Methods and Technical Key Points for Top Grafting and Variety Conversion of Walnut Trees

Among the walnuts currently cultivated on a large scale, seedling trees are in the majority. These seedling trees have mixed populations, severe varietal degeneration, low yields, and poor quality. By adopting the top grafting and variety conversion method, early-bearing walnut varieties bear fruit 1-2 years after top grafting, and late-bearing varieties bear fruit 2-3 years after top grafting, which can significantly improve yield and quality.

1. Timing of top grafting. This varies with climate, and each location can choose a suitable top grafting period according to the local walnut phenological characteristics. Since walnut top grafting and variety conversion mainly uses branch grafting, generally the graft is made when the rootstock tree buds begin to swell and the leaves first unfold. In the south this is generally in February, and in the north generally from late March to mid-to-late April. However, scions must never be collected and cut after bud break, so they must be collected and cut early.

2. Early collection and cutting of scions. Scions should be collected from superior varieties, such as Wen 185, Liaohe No. 1, Xiangling, Baokexiang, Baofeng, Xiluo No. 3, etc. They are taken from the middle or basal section of vigorous growing vegetative branches, with a diameter of more than 1 cm, small pith, smooth, and free of diseases and insect pests. The best time to collect scions is from after leaf fall to mid-February of the following year. Each scion should retain 3 plump buds, be sealed tightly with 95-100°C paraffin, tied in bundles of 50 or 100 by variety, buried in wet sand or wet sawdust at a thickness of 15-20 cm, and stored below 10°C for later use.

3. Grafting method. The rootstock should be selected from vigorous seedling trees or inferior varieties 5-25 years old. Seven days before grafting, the graft heads should be sawn according to the original canopy subordination relationship, and the sawn heads should be 20-30 cm from the base. At the same time, at the base of the trunk 20-30 cm from the ground, drill three or four saw cuts in a spiral pattern, reaching about 1 cm into the xylem, to facilitate the outflow of bleeding sap. If there is too much bleeding sap, 1-2 more drainage cuts can also be sawn at the base of the graft head. When grafting, saw off the trunk layer of the rootstock, then peel off a long tongue-shaped strip of the outer bark of the rootstock. Cut the scion into a tongue-shaped cut surface 5-6 cm long, and use your hands to pinch apart the bark and xylem of the tongue-shaped part. Then insert the tongue-shaped xylem part between the bark and xylem on one side of the rootstock cross-section, so that the tongue-shaped outer bark layer of the scion lies on the tongue-shaped cut surface of the rootstock bark. Finally, bind tightly with plastic tying strips.

4. Management after grafting. 25-30 days after grafting, the scions sprout one after another and grow rapidly. When the new shoots grow to 20-30 cm, support sticks should be tied to train them, to prevent wind breakage, and the sprouts on the branches and trunk should be cut off at any time. For scions that do not survive, choose 2-3 appropriately positioned sprouts growing on the rootstock stump and keep them, and perform bud grafting in July-August. Check the survival rate 60 days after grafting, and remove the bindings. In late autumn or early spring, in conjunction with collecting scions, properly prune the surviving grafted scion branches on the rootstock. At the same time, strengthen fertilizer and water management and disease and pest control after grafting.