Walnut tree

Juglans regia

1. Identification and origin

The walnut tree (Juglans regia L.) is a tree species of the Juglandaceae family, native to Central Asia, namely the mountainous regions of Iran, Afghanistan and the Caucasus. Its spread to Europe occurred several centuries ago, and it is currently cultivated in temperate regions throughout the world. In Portugal, it is found both in commercial orchards and as isolated trees on agricultural holdings.

2. Economic importance

The walnut tree is a crop of high economic value due to the production of the walnut, a nut highly appreciated by the national and international market. Walnut timber is equally valued for its quality, being used in furniture and fine cabinetmaking. In Portugal, production is concentrated mainly in the Centre and North, with growing interest in modern irrigated orchards. Worldwide, the main producers stand out as the United States (California), China, Iran and Turkey.

3. Botanical characterisation

A large tree, which can reach 20–30 m in height, with a broad, rounded crown. The leaves are compound, imparipinnate, with 5–9 ovate leaflets. The species is monoecious, with male flowers in pendulous catkins and female flowers in small terminal groups. The fruit is a drupe whose outer husk (epicarp) separates at ripening, releasing the walnut, formed by a hard shell (endocarp) that surrounds the edible kernel. The longevity of the tree can exceed 80–100 years.

4. Soil and climate requirements

The walnut tree adapts to temperate climates, requiring an adequate number of winter chilling hours for good budbreak. It is sensitive to late frosts, especially during flowering. It prefers deep, fertile, well-drained soils, with a pH between 6.5 and 7.5. It tolerates some summer dryness, but regular irrigation significantly improves the size and yield of the walnuts. It is sensitive to waterlogging and soil compaction.

5. Main pests

• Walnut husk fly (Rhagoletis completa): damage to the epicarp and loss of quality
• Codling moth (Cydia pomonella): perforations and destruction of the kernel
• Aphids (Chromaphis juglandicola): leaf curling and honeydew
• Mites (Tetranychidae): chlorosis and reduced photosynthesis
• Weevils and other xylophagous beetles: damage to wood and branches

6. Main diseases

• Anthracnose (Ophiognomonia leptostyla): leaf spots and early defoliation
• Bacteriosis (Xanthomonas arboricola pv. juglandis): necrosis on leaves, fruits and branches
• Woody cankers: opportunistic fungal agents
• Root rots associated with waterlogged soils
• Armillaria spp.: white rot of the roots in contaminated soils

7. General crop management

Management of the walnut tree includes choosing cultivars adapted to the local climate and to the number of chilling hours available. Pruning should be moderate, favouring initial formation and the removal of dead or poorly positioned branches. Regular irrigation, especially in summer, is essential to ensure good size and filling of the walnuts. Fertilisation should be balanced, with attention to nitrogen and potassium. Control of the walnut husk fly and of bacteriosis is fundamental in regions of greater pressure. Harvesting is carried out when the epicarp opens naturally, allowing the walnuts to fall or be easily removed.

Bibliographic references

• EPPO Global Database – Juglans spp.
• CABI – Crop Protection Compendium – Juglans regia
• Solar, A., et al. (2017). Walnut production and management. In: Advances in Nut Tree Crops. Springer
• Pollegioni, P., et al. (2014). Genetic resources of Juglans regia. In: Forest Genetic Resources Review. FAO
• Ramos, D. (1997). Walnut Production Manual. University of California