Walnut

Family

Juglandaceae

Habit

Tree

Height

25 m

Spread

25 m

Growth

fast

Lifespan

150 years

First yield

after 8 years

Yield per plant

up to 60 kg a year

Yield in cultivation

up to 1.8 t/ha

Energy yield

1300 kcal per m² a year

Hardiness

zone 6–9

Shade tolerance

full sun

Soil

Sand, Silt, Loam, Clay

Soil moisture

dry – very moist

pH

5 – 8.5

Synonyms

Walnussbaum

Planned in gardens

8×

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

Edible: Pickled fruit, raw and germinated seeds – 1.8 tons per hectare, equivalent to around 1,290 calories per square meter. Oil can be obtained from the seeds. The tree sap is useful, and the leaves can be used for tea or as a pepper seasoning.

Medicinal: Walnut leaves and the green husk have long been used in European folk medicine. They contain tannins and juglone and were traditionally applied externally in washes and compresses for weeping rashes and eczema; as an infusion the leaves were regarded as an astringent. The kernel itself is not a medicine, but is nutritionally notable: 40 to 65 percent fat, of which around 70 percent are unsaturated fatty acids with a high proportion of linoleic and linolenic acid, plus almost 15 percent protein, potassium, calcium, phosphorus and iron at a very low sodium content.

Other uses: Dyeing: Catkins and fruit skin turn yellow, while nutshell and leaves become brown. Nut oil and wood polish extracted from the kernel are used to fill imperfections. Fruit skin is dried for wood painting, and leaves serve as insect repellents. The bark, rich in tannin, also acts as a tooth cleaner. The wood is heavy, hard, durable, and quick to dry.

The walnut needs space and depth: at least 120 centimetres of rootable, fertile, calcareous to moderately acid loam at pH 6.0 to 8.0, fresh to alternately moist but never waterlogged. It does best where summers are dry and hot and winters only moderately severe. The wood is sensitive to cold and late frosts destroy the flowers, so late-leafing cultivars such as Mars or Geisenheim 26 are the safe choice on frost-prone sites. A mature tree claims eight to ten metres square. Although the walnut is self-fertile, male and female flowers often open at different times, so a second cultivar secures the crop. No pruning is needed beyond the planting cut; any work belongs in late summer, because the tree bleeds heavily in spring. Wet summers bring leaf blotch and bacterial blight, and the walnut husk fly has been spreading since 2009.

Seed-grown walnuts first fruit after twelve to fifteen years and do not come true — what nut a seedling will eventually bear cannot be predicted. Sowing is therefore worthwhile only for raising rootstocks, or when deliberately selecting new types as was once done in traditional orchards. The nuts are heeled in during autumn and overwinter cold and moist before germinating in spring; they must be protected from mice, squirrels and crows. Cultivars are propagated by scion grafting or patch budding, usually under glass with controlled warmth, since the walnut unites poorly in the open. Rootstocks are one- to two-year-old Juglans regia seedlings. Tissue culture of adult cultivars is now possible as well.

Provides pollen and nectar for various insects, especially during spring flowering. The leaves, rich in tannins, are less attractive as food for many insects. The fruits serve as food for birds and small mammals. Its dense crown offers shelter and nesting sites for birds. The fallen leaves and chemical compounds in the soil affect surrounding vegetation and indirectly influence local wildlife.

Special features: One crop was able to completely supply itself with the walnut tree. Half-ripe nuts and early summer leaves contain 1% vitamin C! The leaves are very rich in tannins and are allelopathic, meaning they compost slowly and are avoided by insects. The leaves only shade the trees in early summer. Scandinavian walnuts only bloom from June onwards, meaning there is no risk of late frost. They prefer warmth and wind protection. They have a very good ratio of omega 3:6 fatty acids, ~2.1% in German forests; 1500 kWh/m²; 4.1 kWh/kg.

South-eastern Europe to Western Asia. The natural range of the common walnut is taken to be the Balkans and Western Asia; the occurrences from the Caucasus to western China, by contrast, may go back to human activity. It is the only walnut species native to Europe. Remarkable are the walnut wild-fruit forests of Kyrgyzstan around Arslanbob, regarded as the largest continuous stands of the species and discussed as an ice-age relict. The species reached Central Europe through Roman cultivation and later through monastery gardens; today it is widespread here and escapes regularly, because crows and squirrels carry off the nuts and bury them and so take over its spread.

The walnut is a tree of positions with warm summers and mild winters on deep, moist, nutrient- and base-rich soil. In its native range it grows in open mixed deciduous woodland of the mountain forelands and in stream valleys, where water and fine soil collect; in Kyrgyzstan it even forms closed walnut forests there. It avoids waterlogging, heavy clay and acid raw soils as much as frost pockets, since its new growth is vulnerable to late frost. Light-demanding, it develops a broad, deeply shading crown when free-standing and cannot hold its own in closed forest. In Central Europe its real habitat is therefore the cultural landscape: farmsteads, traditional orchards, field margins and woodland edges in warm valley and basin positions.