Walnut, <em>Juglans regia</em>: planting, pruning and care

Description and botany

Genus Juglans belongs to family Juglandaceae and includes about twenty species, native to south-east Europe, eastern Asia, northern India and North and South America. Common walnut, also called English walnut, Juglans regia, source of main cultivated varieties in France is native to a vast area stretching from south-east Europe to China. It is cultivated both for its walnuts and its high-quality wood.

English walnut is a large tree with a deep root system that withstands severe cold (-30 °C) since it occurs naturally on Himalayan foothills. However, it does not like overly hot situations such as Mediterranean south and its young shoots are sensitive to spring frosts common in valleys. Choose in that case a late variety such as Franquette or Parisienne.

Its stocky outline gives impression of a robust tree with strong branches, 15 to 20 m high with a 9 to 18 m spread. Growth is relatively slow as tree reaches 10 m after 20 years. Good rich soil composed of alluvial deposits, fresh, calcareous and well drained allows it to live for more than 200 years.

The young bronze-purple leaves become bright green at ripeness before yellowing in autumn. The alternate leaves, borne on a petiole of 3–4 cm, comprise 5 to 9 pointed leaflets with an ovate base, the terminal leaflet being more developed.

Flowering is clearly visible in spring – between April and June depending on variety – in the form of yellow-green male aments measuring 5 to 10 cm long in a lateral position along the shoots.

Juglans regia – botanical illustration by P.J. Redouté

The female flowers, rather inconspicuous, appear at the tips in groups of 3 or 4. This tree needs a long establishment period to start producing walnuts, between 15 and 25 years, but there are fast-producing varieties such as Franquette which flower after 5 to 7 years. Production is fairly variable from year to year but can continue until age 70. It is advisable to plant another variety nearby to encourage fertilization of the flowers because male aments do not mature at the same time as the female flowers.

The spherical fruit 4–5 cm in diameter has a fleshy, smooth green husk, very staining to the touch, called brou which encloses the stone corresponding to the walnut protected by its beige shell. The fruit opens in autumn to release the seed whose shell, more or less hard, is formed of two hemispheres with a sinuous surface. The cracked walnut reveals two whitish kernels covered with a thin beige skin slightly bitter when walnut is fresh, which suggest the hemispheres of a brain. They are eaten fresh or dried, grated or pressed for oil production.

The black walnut (Juglans nigra) is also valued for its nut and the quality of its wood. It has the advantage of resisting armillaria root rot.

According to the doctrine of signatures, very popular in the Middle Ages, nature would give keys to knowledge by its physical appearance. People thus imagined, rightly, that eating walnuts benefited the brain by referring to the seed’s appearance in two sinuous hemispheres.

Walnut: male ament, female flower, foliage and fruit, walnut

Main walnut varieties

Common Walnut Franquette - Juglans regia

Variety widely grown in our territory, part of AOP Walnut of Grenoble varieties, resistant to drought and disease. Walnut of medium calibre with an elongated hard shell, well filled, with kernels of good flavour. Majestic, large tree that bears fruit from its fifth year. Franquette is self-fertile, very productive and late-ripening; it can be grown alone but yield will be better with other walnut trees nearby.

• Flowering time June
• Height at maturity 10 m

See the product sheet

Common Walnut Parisienne - Juglans regia

Variety belonging to AOP Walnut of Grenoble, traditionally cultivated around Grenoble. Large-calibre walnut with a thin, tender shell that breaks easily by hand. Good, fragrant walnut, to be eaten fresh as dessert. Begins to bear fruit from its eighth year. Self-fertile and late-ripening; it can be grown alone but yield will be much better with other walnut trees nearby.

• Flowering time June
• Height at maturity 10 m

See the product sheet

Planting

Where to plant walnut?

Plant walnut in isolation, in very sunny, not too windy position, in deep, fairly loose soil so it can extend its roots even if it tolerates clay.

Walnut dislikes both stagnant moisture and prolonged drought. Provide fresh, deep, well-drained soil, even calcareous. Despite excellent hardiness (-25°C), an overly cold winter limits fruiting and can weaken the tree. Its sensitivity to late frosts means that a walnut will not develop properly north of the Loire whereas it grows well up to 1,000 m altitude in southern France. Drôme, Isère and Périgord are main French production regions for walnut.

When to plant?

Prefer autumn (October–November) to plant walnut trees, or March–April.

How to plant?

This plant is easy to grow but bear in mind that nothing grows under its foliage because it produces juglone, a compound aromatic toxic to most plants. Its shade is dense. People say you mustn’t fall asleep under a walnut, or you’ll catch a cold!

Allow spacing of 8–10 m between each walnut.

• Dig a spacious planting pit at least three times the width of the rootball.
• Submerge pot in a bucket of water to soak it thoroughly.
• Add a few shovelfuls of sand and gravel to ensure good drainage around roots, and some compost to lighten soil. On very acidic soil (pH below 6.5), add dolomite or ash.
• Place a stake at a distance from the rootball.
• Place the plant in the planting hole, avoiding burying the collar.
• Replace soil and firm down lightly.
• Tie the trunk of the walnut to its stake without overtightening.
• Water then mulch.

Care

• During the first few years, water your walnut during periods of severe drought.
• Apply compost or organic fertiliser in autumn or spring under the crown. After three years, once your walnut is well established, it requires little care.
• Shape your tree by selecting three or four main scaffold branches when the desired height is reached to give a goblet habit. You can also let it grow taller if there is sufficient distance between the walnut and the house. In that case, keep the leading shoot and then gradually raise the lower branches.
• Minimise pruning of your walnut; only remove dead wood and crossing branches when they are of small diameter.

Walnut harvest and storage

The husk splits open on the tree and drops the walnuts to the ground between September and November depending on the earliness of the variety. Use a pole to shake the branches and speed their fall if necessary.

Keep walnuts that are not blackened and show no wormholes.

You can eat them fresh or dry them on racks by laying them in a single layer in a cool, dry and well‑ventilated place.

Almost half of a walnut’s moisture evaporates, allowing storage for 1–2 years without problem.

Green walnuts can also be harvested in June, on St John’s Day, for making walnut wine.

→ Learn more in Solenne’s tutorial: Harvesting and storing walnuts

Propagation

Sowing in spring is the easiest way to propagate walnut trees unless you want to grow specific varieties, in which case layering by stooling or grafting onto a seed-grown walnut is recommended.

Sowing a walnut

• Harvest walnuts in autumn,
• Remove husks (wear gloves to avoid staining hands).
• Stratify them in sand in a pot placed facing north, covered with 15 cm of sand.
• Place a wire mesh over the pot to stop rodents getting at them.
• Walnuts that have produced a radicle, usually around March, are then planted out in open ground at their final position in groups of three or placed in a deep pot filled with garden soil.
• Bury them in situ in groups of three, 5 cm deep, spacing groups 8 to 10 m apart.
• Select the most vigorous of the three seedlings the following autumn. You can graft a variety after one year by a cleft graft performed at the start of the following spring.
• Take scions about 1 cm in diameter in winter and insert them into sand placed facing north.
• When sap rises, cut back the rootstock stem and split it to insert a scion shoot, cut with a double bevel, into the cleft.
• Tie the graft with damp raffia and seal with grafting wax.

Grafted young plants reach ripeness faster than young plants grown from sowing.

Uses and associations

Few plants will grow under walnut trees, but you can plant it against a backdrop of country hedgerow composed of European spindle, cornelian cherry, laurustinus and other plants for calcareous soil if your soil is alkaline. Space walnut trees at least 8 to 10 m apart if planting several. Let branches develop naturally or train to an open-centre form to harvest walnuts more easily.