Root Systems of Macadamia as Influenced by Nursery Practices*

J. H. Beaumont and E. T. Fukunaga*

Reprint from CMS Yearbook 1958

The Macadamia, because of its evergreen habit and rapid growth, develops a very large and heavy top which seems to be out of balance with the strength of the root system. Invariably, a Kona storm or other unusual wind will blow over many apparently well established orchard trees, necessitating extensive repair, wire bracing or replacement. Observation of the root systems of many uprooted trees in earlier years of the Macadamia research program in Hawaii led to the conviction that the nursery practice employed at the time of growing trees in gallon size tins was the principal cause of the weaknesses found. The tap root was usually bent and had failed to develop strongly after transplanting; even worse, the lateral roots tended to grow from the main axis in a spiral, thus offering little lateral bracing and support. Such a tree might be compared to a stick held upright by having one end held by and resting upon coiled spring. The tap root of such a tree if present might break before the lateral root failed as illustrated by Beaumont and Moltzau (1).***

Even with improved nursery methods, young trees need to be supported until they become well established, and still some will blow over. It must be recognized that any nursery practice other than seedling and grafting in place in the orchard which would preserve the natural tap and lateral root system of the seedling must modify the root system of the orchard tree in some degree.

Recognizing the importance of strong, wind resistant orchard trees and the necessity for developing the best possible root system to support the tree, a more or less elementary comparison of root systems developed under various nursery procedures was undertaken. Materials and Methods

Seed of the Pahau variety (425) was harvested in March 1948, husked by hand, and planted in the sand box, the nursery row, or in the test plot for the various tests. Scions of the variety Keauhou (246) were used for grafting and for growing rooted cuttings.

Seedlings germinated in the sand box were transplanted to the nursery in October 1948. All graftings were made on September 23, 1949. Final transplanting to the test plots was made on November 15, 1950. Rooted cuttings were started in January 1948, but otherwise followed the nursery and field planting schedules. Trees were dug and examined December 16, 1952, approximately 41/2 years from seed, 3 years from grafting, and 2 years in the field.

The following treatments were compared:
1.Seeded in nursery; grafted; dug; balled in burlap; re-set in nursery; transplanted to test plot.
2.Seeded in test plot; grafted in place.
3.Rooted cutting; transplanted to nursery; transplanted to test plot.
4.Seeded in nursery row; grafted; transplanted in nursery ("heeled in"); transplanted to test plot. (Comparable to Treatment 1.)
5.Seeded in sand box; transplanted to shallow trench in nursery with bending of tap root; grafted; transplanted to test plot.
6.Same as 5 above except deep trench; tap root not bent.
7.Same as 5 and 6 above except tap root pruned to 3-4 inches.
8.Current practice seeded in sand box; transplanted to nursery; tap root not pruned or bent; grafted; root pruned; transplanted to test plot.

A second test, initiated in October 1948, dealt with a method of handling rooted cuttings. In October 1948, rooted cuttings of Keauhou (246) were transplanted to the nursery, setting them rather deeply. In August 1951, the cuttings were transplanted to the test plot. At this time, all the trees except 2 were ringed by removing about an ¼-inch ring of bark 4-6 inches above the whorl of roots at the base of the cutting. The trees were dug and the root systems examined on June 19, 1955.

At the time of digging, only the recovery of the primary lateral and tap roots was attempted. A brief description of each tree and a photograph of typical and unusual type root systems was made. Before digging, an effort was made to evaluate the anchorage of the tree or strength of the root system by measuring (a) the pressure required to bend the tree until the root system began to give away as evidenced by breaking and releasing of pressure, or (b) the angle of bending under 100-pound pressure when the root system did not fail. Pressure was applied 3 feet above ground using a compound block and tackle and a spring balance except that with No. 2 treatment the pressure was applied 4 feet above ground in order to obtain significant deflections.

Assuming that the principle of the lever would apply, the pressure on the root system was very considerable. Obviously, any large deflection from the vertical would mean that the trunk of the tree was bending or that the root system had given way in part at least. In only one or two instances did the trunk bend. Results and Discussion

The results are presented in Table 1 in which the trees in each treatment are divided into two groups representing (a) trees withstanding 100-pound pressure without root breakage, and (b) those suffering damage before 100-pound pressure had been applied. The angle of bending at 100 pounds and at the time of root breakage is recorded for each group. The height and trunk circumference are given also and indicate uniformly good growth of trees in all treatments.

Table 1-- Effect of various nursery practices upon growth and strength of Macadamia trees.  

1; See text for descriptions of treatments.
2; See text for descriptions of groups A and B.      3; Pressure Applied 4 feet above ground

Figure 1

Root systems of Macadamia trees developed under various nursery practices. (A) Seeded and grafted in place in the orchard (B) a rooted cutting (C) tap root bent but without development of new tap root (D) tap root bent with subsequent development of another tap root.

It is obvious that the data are limited and subject to the errors inherent in such data. A tree could be tested only once and, in some cases, it was apparent after digging that the tree would withstand considerable pressure from one direction and not from another. However, certain striking differences are apparent.

The 10 trees of treatment No. 2 (seeded and grafted in place and never transplanted) were outstanding not only in that they were the largest and most vigorous but also that, without exception, all withstood 100 pound pressure at 4 feet rather than at 3 feet above ground. The root system of a typical tree is illustrated in Figure 1-a. Invariably, these trees had a tap root one foot or more long and the lateral roots were well distributed around and along the tap root.

The 10 trees of treatment No. 3 (rooted cuttings) while of average height and trunk circumference, had the lowest resistance to lateral pressure. On the average, only 39 pound pressure was required to deflect them an average of 18^° from the vertical at which point the root systems were breaking. None could withstand the 100 pound maximum. The reason is clearly shown in figure l-b, which illustrate the typical root system.

Treatments 1 and 4 are comparable in that both were direct seeded in the nursery, grafted in place, dug and balled in burlap and re-set in nursery (Treatment l) or merely dug and re-set in nursery (Treatment 4) prior to setting out into the test plot. Of the 13 trees in the two treatments, only 3 could withstand the maximum pressure test, although the other trees seem reasonably satisfactory. Examination of the photographs of the root systems shows generally fewer but heavier lateral roots and, as indicated in figure 2-a, there may be a tendency for some of the lateral roots to be somewhat bent due to bailing or "heeling in." It would seem that the root pruning at time of the first transplanting after grafting, when the rootstocks were 2 3/4 years of age, may be more difficult mechanically because of the generally heavier tap and lateral roots. The tap roots of many trees had been cut too short and new tap roots did not develop after transplanting to the field.

With treatments 5, 6 and 7, the seedling stocks were started in a deep sand box, but in transplanting to the nursery row, the tap root was (Treatment 5) bent sidewise, (Treatment 6) not bent, and (Treatment 7) pruned at 3-4 inches to avoid bending. Photographs and notes indicate that transplanting the young seedlings in a shallow trench and bending of the tap root (Treatment 5) in some instances resulted in a root system similar to that of a rooted cutting, in that the lateral roots arose in a very shallow plane. (Figures l-b, l-e, and 2-e.) Only 1 of the 6 trees withstood the 100 pound test and this tree had developed a multiple tap root system (Figure l-d).

Treatments 6 and 7 are probably about equal. Five of the 11 trees in the two treatments withstood the 100 pound test and, with the exception of one weak tree, all were reasonably good. In no case, however, could a tree withstand the 100 pound test unless it had a tap root or a root system in at least 2 planes (Figure 2-b). Treatment 8, which utilizes all recommended practices for nursery growing, was not perfect. Four of the 8 trees withstood the 100 pound test, 2 withstood 85 pound pressure before root breakage, and 2 withstood 50 pound pressure.

Figure 2 Root system of Macadamia trees developed under various nursery practices. (A) Seeded in place, grafted, transplanted. (B) Seeded in sand box, tap root pruned. (C) Same as figures lC and 1D but illustrating effect of poor and shallow planting in addition to bending of tap root. (D) Rooted cutting ringed at time of setting in test plot showing development of an upper scaffold of roots where the ring of bark was removed.

It seems obvious that with the best of care in nursery growing, transplanting, and management, it is difficult to attain the fine, sturdy trees developed when the seedling rootstocks are grown and grafted in place in the orchard. In the Station orchard of transplanted trees, inarching as described in horticultural textbooks (2) has been adopted as a routine practice even though it is in one of the least windy locations in the Territory. However, it is admitted that better methods may be developed and that some of the trees that may be weak in early years may develop a stronger root system later.

The practice of growing trees from rooted cuttings has much to commend it. However, the use of such trees in the commercial orchard would be foolhardy without certain precautions if the data in Table 1 and the habit of growth illustrated in Figure 1-b are of any significance. Inarching suggests itself as a logical solution if such trees are to be used and the practice certainly is to be generally recommended for Macadamia. The practice of removing a ring of bark about 1/8 inch wide and 6 inches above the basal whorl of roots at time of transplanting the rooted cutting to the orchard and of planting the ringed tree 8 to 10 inches deeper than it was grown in the nursery has been tried with 100 percent success. A tree grown by this method is illustrated in Figure 2, lower right, in which the two whorls of roots are clearly shown. While the pressure required to bend these trees over was not determined, none of the ringed trees required any support and none was damaged in any storm. Two unringed trees were no better than those described in Treatment 3, Table 1, Figure l-b, and both required artificial support to hold them in an upright position.

Further research on strengthening the root systems of nursery stock developed from rooted stem cuttings holds much interest and promise. It would seem that a greater distance than 4 to 6 inches between whorls of roots developed from the base of the cutting and from the ring would offer great advantages, particularly if the roots could be initiated before transplanting to the orchard, thus removing the uncertainty that the lower scaffold of roots might die because of the ringing. Perhaps ringing or notching above ground followed by deep mounding, or even a form of marcottage, while the trees are in the nursery, row might be practical.

In Hawaii where the winds are an important factor in Macadamia culture, the best root system is one with a long, strong tap root with strong lateral roots in several planes. In the absence of a tap root, lateral roots in two widely spaced planes will make a strong tree. Good insurance is to inarch nursery grown trees as quickly as possible after they are set in the orchard. LITERATURE CITED Published with the approval of the Director of the Hawaii Agricultural Experiment Station as Technical Paper No. 391.

**Senior Horticulturist and Associate Agriculturist, respectively, Hawaii Agricultural

Experiment Station. ***Figures in italics refer to literature cited.

1. Beaumont, ]. H. and R. H. Moltzau. 1937. Nursery Propagation and Top working of the Macadamia. Hawaii Agr. Exp.. Sta. Cir. 13. 28 pp.

2. Garner, R. ]. 1949. The Grafter's Handbook. Oxford University Press. New York.