G. F. Ryan and E. Frolich Dr. Ryan, is an instructor in Subtropical Horticulture, UCLA, Los Angeles. Mr. Frolich is laboratory technician in Subtropical Horticulture, UCLA, Los Angeles. Unpublished data of S. H. Cameron and O. R. Hagaman, University of California, Los Angeles. Reprinted from the California Macadamia Society Yearbook, 1956
Macadamia varieties may be propagated in two ways, by grafting or by rooting of cuttings. Since little is known at present about rootstock effects in the Macadamia, the cost of producing a tree will to a great extent determine whether grafted plants or rooted cuttings will be used. Rooting of cuttings is generally a less expensive operation than grafting for varieties, which root readily and grow well from cuttings. This method will be considered first.
Two types of cuttings have been used in propagating macadamias: leafy semi-hardwood cuttings of mature current season growth, and hardwood cuttings of one-year-old or older stems. The semi-hardwood cuttings are preferred for at least two reasons: (1) In general they produce a stronger root system, and (2) more cutting material is available from a given source, with less damage to the tree.
Semi-hardwood cuttings are usually 3- to 4-inch long tips with mature leaves. The stem may be either brown or green in color. The lowest leaves are removed if necessary in order to insert the cuttings in the rooting medium, and sometimes the remaining leaves are trimmed to conserve space.
Treatment of the base of the cutting with root-inducing preparation results in a better root system than without treatment, and rooting occurs in less time. Some varieties respond well to commercial preparations such as Hormodin 3, which contains the plant regulator indolebutyric acid (IBA), mixed with talc at a concentration of 0.8 percent. Other varieties require a stronger treatment to give satisfactory rooting. Cuttings of Arcia, for example, failed to root satisfactorily when treated with Hormodin 3, but comparable cuttings of this variety rooted well after basal application of IBA in approximately 50 percent alcohol at a concentration of 10 milligrams per milliliter.
For this treatment dissolving the acid in ethyl or methyl alcohol at twice the desired concentration first makes a stock solution. The final solution for treating cuttings is made up as needed by diluting the stock solution with water. Approximately one-fourth inch of the base of each cutting is immersed in the solution for 2-3 seconds, after which the cuttings are inserted into the rooting medium.
Naphthalene acetic acid (NAA) is a much less expensive plant regulator than IBA, and on the basis of experience with other plants, it would be expected to be about as effective as IBA in stimulating rooting of macadamia cuttings. The cost per cutting is insignificant in either case.
It should be pointed out that only by experience with a particular variety could the optimum concentration be determined. Cuttings of a variety, which responds well to a weaker treatment, are likely to be damaged by a stronger treatment.
The cuttings may be rooted in any of the commonly used media such as vermiculite, sand, or perlite, mixtures of these, or mixtures with peat. The choice of the rooting medium will depend on various factors including temperature and humidity conditions, cost, and personal preference. The beds or flats of cuttings should be in a closed propagating case or under intermittent mist. Bottom heat should be used when necessary to maintain a temperature of approximately 75 F. in the rooting medium.
Variety differences in ease of rooting have already been mentioned. In general it has been observed that cuttings of M. tetraphylla varieties produce better root systems and root in less time than M. ternifolia varieties. The Australian varieties J3, J4, J6, H3 and Fl, and the California varieties Hall, Santa Ana and UCLA No. 23 are examples of M. tetraphylla which have been found to root more readily than M. ternifolia varieties such as Arcia, H.E.S. 246 (Keauhou) and H.E.S. 475 (Wailua).
The rootstocks for grafted trees are usually seedlings that are grown from any available seeds which germinate readily and produce vigorous plants. Seedlings obtained from different seed sources have been found to vary in their susceptibility to chlorosis in the seedling stage. At present not enough information is available on this deficiency symptom in the Macadamia to know how important differences in seedling susceptibility may be in obtaining satisfactory trees. Studies are in progress to determine the relative importance of the scion and the stock in producing chlorosis in the grafted tree (Wallace and Frolich, 1956). A safe precaution would be to avoid any seed sources known to produce a large number of susceptible seedlings when grown in soil favoring appearance of chlorosis.
Fresh seed should be used when possible. Viability decreases rapidly if the nuts are held at room temperature, but limited experiments have indicated that germination may be fairly good after as long as 12 to 14 months of storage at 40F. Cracking of the shell to hasten or improve germination has been tried, but is not successful. Fungi, with resulting decay quickly attack embryos exposed by cracking or removing the shell.
The seeds may be germinated in a seedbed or in deep flats in vermiculite, sand, perlite or other convenient medium. Maintaining a temperature of 75F or higher by means of heating cable will hasten germination. The seedlings are ready for transplanting when 3-6 inches tall or when the first true leaves are formed. If grafting is to be done in the nursery, they may be transplanted bare-root directly from the seedbed or flat to the nursery row, or they may be grown to a larger size in small tar-paper containers before being set out.
Various common grafting methods appear to be equally satisfactory in the nursery row. The whip-and-tongue, splice, cleft and side-wedge grafts are all successful, provided leaves are left on the seedling when the graft is made. Complete failure has resulted with these methods when all of the leaves have been removed from the stock.
Leafless scions of 1/4inch or greater diameter from one-year-old or older wood may be used. Strips of polyvinyl plastic about one-half inch wide are convenient for wrapping and tying the grafts. All exposed cut surfaces should be protected with asphalt emulsion or other grafting preparation.
In Hawaii and in Florida, girdling of branches about 6 weeks before the scions are to be cut has been shown to cause a readily observed increase in starch accumulation, and to increase the success in grafting (Beaumont and Moltzau, 1937; Jones and Beaumont, 1937; Fahmy, 1952; Fukunaga, 1953).
Results of experiments during the spring and summer of 1955 indicated that girdling is beneficial at Los Angeles also (Tables 1 and 2). Branches were girdled in January, and scions were cut for grafting in March and in August. The effect of girdling was less pronounced in the spring than in the summer. In March, when winter temperatures had slowed growth, non-girdled scions were about 65 percent successful. Girdling is unnecessary for scion-wood taken in December to February in the Kona region of Hawaii, because this is the dry period during which the trees are not growing (Fukunaga, 1953).
Scion
Pretreatment
Date
Number Number of
Percent of
Variety
of Scion Wood
Grafted
of Grafts Successful Grafts
Successful Grafts
246
Girdled
3/4/55
9
9
100
246 Not girdled 3/4/55 9 6 67
F1 Not girdled 3/29/55 11 9 82
J3 Not girdled 3/29/55 11 4 36
J3b Not girdled 3/31/55 26 20 77
J4 Not girdled 3/29/55 9 4 44
Total of last four ..........................................................................57 37 65
a. Branches girdled January 1955.
b. Scion wood was larger in diameter than that used March 29.
Table 2. Effect of Girdling on success of grafting in August.
Pretreatment Number Number of Percent
of of Successful Successful
Scion Wood(a) Grafts(b) Grafts Grafts
Girdled ........................................ 12 10 83
Not girdled .................................. 13 4 31
a. Branches girdled January 1955. Grafts made August 23, 1955.
b. Cleft and splice grafts.
In August the beneficial effect of girdling was clearly apparent (Table 2). Starch accumulation, estimated by applying the iodine-potassium iodide test on the cut ends of the stem, was much greater in girdled than in non-girdled wood. It is of interest to note that the grafting was done on August 23, just nine days before the sudden hot spell of August 31 to September 3. The daily maximum temperatures during this period were 102, 108, 104, and 98F.
Where sufficient large-diameter hard scionwood is not available to graft in the nursery row, propagation can be done under glass, using leafy scions of current season wood. For this method, seeds are germinated as described above and the 3-6 inch tall seedlings are transplanted into soil in suitable containers. One-quart juice cans or tarpaper pots of about the same capacity are satisfactory. When these seedlings are approximately four months old they will have a stem diameter of about 1/8 inch and will be large enough to graft.
The grafting procedure is illustrated in figures 1 to 4. If the wedge at the base of the scion is made as shown (Fig. 1), with one cut at a sharper angle than the other, the scion will fit into the stock in an upright position, forming a straight union. The scion is tied in securely, but need not be sealed. Number 16 rubber bands make a convenient tie, but various other tying materials may be used. The grafted plants are placed in a closed propagating case where high humidity is maintained by daily sprinkling, and the maximum temperature is kept below 95F by shading.
Grafts made in this way in late June 1953 had formed good unions in 10 to 12 weeks. Only about 50 per cent success was obtained in the first attempt, using either the side wedge graft or the splice graft (as used in tipgrafting avocados), and similar results were obtained in the summer of 1954. When grafting was done by this method in January, 80 percent of the grafts were successful. The scion-wood was not from girdled branches in any of these trials.
A variation of the above-described method, which might be more economical, has been tested experimentally with fair success. The grafting was done on smaller seed