Walnut is the most important temperate nut fruit of the country with incredible health benefits. North Himalayan region is popular for walnut cultivation, The seeding trees attain giant size and start bearing nuts of the variable sizes and shapes after 10-15 years where as vegetative propagated plants are true to type on produce almost uniform sized nuts after 45 years. They remain within manageable size. But the major constraint is low success in vegetative propagation. Limited availability of scion material form desired trees resulted very few vegetative propagated plants.
Climate and Soil
Climate Sensitivity
Walnut trees are sensitive to extreme weather conditions. Key considerations include:
- Low Temperature: Late spring frosts can damage young flowers, while early autumn frosts may cause shoots to fail to leaf.
- High Temperature: Walnut trees should not be exposed to extreme heat during summer as it affects growth.
- Annual Rainfall: Well-spread rainfall of around 75 cm or more is ideal, and a temperature range of 29-32°C at harvesting leads to well-filled kernels.
Soil Requirements:
Walnuts flourish in well-drained, silty loam soil with rich organic matter. Avoid rocky, alkaline, and impervious clay soils as they hinder growth.
Varieties of walnut
Some popular varieties include:
- Chakkrata Selection
- Govinda
- Eureka
- Placentia
Propagation and Rootstock of walnut
Propagation via Seedlings
Walnut seedlings are commonly used as rootstocks. The nuts should be procured from the current season’s crop and either sown directly in seedbeds during November or stratified in moist sand until the soil is suitable for sowing. For direct sowing, maintain a spacing of 25 cm between nuts and 75 cm between rows.
Grafting and Budding
For budding, opt for seedlings with more than pencil thickness, and use 5-6 months old scions, defoliated 10-15 days earlier. Patch budding or veneer grafting in July/August yields good results. In spring, practice tongue or cleft grafting using rootstocks of similar diameter to scions.
Planting and Spacing
- For Seedlings: Plant trees 10 m apart.
- For Grafted/Budded Plants: Maintain a spacing of 7-8 m.
- Ideal Planting Season: December to March is best for walnut planting using square, hexagonal, or contour systems (for undulating soils).
Training and Pruning
The Modified Central Leader System is ideal, providing good structural strength to the tree’s framework. While pruning is not practical for large seedling-grown trees, it is essential for grafted plants to improve productivity and nut quality.
Manuring and Fertilization
For 10-year-old trees, a recommended dose of 250 g Nitrogen (N), 60 g Phosphate (P₂O₅), and 250 g Potassium (K₂O) is advisable. Follow these steps:
- Apply the full dose of P and K and half the amount of N two weeks before bloom.
- Split the remaining half of N into two equal doses to be applied three weeks after fruit set and in early July.
- Address zinc deficiency with a 0.4% Zinc Sulphate foliar spray.
Irrigation in walnut
Although walnuts are typically grown under rain-fed conditions, supplemental irrigation is crucial during the 5-6 weeks after blooming. Insufficient moisture leads to poor-quality kernels and reduced yields. Where irrigation is available, it should be applied at 1-2 fortnight intervals based on soil moisture and dry spell duration.
Harvesting and Post-Harvest Management
Walnuts are ready for harvest when the hull color shifts from green to yellowish and begins splitting at the suture. For efficient harvesting:
- Stage for Harvesting: Ensure nuts are harvested at the PTB (Packing Tissue Brown) stage to avoid stick-tight hulls.
- Use of Ethephon: Spray ethephon (2-chloroethyl-phosphonic acid) at 1000-1500 ppm to prevent hull fermentation and ease de-hulling.
- Post-Harvest Handling: Clean the orchard floor and spread tarpaulins before knocking the nuts. Dehull, wash, and dry the nuts properly to prevent mold development. Finally, store the kernels in wooden boxes after shell removal.
Walnut cultivation offers significant economic benefits if managed correctly, from planting to harvesting. By selecting suitable varieties, providing optimal soil and climatic conditions, and implementing best practices in propagation, pruning, and fertilization, walnut growers can achieve high-quality yields.