Shield-budding is most successful when the stocks are small and full of vigor. If the plants are once allowed to cease the rapid thrifty growth with which they spring from the seed, the wood hardens, sap is less abundant, and if the bud unites at all there is great difficulty in forcing it into growth. Those who do not devote their undivided attention to the propagation of the avocado sometimes allow the seedlings to get into this condition before they attempt to bud them, and as a result failures are numerous. It must be stated unreservedly that shield-budding of the avocado, to be successful, must be made the subject of careful and intelligent study on the part of the nurseryman, who must exercise constant vigilance to keep the stock plants in perfect condition. If this is done, and bud-wood is intelligently selected, success is within reach, but the number of failures from neglect or ignorance of these two points might well be enough to discourage the beginner from attempting to bud the avocado. It is only through the closest application to minute details that real success in budding avocados can be achieved, and it may truthfully be said that those who have produced budded trees in quantity have invariably been men who have devoted their best efforts to the work and made it a painstaking study.
As soon as the stock plants are large enough to receive the bud conveniently they should be budded, provided the season is favorable. In southern Florida the best months for budding the West Indian race are November, December, January, and February. Budding can be continued into March with success, but after warm weather commences the percentage of failures is too high to make the undertaking profitable. In California the best time is as soon as the sap has begun to flow freely. This usually occurs late in April or early in May, at which season there is a period of three or four weeks when budding is more successful than at any other time of the year. After this short period, however, avocados are in active growth and the proper sort of bud wood is difficult to obtain, hence it is best to wait until the growth has hardened sufficiently to make good bud-wood. This will usually be late in June or in July, when budding can be recommenced and continued until autumn. October and November are good months, although not quite so favorable as the first-named period in the spring. Buds inserted in autumn frequently push out within five or six weeks and must be protected carefully during the ensuing winter. Unless the work is done very late in the autumn, the buds cannot be held dormant until spring.
Fig. 3. Shield-budding the avocado. On the left, a bud properly inserted; above the knife blade, two buds of proper size and shape ; and on the right, bud wood with good "eyes." The method of wrapping the inserted bud is shown in Fig. 11.
Selection of the proper type of budwood requires more experience and judgment than any other feature of avocado propagation, since the character of the buds differs widely among varieties of the same race. Some kinds make such poor budwood that not more than 50 per cent of the buds will grow even for the most skillful propagator; in other varieties, such as Taft and Fuerte, 95 per cent of the buds can frequently be made to develop into trees. In general, it may be said that the budwood should be of recent growth, not soft enough to snap on bending but beginning to mature. In early spring, budwood must be obtained from mature growth of the previous fall and early winter. In summer it must be obtained from the current season's growth. In some sorts, such as Fuerte, very young budwood can be used successfully, but that which has commenced to mature is usually better. Buds can sometimes be cut from the tips of the branchlets and from 6 to 12 inches from the tip, according to the variety and the condition of the wood. Buds which have broken into growth should be avoided, in the case of most varieties, at least; so should those from which the outer bud-scales have dropped, as this is indicative of old wood, and such buds, when inserted, will frequently "drop their eyes" and leave a blind shield from which a tree cannot develop.
To insert the buds, an incision is made in the stock, as close to the ground as convenient, either in the form of a T or an inverted T. No particular advantage seems to be derived from either form of incision, both being used quite successfully. The bark should not be opened by using the ivory end of the budding-knife, as this injures the delicate tissues below; if the bark does not separate from the wood readily enough to allow the bud to be pushed in easily, the stock is too dry to be budded. The propagator should always aim to have the stock plants in such vigorous condition that he can force the bud into the incision with very slight pressure and without loosening the bark with his knife. The most skillful budders, when making the horizontal cut of the incision, turn the knife blade forward dexterously, forcing the bark away from the stock and leaving a sufficient opening in which to insert the point of the bud. The latter is then pushed in very gently and wrapped immediately with a strip of waxed cloth, raffia, soft cotton twine, or plain tape. This should be wound firmly around the stock, from the bottom upward, and fastened securely at the upper end, above the. incision, by slipping the end through the last loop and drawing it down tightly.
In cutting the buds, an extremely thin-bladed, sharp-edged budding-knife should be used, and it should never be allowed to become the least bit dull. A razor-strop is usually worn by budders, attached to the belt; after ten or fifteen buds have been cut, the knife is given a few strokes on the strop to keep it in perfect condition. It should be the aim of the budder to cut the bud with one sliding stroke of the knife, keeping the blade as nearly parallel with the budstick as possible, so that the cut surface will be flat and not rounded at the ends. Buds which are gouged out do not fit snugly on the stock. It is well to cut the buds somewhat larger than citrus buds, 1 inch being the minimum length, and 1 1/2 inches the ideal for most varieties. This must vary, of course, with the size of the stock and budwood, large stocks sometimes taking a bud 2 inches long.
Plate III. Avocado-growing in the Mexican highlands.
Opinions differ as to the best material for wrapping, some preferring waxed cloth, while others have found plain cloth tape equally good, and still others use raffia successfully. Waxed cloth is doubtless the safest, but the objection to it has been that in hot weather the wax melts and works its way into the bud, sometimes killing it. This can be avoided by using a compound of 1 pound beeswax and 1/4 pound rosin. The cloth, preferably a cheap grade of bleached muslin, should be torn in strips 6 inches wide, made into rolls 1 inch in diameter, and boiled for fifteen minutes in this mixture. It may then be kept until needed, when it is torn into narrow strips of the proper width and length for tying buds.
Three weeks after insertion the buds should have united with the stock, and the wraps must be loosened or they will soon bind the stock, if growth is active. They should not be removed until the end of six weeks or two months. In order to force the bud into growth, the tree should be topped at the time the wrap is first loosened, 3 or 4 inches being removed from the tip. The axillary buds along the stem will then break into growth; some of these should be allowed to develop for a while, to keep up an active flow of sap. In another four or five weeks the top should be cut back farther, but a few axillary buds still left on the seedling to grow and maintain the flow of sap. If the stock is cut back too heavily the first time, the eye may fall from the bud, leaving a blind shield. Lopping, as practiced with many other fruits, is not altogether successful with the avocado.
As soon as the bud has made a growth of 3 or 4 inches, it should be tied back to the stem of the seedling with raffia. Later it must be stake-trained, and when it has reached a height of 24 to 30 inches it should be forced to branch and form a shapely top. The stub which remains from the seedling stock should not be cut off until the bud has developed to the height of one foot. In California it is usually considered best to remove the stub in winter; it should be cut off just above the bud, and the cut surface covered with grafting-wax, or shellac made with alcohol and a little rosin. Common paint should not be used for this purpose.
Field-grown trees, after they have reached the proper size, are either lifted and put into pots or boxes, where they are held until established and then planted in the field; or they are balled at any time after they have gone dormant in late winter, and heeled-in under a plant-shed, where they can be kept until spring and then planted out. In Florida, field-grown plants are usually lifted and set in wooden boxes 5 X 5 X 12 inches in size. As soon as they are placed in these boxes, they must be set in partial shade and watered copiously. When they have become established, which will be within a month or six weeks, they can be transplanted to the orchard.
Transplanting with bare roots has not proved generally satisfactory in California. Regarding his experience with it in Florida, Krome says:
"This may become one of the recognized methods of planting and under certain conditions it has many advantages over setting either boxed or balled plants. Two years ago I moved about four hundred seedlings with semi-bare roots and lost only three trees in the process. The trees were two year stocks averaging four feet in height grown in a 'red-flat' at my own grove. We began transplanting during July but most of the trees were moved in September. We waited until the trees had reached a dormant state between flushes and then defoliated them and pruned back the most tender growth. We moved them only after three o'clock in the afternoon when the greatest heat of the day was over, digging only as many trees as could be carefully planted during the remainder of that day. Before digging we wet down the surrounding soil until it puddled easily. The trees were dug with as much of the root systems as could well be handled and the roots were immediately wrapped in wet burlap and the trees placed in the shade. We did our defoliating and pruning back considerably ahead of the digging and found that trees which had been cut back for a week or more and had just started a new growth could be moved as successfully, and in fact grew off better, than those which had been more recently defoliated.
"Since then we have carried on experiments in this line at our nursery, using trees with roots entirely bare, and have had a very low percentage of loss. Upon our recommendation a number of avocado growers in South Dade have tried the method with a limited number of trees and without exception have expressed themselves as intending to make all their plantings hereafter with bare-root trees.
"The two essentials seem to be getting the tree into proper condition before moving from its original position and plenty of water after transplanting."