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Understanding the walnut tree: exploring its traits and methods for successful cultivation

Walnut tree cultivation boasts a rich heritage. Delve into the tree's attributes and the application of organic methods to successfully nurture these trees.

by BioGrow

Walnut is one of the oldest fruit trees known and cultivated by humans. Due to its majestic size and great longevity, it can be considered the king of fruit trees. Its fruits can be consumed fresh or dried. Furthermore, they can be used for making sweets, extracting oil, and when still green, for making the famous nocino liqueur. The walnut tree also holds significant value for its precious, durable wood. It has always been used in high-quality furniture production. Lastly, let’s not forget the beauty of this tree, valued both as an ornamental plant and for its excellent shading capability. In our country, walnut cultivation has a rich tradition, although unfortunately, over time, it has been declining. Until the 1960s, Italy was the world’s leading producer; however, today, it only holds a small share of production. Sadly, the majority of walnuts on the market are imported, particularly from the United States. This cultivar deserves to be rediscovered and valued, primarily for its tradition, but also for its economic worth.

Let’s delve into the characteristics of the walnut and explore how to cultivate it properly.

Botanical Background and History

Walnut tree

The walnut (Juglans regia) is a fruit tree belonging to the botanical family Juglandaceae. This family comprises about sixty tree species divided into seven genera, with our point of interest being Juglans.
Juglans regia is the most economically important species and is also known as the common walnut or English walnut. The origin of the walnut is rather uncertain. Fossil remains indicate its presence as a wild species already 9,000 years ago in Asia, Europe, and North America. However, due to glaciations, the plant’s geographical range has shifted. Following evolution and adaptation, about 20 species have developed. It’s rare, though, to find original wild walnut trees. In Europe, they are significantly present only in certain areas of Greece. In the rest of the continent, walnuts are mostly cultivated, and the varieties are suited to different climatic environments.
Apart from Juglans regia, there is another species of agronomic interest in this genus, Juglans nigra. This species produces non-edible fruits and is also known as the black walnut. Due to its strong resistance, it’s used as the rootstock for common walnuts.

Characteristics of the Walnut Tree

Young walnut orchard

The first striking characteristic of the walnut tree is its majesty. It can reach a height of 20-25 meters and is a long-lived plant, capable of living for over a century. It’s a deciduous species, shedding its leaves in the winter. It’s also a broad-leaved species, characterized by wide leaves (regardless of their shape). This term contrasts with needle-leaved trees and shrubs.
The walnut is a solitary tree, meaning that no other plants grow around it. This phenomenon, known as allelopathy, is due to the presence of a substance called juglone in its roots, leaves, and bark. Juglone is toxic to other plants and is released into the soil by the tree. This is why walnuts rarely appear in natural forests. The walnut tree has an extensive root system with fibrous roots. Through this system, it absorbs a large quantity of minerals and other elements from the soil. Therefore, its growth is solitary for this reason as well.

Trunk, Leaves, Flowers, and Fruits

Leaves and fruits of the walnut tree

The trunk of the walnut tree is truly impressive. Very solid, tall, and straight, it gives the tree its majestic appearance.
When young (first twenty years), the bark is gray-ashen and smooth, but over the years, it becomes darker and rough. The trunk’s diameter can even measure up to one meter. The walnut has a wide canopy, which can have a diameter of up to 10 meters. The leaves are light green, oval-shaped, with a smooth margin. They are pinnate and grouped in sets of 6-7, with the top leaf being larger than the others.
This tree is monoecious, meaning it bears both male and female flowers on the same plant. Male flowers are about 10 to 15 cm long and appear on the previous year’s branches before the leaves. Female flowers are usually solitary or grouped in sets of 3-4. They form after the male flowers, along with the leaves. The fruit, namely the walnuts, consists of drupes formed by the husk, which is a green, fleshy, and fibrous exocarp. The husk turns black when the fruit is fully mature, revealing the endocarp, which is the actual edible walnut. The endocarp is the woody shell that encloses the edible part, called the kernel.

Walnut Varieties

There are several fruit walnut varieties to choose from when starting cultivation. Let’s see the main ones.

  • Sorrento: This is the most common variety in Italy. It has a dual purpose, serving both as a fruit and wood source. This variety is very vigorous, producing medium-sized fruits of excellent quality. It has a medium to late ripening, occurring in October in the North and late September in the South.
  • Franquette: This variety also has high vigor and a dual purpose. Its fruits are much larger, but the plant requires a certain amount of cold in the winter months to produce well. Therefore, cultivation of this variety is recommended in the North and colder regions of Southern Italy.
  • Hartley: This variety originates from America, with medium vigor and only intended for fruit production. The quality is excellent, and it can be cultivated successfully both in the North and the South.

Walnut Cultivation

Climate and Exposure

Walnut trees are cultivated from the plain to the hills, up to 800-1000 meters. They tolerate winter cold well but fear spring frost, which can damage developing buds. The ideal environment has an average annual temperature between 8 and 15°C, making regions with a mild climate suitable. Adequate precipitation during the spring vegetative phase is also necessary. The absence of rain in May-June can compromise fruit quality. For exposure, a sunny area is recommended, while still being protected from the wind.

Soil

The ideal soil for walnut trees is deep, light, and fresh, with good fertility. Siliceous soils derived from the degradation of granite and crystalline schists are also suitable. In terms of soil pH, fruit walnut trees tolerate slightly acidic soils, with a pH between 6 and 7.5, which corresponds to sub-acidic and neutral soils. It’s crucial that the soil drains well and doesn’t suffer from waterlogging. As such, heavy, asphyxiated, or clayey soils should be avoided when establishing a walnut grove. Such soils are prone to water stagnation, which can lead to root rot, cryptogamic diseases, weakening the plant, and reducing production.

Propagation

The reproduction of walnut trees can be done by seed or through grafting. The most commonly used rootstocks are the “seedling rootstock” or, as mentioned, Juglans nigra. In Europe, seedling rootstocks are preferred as they ensure the tree’s optimal development, even though they delay fruiting. In the USA, the black walnut is preferred due to its vigor, accelerating fruiting.

Soil Preparation, Planting, and Spacing

For planting a walnut grove, typically, plants of at least two years old from nurseries are used. They are planted in the autumn and receive grafting of the desired variety in the winter. Thorough soil preparation is essential, involving proper ground work. Depending on soil characteristics, plowing or subsoiling is recommended. In this phase, creating water runoff channels is advisable to prevent water stagnation. After working the soil, well-rotted manure (about 350 quintals per hectare) should be incorporated through milling. Spacing should consider the tree’s strong vigor. Thus, a distance of at least 8 meters between rows and trees should be maintained when using black walnut rootstocks. For seedling rootstocks, 10 meters is necessary.

Irrigation and Fertilization

Irrigation is not strictly required for walnut trees. It’s a plant that can grow lush with normal precipitation. However, a hot and dry season can stress the tree and compromise fruit quality, especially when the plant is young. Ideally, water should only be applied when truly necessary; irrigated soil is advantageous in this regard. Similarly, fertilization is straightforward. After the initial fertilization during planting, applying mature manure or compost every two years during the tree’s first 10-15 years of life is sufficient.

Cultural Care, Light Cultivations, and Ground Cover

Grass-covered walnut orchard

Grass-covered walnut orchard

During the initial years of walnut cultivation, attention should be given to soil management and weed control. Some adopt the practice of light surface cultivation using a tractor, which certainly keeps the soil clean. Unfortunately, these operations expose the soil to erosive agents, and if not carried out correctly, they can cause damage to the root system or the formation of a compacted layer. Ground cover, the practice of maintaining a green layer between plants, is widely used in organic farming.
This technique involves keeping the cultivation soil covered with vegetation. In the case of walnuts, native flora is utilized, or a mix of seeds not affected by allelopathy is used. Generally, for the latter case, grasses such as Lolium perenne, Festuca ovina, Festuca arundinacea are used. The grass is cut to about 5 cm from the ground in early spring. Then, in late summer, it’s cut again when the grass cover has reached 15-20 cm. The cut grass is left on the ground to contribute organic matter. Ground cover improves soil physical properties, increases porosity, and enhances available water reserves. Additionally, on sloping terrain, erosion is prevented.

Pruning Walnut Trees

In the initial years of walnut tree pruning is focused on creating the definitive shape. Two green pruning interventions are carried out. The first, during vegetative growth in spring, involves removing water sprouts and shortening other branches to 2 leaves from the central axis. The second intervention is in July, during the second vegetative growth, again aimed at promoting the central axis’s development. These interventions continue until the tree reaches a permanent framework about 2.5 meters above the ground. In the subsequent years, pruning interventions are relatively limited. Fruit walnut trees, in fact, do not tolerate drastic pruning and self-regulate growth. During pruning, water sprouts, damaged or poorly positioned branches in the canopy, and few return cuts are removed.

Production

Walnut trees enter production after a certain number of years. Plants grafted onto seedling rootstocks usually take about 8-10 years before producing satisfactory yields. With grafts onto Juglans nigra, this time is shortened, and the tree starts producing in the 5-6th year. The peak of production occurs from the twentieth to the seventieth year of age, illustrating the prolific nature of this tree. In optimal conditions, a single tree can yield up to 50 kg of walnuts.

Pests and Challenges

Walnut fly infestation

Walnut fly infestation

Walnut trees can suffer from pest attacks and some challenges related to cryptogamic diseases. Among insects, attention should be paid to the presence of defoliating Lepidoptera such as the Walnut Caterpillar, which can be controlled using Bacillus thuringiensis. The Walnut Husk Fly, Rhagoletis completa, can also cause problems, which can be effectively managed using chromotropic traps in yellow. Wood-boring insects like the Wood Borer can cause serious damage. This pest is combated using sexual confusion traps, which eliminate male specimens, effectively stopping proliferation. Another method, suitable when larvae are active, is removing them using wire to penetrate the galleries formed in the wood. This mechanical method has excellent success, but it’s labor-intensive in cases of severe infestations.
Another insect to watch for is the Codling Moth (Cydia pomonella). This moth damages fruits, often leading to the presence of worms inside walnuts. It’s a problematic pest not only for walnuts but also for all pome fruits. It requires a comprehensive defense strategy. A detailed study of the insect and its biological control is recommended for the future.
Among challenges, mention should be made of ink disease and anthracnose, which can be prevented in high-risk areas by avoiding water stagnation and applying copper treatments during winter.

Ornamental Cultivation

The considerations provided thus far apply to medium to large-scale walnut cultivation and planting. Naturally, walnut trees can also be cultivated individually or in small numbers for ornamental purposes or personal production. Being a long-lasting and space-occupying tree, careful consideration should be given to selecting a suitable location on your property. We recommend this tree since it’s relatively easy to cultivate, visually appealing, and can provide great satisfaction in terms of production. Lastly, regular consumption of walnuts seems to improve cognitive abilities.

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