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Methods for changing cultivars
Top-working
Top-working is the process of changing a tree from one variety to another by means of grafting. It is usually done in the spring, shortly before new growth starts. The exact time depends upon the method to be used. The cleft graft is performed before the bark is slipping. The bark graft is done when the bark is slipping, but before the buds of the stock begin to grow. Several advantages of top-working include shorter time for trees to come into bearing, avoidance of replant problems and an already established root system. The trees that are selected to be top-worked need to be 'healthy' and in a site that has no problems such as poor drainage. It is not cost effective to invest time, resources, and money into trees that will never be fully productive In addition, you must remember that any cultivar or variety that is patented, such as Honycrisp, require propagation agreements. The following information appeared in Kevin Iungerman,'s letter, the Northeast Tree-Fruit, which I have copied with his permission
Cleft grafting is probably the most common grafting method used to change the bearing area of apple trees, being especially adapted to top-working trees, either in the trunk of a small tree or the scaffold branches of a larger tree. The ideal time to cleft graft is in the early spring either right before or right after active growth starts, preferably before the buds in the stock begins to break.
I have seen this grafting technique, with some modifications, used around the state. Russ Allen has top-worked many trees at various orchards with a very high success rate.
Preparation of Scion
Use healthy, vigorous, one-year-old dormant wood. Watersprouts are ideal since they tend to stay dormant longer than other shoots, but avoid watersprouts that developed in shaded areas.
Scion wood should be collected while dormant, and stored in a moist medium, wrapped in polyethylene plastic sheeting. The refrigerator is a good place to store the scion wood (about 40F). If you are planing to store the wood for a period of 1 to 3 months then keep the wood at 32F. Do not store in your freezer. This temperature is too low and may cause damage.
The scion piece needs to be between 1/2 to 1/4 inches in diameter. Use only the central portion of the scion stick for the graft.
Cut a wedge at the butt end of the scion piece. Start at about 2 inches from the end, and make a smooth, straight cut that has a surface of about 1/2 inches long. Repeat the same procedure on the other side of the stick (Fig. 1a).
After cutting the wedge, shorten the scion to about 3 buds. The top cut should be made just above a bud.
Preparation of the stock
Select a 'nurse' limb and cut 3 to 5 inches above this limb. Avoid areas where callus has formed (usually where a thinning cut was made).
Make the cut at a slight right angle from the main axis To expose the position of the cambium, smooth the stub with a sharp knife. Avoid ripping the bark.
Split the stub with a grafting chisel to a depth of 3 to 4 inches. Do not split too far down (Fig. 1b). Keep the stock open by a wedge for insertion of the scions.
Inserting the scion
Insert the scion on the side of the stock making sure there is cambium contact between both the stock and the scion (Fig. 1c). Usually the scion will be set in somewhat because of a thicker bark on the stock. Usually, 2 scions are used if the stock is thick enough. This will double your chances of a successful take.
Be sure to push the scions down into the crack far enough to hold the scions in place (Fig 1d). If necessary use grafting tape to hold the scions in place.
All of the cut surfaces should be covered with grafting wax (Fig. 1e). Do not allow the cut to dry out or water to enter into the crack. Re-coat as needed.
If both scions take, it will be necessary to head back the weaker after the first growing season. Eventually, the weaker scion should be removed completely.
Shoots growing from below the graft should be removed completely. As the new graft develops, one should do some pruning to establish the desired shape of the tree.
The nurse limb is removed the year after the graft was made.
The Modified Bark Graft (Fig. 2).
This graft can be used in place of the cleft graft, but it can only be made when the bark is slipping around tight cluster and bloom (remember that the scion wood must be dormant). It is supposed to be faster than the cleft graft. However, not much scientific research has been conducted in comparing the cost and productivity of these two grafts. One disadvantage to this graft is that the scion must be supported.
Recently, I attended a top-working demonstration at Chazy Orchard (New York) where Kevin Inguerman, Dave Rosenberger (both from Cornell) and John Whightman demonstrated a modified bark graft that is being used to top-work some cultivar to Honeycrisp. John has been very successful with this grafting technique.
Preparation of Scion
The scion wood should be collected as for the cleft graft
A long cut (1 1/2 to 2 inches long) is made to the scion with a shoulder on one side
This is the side that will be placed against the wood of the stock
A shorter cut is made on the opposite side
Preparation of the stock
Select a 'nurse' limb and cut 3 to 5 inches above this limb. Avoid areas where callus has formed (usually where a thinning cut was made).
The scion is cut at slight right angle from the main axis To expose the position of the cambium, smooth the stub with a sharp knife. Avoid ripping the bark
A vertical cut (1 to 2 inches long) is made through the bark to the wood or make two parallel cuts the width of the scion
The bark on both sides of the cut is separated from the wood
Inserting the scion
There are several ways to insert the scion. One method is to cut a slit, about 2 inches in length, through the bark of the stock. Separate the bark from the wood on both sides with a knife blade, and then slide the scion, with the cut surface next to the wood, down to the proper depth.
More than one scion may be inserted on the if the stock has a large diameter Use nails to secure the scion to the stock. (John uses a brad gun to accomplish this task).
The following steps are the modifications to the standard method:
Once the scion is secured in place by the brads, use a plastic bag to cover the scion and the stock. Sandwich bags work well. Make a very small opening at the top of the bag to slip the scion. If you have more than one scion stick, make sure that the bags fits loosely around the scion stick. Otherwise, the bags will tear and this will allow for drying of the callus tissue.
Cover the plastic bag with aluminum foil. This will ensure that the developing callus tissue will remain moist. Tape the foil securely with electrical tape.
Cover this with a light weight paper bag by making a small hole at the bottom of the bag and slipping it over the scion/stock union. The paper bag will decrease the amount of bird damage caused by pecking.
Let the healing process continue till the scion grows 4 to 5 inches, at which time you will uncover the graft union.
A few days after uncovering, you might want to paint this union with Tree Limb Coating® This compound contains copper, thus it decreases the incidence of fungal attacks to this tender tissue.
Provide support for the growing scion.
The nurse limb is removed the year after the graft was made.
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