Detailed Explanation of the Top-Grafting and Variety-Conversion Method for Chestnut Trees and Post-Grafting Management
Chestnut (scientific name: Castanea mollissima), also known as li, chestnut, chestnut seed, and fengla, is a plant of the genus Castanea in the family Fagaceae. It is native to China and distributed in Vietnam, Taiwan, China, and the mainland region of China. It grows in areas at an altitude of 370-2800 m, is mostly found in mountainous areas, and has been widely cultivated by humans. Chestnuts are rich in nutrients; their vitamin C content is even higher than that of tomatoes, and more than ten times that of apples. The minerals in chestnuts are also comprehensive, including potassium, zinc, iron, and others. Although the content is not as high as in hazelnuts, it is still much higher than in ordinary fruits such as apples, especially the potassium content, which is 3 times higher than that of apples.
1. Variety selection
The large trees to be top-grafted can be seedling trees or the original grafted trees. The main cultivar and secondary cultivar in the same orchard should be reasonably matched. One high-yielding improved variety can be selected as the secondary cultivar and top-grafted as a pollinizer to increase the fruit-setting rate.
2. Top-grafting and variety-conversion method
For the selection of scions, first of all, the variety must be pure. Second, branches that are well developed, have plump buds, and are free of pests and diseases should be selected from vigorously growing trees. Male flower segments without buds cannot be used as scions. Collect scions during the dormant period of chestnut and store them properly in moist sand for later use. Grafting is generally done in early spring. Note that the scion and rootstock should be cut and grafted immediately; the cut surface should reach deep into the xylem; the scion must not be soaked in water after being cut; and the grafting action should be rapid.
Before top-grafting, select 3-4 or even more than 10 evenly distributed main branches or secondary main branches in preparation for top-grafting. Heavily retract the selected main branches or secondary main branches while retaining a certain number of auxiliary nurturing branches. In principle, the number of auxiliary nurturing branches retained is greater for weak trees and less for vigorous trees: for trees over 20 years old or old trees, retain 1/3 of the whole tree; for mature vigorous trees, retain 1/4; for trees under 10 years old or young trees, retain 1/6-1/5 or none.
When the top-grafting position is high, growth after grafting is slow, but fruiting is early. Otherwise, the opposite is true. Therefore, it is advisable to use both high-level cut grafting and side grafting. Heavily retract the selected main skeletal branches, saw them off 10-30 cm above the branching point of the main branches and secondary main branches, and use the cut-grafting method at the sawed-off point. On the middle and lower parts of the branches and trunk, use the side-grafting method to graft several more buds.
3. Post-top-grafting management
The wounds at the top-grafting site must be disinfected and protected against decay. Large wounds should be coated with wax or wrapped with plastic film, and the main branches and trunk should be whitewashed with limewater. Top-grafted trees should be checked frequently. If the scions survive and begin to sprout, the sprouts on the rootstock should be wiped off at any time. When the new shoots of the scions grow to 30 cm long, pinch off the growing tips to control excessive vegetative growth and increase the number of branching orders. When the secondary branches grow to 30 cm long, pinch off the growing tips again to thicken the diameter of the grafted branches and plump up the lower branches and buds. Using pre-stored scions, after determining that the scions have not survived, cut the rootstock below the original graft union in time and graft again to fill in.