The King of Nuts: The Right Time to Graft Walnut Trees, Compatible Rootstock Species and Methods
Walnut, also known as Persian walnut or Qiang walnut, is a relatively common plant of the Juglandaceae family in China. However, to achieve high walnut yields, in addition to basic management work, grafting and transplanting must also be done well. So when is the best time to graft walnut trees? Which trees can be used for grafting? How to graft?
Grafting time:
1. The suitable time for branch grafting is from early April to early May. Since this technique is not easy to master and the survival rate is not high, this method is not recommended.
2. The best time for bud grafting is from mid-May to late June. At this time, the temperature and humidity conditions are suitable, both the rootstock and scion are growing vigorously, callus tissue forms easily after grafting, and after the buds sprout they grow quickly with a high degree of lignification, which is beneficial for safely overwintering.
3. Walnut tree grafting is carried out from late March to early April. First, collect scions. In mid-March, when the buds are about to sprout, collect 1-year-old branches that are vigorous and free of pests and diseases as scions. After collection, store them by variety in moist sand. In early April, when the walnut rootstock buds begin to sprout, start grafting. The rootstock should have a diameter of about 1.5 cm, and grafting is done at 10 cm above the ground.
During walnut grafting, there is bleeding sap, which makes survival difficult. Within 12 hours before grafting, a cut must be made at the root collar down to the xylem to “drain the sap”, then cleft grafting or bark grafting is performed. After grafting, tie the wound tightly with plastic strips, and coat the upper end of the scion with paint to prevent water evaporation. After survival, promptly remove sprouts and loosen the ties. The ties should be loosened when the new shoots have grown more than 20 cm. When the grafted seedlings grow to about 40 cm, fix them with small bamboo poles or wooden sticks to prevent wind breakage.
Tree seedlings that can be used for grafting:
Generally, only plants of the same family and genus graft well with each other. The best choices for grafting walnut trees are:
1. Grafting onto seedling walnut (seedlings grown from walnut seeds).
2. Juglandaceae Manchurian walnut (Juglans mandshurica) genus can be used for grafting, such as the common Manchurian walnut (mountain walnut), but it tends to develop a small-foot phenomenon (the rootstock is thinner, while the part above the graft union is thicker).
3. Juglandaceae Chinese wingnut (Pterocarya) genus (in the south) can also be used for grafting walnut.
Grafting methods:
1. Cleft grafting: A cleft is made in the rootstock and the scion is inserted, hence the name cleft grafting. Cleft grafting is a main method of branch grafting in spring.
① Rootstock cutting. Saw off the rootstock at a place where the bark is straight and free of scars, smooth the wound with a knife, then make a vertical cleft 4-5 cm deep in the middle of the rootstock.
② Scion cutting. Keep 2-3 buds on the scion, and cut one slant on each side of its lower part to form a wedge shape. The outer side of the wedge at the lower part of the scion should connect with the cambium of the rootstock, while the inner side does not connect. The cut surface of the scion should be 4-5 cm long. The cut surface must be flat and the angle must be appropriate, so that the rootstock at the graft union can connect with the scion both above and below.
③ Joining. The key is to align the cambium layers of both sides, preferably aligning the cambium on both outer side surfaces of the scion. If both sides cannot be aligned, keep the cambium aligned on one side. Be careful not to insert the entire cut surface of the scion into the cleft; leave more than 0.5 cm exposed to promote wound healing.
④ Wrapping. For medium or relatively thin rootstocks, insert 1 scion into the cleft (preferably wax-sealed), then use plastic strips 4 cm wide and 30-40 cm long to tie up the graft union. When tying, completely cover and tightly bind the cleft, the wound and the exposed part.
2. Double-door bud grafting and single-door bud grafting: During grafting, the bark on both sides of the rootstock cut is pried open, like opening two doors, so it is called double-door bud grafting; single-door bud grafting only pries open the bark on one side of the cut. The other grafting methods for the two are the same. It is suitable for grafting varieties that are relatively difficult to keep alive. After the graft survives, it can sprout in the same year.
① Rootstock cutting. Make the length of the bud patch equal to the length of the rootstock cut. Make one cut above and one below on the smooth part of the rootstock bark, so that the width is appropriately greater than the width of the bud patch. Then make a vertical cut in the center, making the cut appear like the character “I”. If it is single-door, make 1 vertical cut on one side, deep to the xylem. Then pry open the bark to form a double door or single door.
② Scion cutting. Make one cut on each of the four sides around the scion bud, and take out a square-shaped bud patch.
③ Joining. Put the scion into the rootstock cut. For double-door bud grafting, cover the bud patch with the left and right sides; for single-door bud grafting, pry open the rootstock bark to cover the scion bud patch, and the bud patch should be pressed closely against the opened side.
④ Wrapping. Use plastic strips 1-1.5 cm wide and 30-40 cm long to tie up the bud patch, exposing the grafted bud and leaf stalk.
3. Square grafting: The bud patch taken during grafting is square in shape, and a corresponding square piece of bark is also taken from the rootstock, so it is called square bud grafting. Square bud grafting has a large contact surface, is relatively suitable for walnuts that are difficult to keep alive with bud grafting, and sprouts easily after grafting.
① Rootstock cutting. During grafting, first compare the lengths of the rootstock and scion cuts, and mark them with a knife. Then make one cut on each of the top, bottom, left and right sides, deep to the xylem, then use the tip of the knife to lift out and remove the rootstock bark.
② Joining. Put the bud patch into the rootstock cut so that its top, bottom, left and right sides fit exactly with the rootstock cut. If the scion bud patch is a little small, that is no problem; if the scion bud patch is too large to fit in, it must be trimmed smaller to make it the right size.
③ Wrapping. Use plastic strips 1-1.5 cm wide and 30-40 cm long to tie up the graft union, exposing the bud and leaf stalk.
Post-grafting management techniques:
1. After grafting: Check for survival after about 10 days. The signs of survival are leaf stalk shedding or bud sprouting. Re-graft those that have not survived in time.
2. Sprout removal: Remove sprouts on the rootstock to promote sprouting of the grafted buds. Generally remove sprouts once every 10 days or so.
3. Rootstock pruning: After the grafted buds survive, cut off the rootstock about 2 cm above the grafted buds to stimulate growth of the grafted buds.
4. Untying: When the new grafted shoots grow to more than 5 cm, the wrapping can be untied. Untying too early or too late will affect growth.
5. Pinching: When the new grafted shoots grow to about 30 cm, pinch off the growing tips to help ensure safe overwintering.
6. Pest control: If pests such as wood-boring looper moths and stinging moths occur, use fenvalerate 2000-fold solution for control.
7. Cold protection: Walnut field grafting requires winter cold protection in the same year. Before the soil freezes, those that can be bent down should be covered with soil for cold protection; those that cannot be covered with soil can be wrapped with plastic film, and in the spring of the following year during the sprouting period, uncover them or remove the wrapping at the appropriate time.