiu' are therefore grafted in late winter onto seedling rootstocks of the same species. The species is monoecious, but male and female flowers mature at different times, so a second tree nearby markedly improves the crop.

As a wind-pollinated tree the Japanese walnut offers flower visitors little: the catkins carry no nectar, and no wild bee counts have been recorded for the species. Its value to wildlife lies elsewhere. The oil-rich nuts are high-grade winter food for squirrels, dormice and corvids, which carry them off and bury them and so take over dispersal as well. The wide-spreading, half-open crown of an older tree carries nests and offers perches; cavities in old wood follow with the years. The juglone is a limiting factor: it suppresses the understorey, so less flower and cover resource develops beneath the crown than under other large trees. For native caterpillars this non-native species is of practically no importance.

Special features: the variety cordiformis (heartnut) splits cleanly into two halves with one blow on the edge and releases the kernel whole; kernels smaller than those of good walnut cultivars; immune to butternut canker, hence planted in North America to replace Juglans cinerea; pinnate leaves up to 1m long, 3-10 fruits per spike; hardy to -34°C; juglone suppresses the understorey; prune only in late summer or it bleeds heavily; monoecious but dichogamous -> a second tree raises the crop

East Asia: Japan and Sakhalin, where it grows in temperate deciduous mountain forests and along watercourses. In the northeastern United States and eastern Canada it has been widely planted since the early 20th century because it is immune to butternut canker (Sirococcus clavigigenti-juglandacearum), the disease that has destroyed native Juglans cinerea across much of its range; most named cultivars come out of that planting history. In Central Europe the tree is rare and seen almost only in collections, arboreta and permaculture plantings, where it is valued as a robust complement to the common walnut.

The Japanese walnut is a tree of temperate deciduous mixed forest in climates with summer rainfall. In Japan it favours moist, deep soils in valleys, along streams and at the foot of slopes, where water and nutrients collect; it avoids markedly dry ground. Light-demanding and fast-growing, it takes gaps and woodland edges, quickly building a half-open, wide-spreading crown out of pinnate leaves up to a metre long. Like all walnuts it enriches the soil beneath it with juglone and so keeps competition in the root zone down. It roots deeply and is hardy to about −34 °C.