What are the common methods of walnut grafting?

  There are many methods of walnut grafting. According to the grafting time and the type of scion, they can be divided into two categories: branch grafting and bud grafting. A grafting method that uses a section of a shoot with buds as the scion is called branch grafting, while one that uses only a single bud piece as the scion is called bud grafting.

  Common branch grafting methods include cleft grafting, bark-tongue grafting, bark grafting, and side grafting; bud grafting methods include square patch budding, T-shaped budding, ring budding, and I-shaped budding. Square patch budding is currently the most commonly used method in production for mass propagation of nursery stock. Branch grafting is mainly used for top grafting of large trees to improve varieties or for supplementary grafting in the second spring when bud grafting has failed to survive.

  1. Cleft grafting. In northern regions, it is mostly carried out from late March to late April, after the buds begin to swell and up to leaf expansion. Select rootstock with a diameter of more than 2 cm, cut or saw it off about 10 cm above the ground, smooth the saw cut, use a cleft grafting knife to split the rootstock in the middle to a depth of about 5 cm. The scion is wax-sealed, retain 1-3 buds, and on the side opposite the first bud, cut a 3-5 cm slanted surface on each side, with both slanted surfaces equal in length. Insert the scion into the split in the rootstock so that the base of the scion's cut surface protrudes slightly, forming a half-moon shape. Be careful to align the cambium of the rootstock and scion. When the rootstock and scion are of the same thickness, the cambium on both sides can be aligned; if the rootstock is thicker and the scion thinner, align the cambium on one side. Then bind tightly with plastic strips. The timing of branch grafting is very critical; generally after leaf expansion there is less bleeding, and the survival rate is higher.

  When grafting, note that after tying, the scion must not be loose; it should be firm enough that it does not easily move even when shaken by hand. Beginners often fail to tie tightly enough, and grafting often fails because the scion is loose. The main reasons for a loose scion are: first, the angle of the scion's cut surface is inconsistent with the angle of the rootstock opening; second, the scion's cut surface is uneven. If the angle of the scion's cut surface is too large, the front end cannot be clamped tightly; if the angle is too small, the rear part cannot be clamped tightly. The angle of the scion's cut surface can only be mastered through much practice. In addition, failure to align the cambium is also one of the reasons for grafting failure.

  2. Bark-tongue grafting. After sawing off the rootstock, select a smooth area and cut away a strip of bark from bottom to top, 5-7 cm long and 1-1.5 cm wide, exposing the cambium, and make a vertical cut in the middle. Cut the scion into a 6-8 cm single cut surface in the shape of a horse ear, use your fingers to pinch open the cambium on the back of the cut surface to separate it from the xylem, insert the xylem of the scion's cut surface between the xylem and bark where the rootstock bark has been removed, cover the rootstock's cut surface with the bark pinched open on the scion, and finally bind tightly with plastic film. In spring, the scion bark does not slip and is difficult to pinch open, so scions used for bark-tongue grafting must first undergo waking treatment to make the bark slip. The method is the same as seed germination, and attention should be paid to the treatment time; if the waking time is too long, the scion will sprout, reducing the grafting survival rate. The bark-tongue grafting method is slightly more troublesome, but it is the branch grafting method with the highest survival rate for walnut.

  3. Bark grafting. Also called under-bark grafting, it is a branch grafting method with a very high survival rate. It must be carried out after the rootstock bark "slips" (that is, the cambium begins to become active). First saw or cut off the rootstock, smooth the cut, and on a smooth part of the rootstock make a vertical cut from top to bottom, reaching the xylem, with a length equal to the scion's cut surface, while using the knife to pry the bark open to both sides. The scion's cut surface is in the shape of a horse ear, 6-8 cm long, then on the back of the cut surface at the tip, lightly cut a small slanted surface 0.5 cm long, or make two small slanted surfaces by cutting on the left and right, to facilitate inserting the scion downward. You can also lightly scrape off the wax layer and bark on the back of the cut surface with a knife to expose the white-green phloem, which can increase the contact surface, facilitate water and nutrient transport, and promote callus formation.

  After the scion is cut, insert it into the small opening in the rootstock, with the base of the cut surface exposed and forming a half-moon shape. Finally wrap it well with plastic film. Bark grafting has a large contact surface between rootstock and scion, a high grafting survival rate, and is widely used in production. However, after successful grafting, the mechanical bearing capacity of the callus tissue between rootstock and scion is relatively poor, and after the scion sprouts it is easily broken by wind, so timely support is needed.

  Notes on branch grafting: a. Do not irrigate before grafting. 3-5 days before grafting, saw off the rootstock in advance to release water, or at the time of grafting open several spiral water-release openings at the base of the trunk, so as to avoid excessive bleeding affecting the grafting survival rate. b. Choose the grafting time well, because callus formation requires a certain temperature range, with the optimum temperature around 26°C. Branch grafting should be carried out in spring from budbreak to leaf expansion.

  4. Square patch budding. This method has a high grafting survival rate and has been the most widely used walnut grafting method in recent years. The specific operation method is: first use a knife to cut off the petiole, leaving about 0.5 cm, make a horizontal cut 1 cm above and below the bud, and make a vertical cut 0.5 cm on each side of the petiole of the bud, intersecting the horizontal cuts. Use the thumb and index finger to press the petiole area and peel it off horizontally, removing a rectangular bud piece. Be careful to include the growth point, a small piece of bud flesh tissue in the concave area below the bud base on the inner side of the bud piece. On a smooth part of the rootstock about 20 cm above the ground, make a horizontal cut, then make another horizontal cut above it, with the distance between the two cuts equal to the length of the bud. At one end of the two horizontal cuts, make a vertical cut connecting the upper and lower horizontal cuts, pry open the bark to make a "door," insert the bud, with one side pressed tightly against the vertical cut, and according to the transverse width of the bud, tear off the lifted rootstock bark. Note that the removed bark should be 1-2 mm wider than the scion bud piece, so that the bud and cambium can combine tightly. Cut thick plastic film into 3 cm wide plastic strips for tying, being careful to wrap the cut surface of the petiole tightly and expose the bud point. Finally, use pruning shears to cut off the rootstock shoot above the union, leaving 1-2 compound leaves, to control the vegetative growth of the rootstock and favor survival of the bud. Square patch budding should be carried out from late May to mid-June, and the bud can sprout and become a seedling in the same year. Some grafting is also done in July-August, but after grafting the bud does not sprout that year (dormant bud), and only in the second year is the rootstock cut and the bud allowed to sprout into a seedling. This is mainly used for supplementary grafting of rootstock seedlings where grafting in May-June did not survive.

  For square patch budding, the rootstock cut can also be made in an I-shaped opening, called I-shaped budding. The scion cutting method is the same as for square patch budding. First make two horizontal cuts on the rootstock, and for the vertical cut, cut in the middle between the two horizontal cuts, pry the bark of the rootstock open to both sides, insert the bud, and tie. I-shaped budding does not remove the excess bark; this method is also called open-door grafting.

  In some places, people are accustomed to using a double-bladed knife for grafting, which is convenient to operate and gives a high survival rate. You can make a double-bladed budding knife yourself: take 2 sections of 10 cm long hacksaw blades and grind the cutting edges with a grinding wheel, find a small wooden strip 4 cm wide, about 1 cm thick, and 10 cm long, and use cloth strips to tie the blades made from the saw blades to both sides of the wooden strip. The grafting operation is similar to that with a single-bladed knife, except that the two horizontal cuts are completed in one action, and it is easy to make the rootstock cut and the scion bud piece equal in length, which can increase grafting speed and improve the survival rate.