Plant Propagation: Grafting
Many methods of grafting, some of them more elaborate than useful, have been employed to make the cuts on both stock and scion, with the result that the operation appears to be very involved. Therefore only the more simple methods are mentioned here in detail, and the others are listed for reference.
Whichever method is used, the essentials for a successful union are the same. The vital part of both stock, and scion is the cambium — a thin layer of tissue lying between the bark and the wood, which is clearly visible as a green line when a young living stem is cut through diagonally. The fusion or joining together of the cells of the cambium layer of both scion and stock makes grafting possible, so that it is essential that these layers are placed in direct contact with each other if the union is to be successful.
Whip and tongue grafting:
Generally used where the stock and scion are of approximately the same thickness. Nurserymen employ it for grafting young. First cut down the stock to about 4 in. above ground level, and on the smoothest side make a slanting upward cut about lj in. long, followed by another smaller downward cut starting just below the top of the upward cut to form the tongue. Prepare the scion, which should be about 4 in. long, by making a corresponding slanting cut at the basal end and a second small upward cut from just above the base of the slanting cut to form a tongue. The cuts on the scion should correspond with those on the stock, but in reverse. Place the tongue of the scion gently into the tongue of the stock, at the same time make sure that the bark and cambium layers of scion and stock are opposite to each other.
Press the scion down until the long, slanting cuts are flush with one another. The tongue will then hold the graft firmly in position.
Make sure that the cut surfaces are in even contact. To form a union the cambium layers must meet on one side, if not on both. Where the stock Is larger than the scion careful attention must be given to this point. Then bind the cut surfaces together with moistened raffia and apply grafting wax to cover and seal the graft area to exclude air and moisture. The cut top of the scion should also be given a spot of grafting wax to keep out moisture.
Similar to whip and tongue grafting, but simpler because no tongue is made. Make two slanting cuts of corresponding size, one on the stock and the other on the scion. Then place the cut surfaces together, making sure that the cambium layers are in contact on one side at least if the stock is larger than the scion. As there is no tongue, hold the splice firmly in place while the tying is done. Afterwards wax the graft as already described. The limitation of this method is that the splice can easily move while it is being tied, with the result that the vital contact of the cambium layers may be lost and the graft fail to grow.
Crown or rind grafting:
Chiefly used for reworking or rejuvenating old fruit trees. Its name refers to the appearance of a limb or branch that has been cut back to expose a circle of wood and a ring of bark, which when worked with a number of scions round the edge gives a crown effect.
Head back the larger branches of the tree to a convenient point, usually l ft. or so above a crotch. After trimming the bark and wood to make it smooth, prepare a number of scions by making long, slanting cuts of about l-1/2 in. Then make a cut in the bark of the prepared limb extending down the side for about 2 in. from the edge of the bark. By slightly lifting the edges of the bark where the cut is made, a prepared scion can easily be pushed down inside the bark, with the flat surface of the slanting cut resting against the inner wood of the stock. The scion is held firmly in position by the tension of the thick bark of the limb or branch, and it is easy to bind round the graft with raffia and then cover the whole cut area with grafting wax. The number of scions inserted round the edge of a headed-back limb will depend on its size, but usually two scions are sufficient, one opposite to the other.
A method of crown grafting that used to be popular because, when properly done, it was easy and reliable although rather crude and rough. Cleft grafting is used on cutback stocks that are considerably larger in section than the scion. Split the stock with a small axe or chisel and use the chisel to keep the split open until the scion (or scions in the case of a large stock) is ready for insertion. Prepare the scion by making two slanting cuts, one on either side to form a wedge-shaped base. Then insert the scion in the cleft or split with one side of the bark of the wedge corresponding to the split bark of the stock. Usually, if the stock is 2 in. or more in diameter, two scions are inserted, one on each side of the cleft, where they are held firmly in position by the pinching effect of the split stock. It is wise, however, to bind round the stock with raffia and then cover the entire surface and sides of the graft with wax. The splitting of the stock across the centre and the insertion of two scions leave an open wound, which must be filled with wax or clay to make the grafts airtight and prevent water entering.
Another way of rejuvenating an old, but the tree is prepared in a different way and numerous scions are used to provide a new framework of side and tip branches. The main branch system of the tree, with the exception of badly positioned branches, is retained to provide a suitably spaced framework from which all side branches, twigs, etc. are cleared completely and replaced by scions of the new variety to be grafted on the tree.
The various methods of frame-working are known as stub grafting, side grafting, inverted I. bark grafting and slit grafting.
Stub grafting consists of grafting on to the stubs of spurs or small side branches and as close as possible to the main branch, whereas in the other three methods the scions are inserted into the main branch itself.
Slit grafting is the easiest of these methods because it involves only one oblique 2-in. cut into the bark with the point of the knife just meeting the wood. The scion, prepared by cutting the basal end to form a wedge with the cuts about 1 in. or more long, is pushed into the slit cut of the bark and, with a little pressure, embeds itself between the bark and the wood. At the same time the scion can be positioned by pushing it up or down to obtain the angle required for the side branch it is to form. As the scion is held firmly in position by the bark no tying is required, but the cut is sealed with grafting wax to make it airtight and waterproof.
Each prepared branch of the tree is worked in this way, the scions are spaced round and along the branch to provide an even and well-positioned framework of side branches. It is wise to put on more than will ultimately be required because unwanted ones can be easily cut out if there is danger of overcrowding.
Frameworking by grafting into the bark can be done only where the bark is reasonably thick, usually on wood of 2 in. diameter and over. The extremities of the branches therefore have to be tip grafted by one of the other methods already mentioned, that is, by crown or rind grafting.
With frameworking a large number of scions are required and should be prepared with more buds, preferably seven or eight, as the growth from scions inserted into an old tree probably having a vigorous root system will be strong. If scions containing only three or four growth buds were used, strong growth would result from them and would only overcrowd the tree, long scions with up to eight buds will not grow so strongly and will only produce growth from the top two or three buds, while the lower buds will produce short spurlike growth bearing fruit buds. In this way the newly grafted tree is productive within a few years.