Large numbers of seedling avocados have been planted in Florida and California. Many of these produce fruits inferior in quality to the best budded varieties, while quite a number do not produce at all. It is often desired, therefore, to convert such avocados into budded trees of choice varieties, and this can easily be done.
Several methods of top-working are employed, the most satisfactory one being shield-budding. When trees are to be top-worked by this means, they should be cut back in November or December in Florida, February or March in California, removing three-fourths of the main limbs a foot or two from their union with the trunk, the remainder being left to keep the tree in vigorous condition. The limbs should be cut off with a sharp saw, to avoid splitting or tearing on the lower side. The stubs should be covered with a good coating of grafting-wax.
When growth has commenced, in early spring, numerous sprouts will appear around the upper ends of the stubs. Only three or four of the strongest should be allowed to remain on each stub, and when these have reached the diameter of one's little finger, they may be budded in the same manner as seedlings, with a large bud, preferably from growth which is not mature. The exceedingly vigorous growth of these sprouts makes success much more certain than in budding seedlings in the nursery. Because of the rapid growth, it is necessary to loosen the wraps frequently to keep them from binding. They should not be removed entirely before the buds have developed to a length of 6 or 8 inches. The sprouts rising from the upper side of the stub form stronger unions with the latter than do those from the lower side.
Cleft-grafting, another method employed in top-working old trees, is most successful with seedlings two to four years old, but can also be used on older trees. While it has not been practiced extensively, it has given good results in the grove of W. J. Krome, at Homestead, Florida. Krome has worked out the method here described.
The trees to be grafted should be sawed off 2 to 4 feet from the ground, according to size, this work being done during November and December in Florida, though it has been successful as late as March. With two-year-old seedlings the trunk itself is sawed off; on larger trees it is well to go above the trunk and saw off the main branches a foot from their union with the trunk. A cleft is then prepared in the stump, not by splitting it with a grafting tool as is usually done with fruit-trees in the North, but by using a saw. After sawing to a depth of 4 to 8 inches, depending on the size of the stub, the saw is removed and a soft wooden wedge is inserted in the top of the cleft and driven down until the lower end of the cleft begins to split. This produces the steady pressure necessary to hold the cion firmly in place.
Cions are cut from wood of larger size and more mature growth than is used for budding, branches about 1/2 inch in diameter being preferable. The cion, which should be 6 to 9 inches long, is trimmed on two sides throughout the lower half to a slender tapering point at the bottom. It is then placed in position in the cleft and forced downward until the upper end of the cut surface is flush with the top of the stub. One cion is placed in the cleft at each side of the stub, nearly even with the surface of the bark on the outside. The wedge which has been used to keep the cleft open is now partly withdrawn until the cions are clamped firmly by the pressure of the two halves of the stub, when it is sawed off flush with the top of the stub and allowed to remain in place so that the pressure on the cions will not become too great.
After the cions are properly placed, the cleft is filled with plastic grafting-wax so that air is excluded. Wax is also rubbed over the outside of the cion where it fits into the stub. The stub is then firmly wound with strips of waxed cloth, covering the top as well as the sides. A collar made of builder's paper is then tied around the stub, extending an inch above the tops of the cions. This collar is filled with sand. Particular attention must be given to insuring a layer of sand between the cions and the side of the collar, since otherwise the latter transmits heat from the outside and kills the cions. Vent holes should be made in the paper near the top of the stub to drain off the water which collects within the cup.
Nothing more remains to be done until the cions have had time to unite with the stock. Two or three months after growth has commenced the sand may be removed and the collar taken off. As a rule, only the stronger of the two cions develops. Both may start to grow but one eventually outstrips the other in most cases, and the weaker one succumbs.
This method appears to produce vigorous trees. Its use has been attended by excellent results at Homestead.