WALNUT: CHARACTERISTICS, HABITAT, USES, CULTIVATION - BIOLOGY - 2025

The walnut (Juglans regia) is a large, monoecious tree with deciduous foliage belonging to the Juglandaceae family. Known as common walnut, Spanish walnut, European walnut or Castile walnut, it is a species native to Europe and Asia.

As its name indicates, the walnut is a regal and large tree, reaching 25-30 m in height. In addition, it is crowned by a wide and dense crown that casts a closed shadow under its canopy.

Walnut (Juglans regia). Source: pixabay.com

The compound leaves are formed by 5-9 oval leaflets with entire margins and arranged alternately. The small and inconspicuous female flowers are not very showy, while the male ones are arranged in hanging catkins that favor anemophilic pollination.

The fruit is a drupe that has a fleshy, green cover that dries up when ripe and reaches a brownish hue: walnut. Walnut is grown primarily for obtaining fruits, although its wood is also used in carpentry due to its excellent quality.

General characteristics

Appearance

Walnut is a deciduous tree species that can reach 25-30 m in height and reach 2 m in diameter. The short and robust trunk when young has smooth and grayish bark, when adult it becomes rough and fissured with a silver-gray tone.

The stem is widely branched from the base with thick, arched and corpulent branches. In turn, numerous thinner and curved secondary branches emerge from the main branches.

The rather wide crown, spheroidal and domed, is made up of dense, bright green foliage. From its branches, glabrous shoots or broad and short buds of dark brown almost black color emerge that promote lateral growth.

Leaves

The compound and alternate leaves, 20-35 cm long, are arranged in odd-pinnate leaflets of 5-9 oval leaflets of 6-15 cm. Usually the leaflets at the apex are larger, decreasing in dimension towards the base of the leaf.

The margins appear whole when adults and slightly serrated in tender leaflets, with an asymmetric base and an obtuse or acuminate apex. They are leathery, glabrous, and bright green in color, with a short petiole initially reddish and then dark green.

Leaves and fruits of the walnut (Juglans regia). Source: pixabay.com

flowers

Walnut is a monoecious plant that blooms between April and June. The male flowers are grouped in cylindrical and pendant catkins of greenish-yellow tones, 15 cm long and in groups of 1-3 units.

The female inflorescences are grouped into 2-5 hirsute flowers with yellow stigmas at the ends of the young shoots. Globular fruits covered by succulent green bracts emerge from the female flowers, inside which the walnut is located.

Fruit

The globose, smooth and greenish fruits 4-5 cm long are grouped in 1-4 units on a short peduncle. The fruit of the walnut is a walnut or drupe, with the fleshy mesocarp and the firm endocarp furrowed in two valves.

The interior of the drupe is divided into 2-4 cells that contain the cerebriform seed composed of wrinkled cotyledons and divided into 2-4 lobes. The seed is edible, it has a sweet and very pleasant taste.

Chemical composition

The walnut seed is a food of high nutritional value, whose average caloric intake ranges between 650-690 kcal / 100 gr. In addition, it contains linoleic acid (omega 6), folic acid (vitamin B ₉), sucrose, dextrins, dextrose, starches and almost 50% of its weight is fat content.

The walnut contains 4% water, 15% carbohydrates, 5% fiber, 15% protein and 60% lipids, of which 30% is linoleic acid. It also has significant amounts of calcium, phosphorus, iron, sodium and potassium, as well as thiamine (vitamin B ₁) and riboflavin (vitamin B ₂).

Female inflorescences of Juglans regia. Source: H. Zell

Taxonomy

- Kingdom: Plantae

- Division: Magnoliophyta

- Class: Magnoliopsida

- Order: Fagales

- Family: Juglandaceae

- Subfamily: Juglandoideae

- Tribe: Juglandeae

- Genus: Juglans

- Species: Juglans regia L., 1753

Etymology

- Juglans: the name of the genus comes from the Latin «J ǒ vis glans», abbreviated as «Jūglans» which means «fruit of Jupiter».

- regia: the specific epithet comes from the Latin «rēgǐa», which means «royal or king».

Synonymy

- Juglans duclouxiana Dode

- Juglans fallax Dode

- J. kamaonia (DC.) Dode

- J. orientis Dode

- Juglans regia var. sinensis DC.

- Juglans regia subsp. fallax Popov

- J. regia var. kamaonia DC.

- J. sinensis (DC.) Dode

Male inflorescences of Juglans regia. Source: pixabay.com

Habitat and distribution

Walnut grows on fertile, deep, well-drained and low-acid soils, which facilitates the deployment of its extensive root system. In wild conditions it tolerates cold well, but is susceptible to occasional frosts, lack of moisture, and severe pruning.

It is common to observe that the area occupied by the shadow cast by the top of the walnut tree lacks other plant species. Indeed, an allelopathic effect occurs, since the tannins present in the falling leaves inhibit the germination and development of other plants.

Walnut is native to Southeast Europe and West Asia. Its current distribution is located from central and southern Europe, to central and south-western Asia.

In the Iberian Peninsula it is located up to altitude levels of 1,500 meters above sea level. Above all, in soils of calcareous origin, deep, loose and with a high content of organic matter.

It is a tree adapted to conditions of half shade or full sun exposure, which requires humidity and regular watering without being excessive. In the initial stages of growth it is susceptible to cold and frost, being a very long-lived species, even centuries-old.

Fruits of the Walnut (Juglans regia). Source: George Chernilevsky

Applications

Properties

The walnut is a dry fruit of high nutritional value, energy and fat content. Indeed, it contains polyunsaturated fats (68%), monounsaturated fats (16%) and saturated fats (11%), among which the essential fatty acid omega stands out.

On the other hand, it contains proteins (14%), high levels of fiber, phosphorus and magnesium, and a lower proportion of calcium, iron, potassium, selenium and zinc. As well as vitamins B ₁ (thiamine), B ₂ (riboflavin), B ₃ (niacin), B ₆ (pyridoxine) and B ₉ (folic acid).

Benefits

The balance of the supply of monounsaturated and polyunsaturated fats in walnuts contributes to various physiological and biochemical processes in the body. Its regular consumption controls blood pressure, diabetes problems, arrhythmias and prevents sudden death.

Omega-3 fatty acids improve symptoms caused by inflammatory diseases such as rheumatoid arthritis and psoriasis. In addition, it reduces the risk of suffering from some type of cancer.

Proteins contain various amino acids such as arginine, an essential element for the prevention of cardiovascular discomfort. For its part, vitamin B ₆ (pyridoxine) influences brain development, and increases the level of red blood cells in the blood.

Applications

Walnut is a species of high economic value, thanks to the production of edible nuts and the quality of its wood. The firm, greyish-brown wood with dark veins is used in joinery and turnery; firewood is used for combustion.

The leaves and the shell of the walnut have a high content of tannins, being used in dry cleaners to dye dark colored fabrics. In traditional medicine it is used to regulate blood glucose, heal wounds and relieve kidney colic. An enamel called "walnut" is obtained from the bark.

Nut. Source: Rasbak

Culture

Spread

Walnut trees are propagated by seed in the wild. However, commercially its multiplication is carried out through shoots or grafts on resistant and vigorous rootstocks.

Seed propagation is a rarely used method due to the enormous time it takes to obtain a new plant. However, the seeds are selected from trees adapted to the environmental conditions of the region, of excellent production and quality.

Walnuts require stratification in sand to loosen the outer bark, then they are macerated to open the shell. 2-3 seeds per point are placed in pots with fertile substrate, under nursery conditions two years are expected until the appearance of the seedling.

Vegetative propagation is carried out in the nursery by bud, spike or patch grafting. Bud grafting is carried out on short rootstocks, in order to avoid burns and the incidence of pathogens through the root.

In larger rootstocks, the graft is made standing, trying to insert the spike below the soil level. In this way, plants are obtained with a straight, robust and firm central axis without lateral branches.

Patch or veneer grafting is done on plants over one year old and fast growing. The process consists of lifting a portion of bark from the stock, which is replaced by a similar part of the graft with 1-2 buds.

In this method it is advisable to pre-ripen the buds, removing the leaves and leaving only the rachis about 8-10 days before the process. As well as using plastic tape or rubber bands to firmly hold the buds to the foot or rootstock.

Juglans regia nut. Source: pixabay.com

Requirements

Walnut adapts to different types of soils, although it prefers fertile, loose, deep and well-drained soils. Indeed, it prefers soils of limestone origin with enough sand and pebbles, as well as a pH of 6-7.5 or slightly alkaline.

Good moisture retention requires soils with a content of 1-2% organic matter and 18-25% clay. Soil pH determines the type of standard used in propagation, for acid soils it is used Juglans nigra and for alkaline Juglans regia.

Regarding environmental conditions, walnut is susceptible to late frosts in spring. Its cultivation should be restricted in places where the temperature drops to -1 ºC, since it affects the formation of new shoots and flowering.

Frosts in spring and autumn interfere with the proper development of the plant and tend to substantially reduce the harvest. In fact, during the juvenile phase, the intense cold can cause the death of all its foliage.

Otherwise, low humidity conditions and temperatures above 38 ºC can cause burns in the young walnuts. As a consequence, walnuts may turn out to be empty, but if the seeds have matured, the walnut tends to become dehydrated, wrinkled, and brown.

The pluviometric requirements of the crop range between 1,000-1,200 mm per year, with a necessary minimum of 700 mm. When rainfall is irregular and insufficient, irrigation must be used to achieve good plant development and increase walnut production.

In cold environments, walnut requires an open location where it receives direct solar radiation. Otherwise, in warm environments it adapts to semi-shady conditions.

Walnut bark adult. Source: User: Gerhard Elsner

Care

Walnut is a fast-growing plant that does not do well on clay soils as it is susceptible to excess moisture. During its growth phase it requires a warm environment without overdoing it, since extreme heat can burn its leaves.

However, during its productive phase, the frequent availability of humidity allows obtaining higher yields during harvest. The application of an organic mulch around the tree favors the conservation of humidity and prevents the appearance of weeds.

In the growth phase, it is advisable to carry out formation pruning, in order to establish a central stem with 4-5 main branches. Severe pruning is not recommended, only maintenance or sanitation pruning during spring or summer.

Poor and humid soils promote the appearance of bacterial diseases, which cause defoliation, fruit drop and plant death. High levels of relative humidity during the growth of leaf shoots favor the appearance of walnut rust.

In regions with a temperate climate, whipped by hot and dry winds from the south, premature foliage defoliation occurs. In addition, it is the favorable environment for the incidence of the carpocapsa moth (Cydia pomonella), a pest that causes the wormy of the fruit.

On the other hand, despite the fact that walnut is a rustic species, it is a plant very susceptible to a lack of humidity. Indeed, it is not recommended to sow it in dry land and in extreme drought conditions.

Diseases

Armillaria mellea

The micellar structure of the fungus penetrates the roots producing a yellowish fluid that can lead to the death of tissues. The external symptoms are manifested by the yellowing of the foliage, dry branches, small fruits and low production of nuts.

Gnomonia leptsostyla

The main symptom is manifested on the leaves as circular brown spots surrounded by a yellow halo. A strong incidence of the disease causes the spots to grow together, causing drying and falling of the leaves.

Gnomonia leptostyla in Juglans regia. Source: Rasbak

Phytophthora cinnamomi

A pathogen present in acidic soils, the fungus affects the roots causing injuries that can destroy the affected tissue. In severe attacks, the injuries reach the neck and trunk area, causing the death of the tree.

Xanthomonas juglandis

A disease caused by a bacterium that reproduces easily in conditions of abundant humidity and high temperatures. The higher incidence affects the production of buds, shoots, flowers and fruits, reducing production substantially.

References

1. El Cultivo del Nogal (2019) © Copyright Infoagro Systems, SL Recovered in: infoagro.com
2. Complete guide for the cultivation of walnut (2016) Portal Frutícola. Recovered in: portalfruticola.com
3. Juglans regia L. (2019) Catalog of Life: 2018 Annual Checklist. Recovered at: catalogueoflife.org
4. Loewe, M., & González, O. (2017). Common walnut (Juglans regia): an alternative to produce high value wood.
5. Walnut or walnut. Juglans regia (Juglandaceae) (2018) Region of Murcia Digital. Recovered at: regmurcia.com
6. Sánchez de Lorenzo Cáceres, JM (2019) Ornamental Flora of Spain. Recovered in: arbolesornamentales.es
7. Polanco Zambrano, DA (2017) Walnut (juglans regia). Nature Paradais Sphynx. Recovered in: Naturaleza.paradais-sphynx.com
8. Rodríguez, P., López Marcos, M., & Sala Galán, J. The graft in walnut (No. CIDAB-: S253-H6-24 / 72). Spain. Department of agriculture.