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Walnut trees benefit greatly from a "drainage" treatment before grafting, which helps reduce the flow of wound exudate. This technique involves making deep cuts at the base of the tree, penetrating into the xylem, so that the sap flows out from the wounds on the trunk rather than the graft site. The closer the cut is to the roots, the more pronounced the root pressure becomes, leading to greater fluid release. By doing this, the amount of bleeding at the graft interface is significantly reduced, improving the success rate of high grafting.
Selecting high-quality scion varieties is essential for successful grafting. Early-maturing types such as Hong Ling, Lu Guang, Liao He 1, Zhonglin 1, Zhonglin 5, Shaanxi He 1, Xifu 1, Zha 343, Wen 185, Yuan Feng, and others are commonly used. Late-maturing varieties like Xiluo 1, Xiluo 3, Zhapin 1, and Jinlong 1 are also popular. It's recommended to use 1–3 main varieties for high-yield production, and arrange them in a ribbon or cross pattern with pollination varieties at a ratio of 5:1 to enhance fruit set.
Establishing a proper scion nursery is crucial. Only vigorous, fully developed branches should be selected for grafting, as hollow piths are not suitable. Since mature walnut trees often have few branches, it's necessary to grow new scions through propagation. Seedlings are first grown from walnuts, then improved using bud grafting techniques. Once established, scions can be collected annually from flat seedlings. These young plants develop strong branches with a large root system, allowing for continuous scion collection. Scions should be taken 20 days before the tree begins to sprout, ideally in late winter or early spring. To minimize bleeding, avoid watering in spring and prune part of the root system. After collection, scions should be sealed with wax and stored in a cool, ventilated place with moist sand or sawdust, or wrapped in plastic and kept in a cold storage at 0°C to 6°C with over 80% humidity.
The timing of grafting is also important. Injurious flow varies depending on when the leaves are left on the rootstock. During dormancy, the flow increases as the tree starts to germinate, peaks when buds begin to open, and decreases after they emerge. Therefore, grafting should be done just after bud emergence. Leaving some lower leaves on the rootstock helps absorb excess water, reducing the amount of sap released from the wound.
Choosing the right grafting method is key. The "bark graft" technique involves selecting well-woody scions, cutting them to 12–15 cm with 2–3 buds, and making a sloped cut on the lower end. The number of graft interfaces depends on the tree’s age: young trees (under 5 years) typically get one, while older ones (7–12 years) may have 2–4, and those 8–20 years old may have up to 7. When performing high grafting, consider the orientation of the main branches. Cut the top first, then make a clean cut with a hand saw, and select a smooth area beneath the cut. Carefully remove the bark from the scion to expose the inner layer, and insert it into the rootstock. Secure the graft with newspaper and hemp, and if the diameter is less than 3 cm, only one scion is needed; for larger diameters, 2–3 scions can be inserted.
After grafting, wrap the interface with a newspaper roll. The cylinder should be about 8–10 cm in diameter and 25–30 cm long. Fill it with moist soil, press lightly, and cover the top with a plastic bag. Secure it tightly and spiral-wrap with an acrylic rope. This creates a mini greenhouse environment, ensuring proper temperature and humidity for both the rootstock and scion.
Post-graft care is equally important. About 20 days after grafting, check the leaves and vent the plastic bag by opening a small hole to allow the tender shoots to grow. Remove any shoots that appear on the rootstock, and provide support once the new growth reaches 30 cm. Protect the graft from wind and ensure regular fertilization and pest control. With proper care, the graft has a better chance of surviving and thriving.