CMS Yearbook 1978

Often after young Macadamia seedlings have been grafted it is seen that some of the scions have formed good unions and begun to grow rapidly while others show no sign of activity and eventually die, this despite the fact that all of the seedlings look alike and all of the scions were cut from a single parent tree. The fact of the matter is that neither all of the seedlings nor all of the scions are in exactly the same state of cambial activity.

If the dead scion is removed from the seedling and examined, it can be seen that there may or may not have been callus tissue formed by the cambium. The same is to be seen if the cut surface of the seedling is examined.

In order for grafts to succeed, the cambium of both stock and scion must be active, differentiating into callus tissues, which grow together to form a good union. If the cambium of the seedling is active but that of the scion is not, the graft is likely to fail. And, vice versa, if the cambium of the scion is active but that of the seedling is not, the graft is likely to fall also.

A procedure for increasing the probability for success is to do as follows. About one month before grafting is to be done, girdle the branches from which scions are to be taken by removing a ring of bark 1/4 - 1/2 inch (6.25 - 12.5 mm) wide, and cut all twigs back to the point where they are of the thickness desired for scions. At the same time, cut the tops off the seedling trees. In a few weeks the buds in the leaf axils of both stock and the girdled branches can be seen to be swelling and beginning to sprout. This indicates a high degree of cambial activity, and that it is a good time for grafting.

The same procedure should be followed for topworking older trees. Prepare the scion branches the same way, and cut the branches on trunks of the older trees back to about 6 inches (about 15 cm) above where the scions are to be placed. Again, when axially buds begin to swell, the time is right for grafting. Cut the branches or trunks back again just below the previous cut and insert the scions.

The method used for grafting seedlings is either largely traditional or a matter of personal preference. Thus in Hawaii, the side-wedge method is used almost exclusively because a high degree of success was achieved by it in early years, whereas, in California the simple whip-graft is used most commonly. In topworking older trees, the two most commonly used methods are the saw-kerf graft and the cleft graft.

In side-wedge and whip grafting, the scion should approximate the width (caliper) of the stock as closely as possible. In other methods in which there may be great disparity between the width of the scion and that of the stock, one should attempt to match the cambial region of each, which separates the wood from the bark, as closely as possible.

The various methods used for grafting seedlings and topworking older trees are not quite so important as the conditions, which make for successful unions and subsequent growth. These are 1, healthy, vigorous trees with active cambiums; 2, scionwood with active cambium; 3, an adequate supply of starch in the scion to serve as foods reserve until union is accomplished. The purpose of girdling is to accumulate starch. Scions taken just prior to flushing from branches, which have been in the resting stage and accumulating starch may graft successfully. However, girdling is recommended as a matter of insurance; 4, cuts should be plane and smooth to minimize air space between stock and scion; 5, scions should be bound firmly in place; 6, the region of the graft and the scion should be coated with a grafting compound, a tree seal, or protected in some other way to minimize water loss; 7, if operations are carried on outdoors, added protection in the form of kraft paper bags may be necessary to prevent sunburn.

Following in chronological order are references to articles relating to grafting that appeared in previous yearbooks.