By Gordon T. Shigeura
Horticulturist, Keaau Orchard, Castle and Cooke, Limited, Hilo, Hawaii. Reprinted from the California Macadamia Society Yearbook, 1956
Training fruit trees, whether apples, peaches, or plums has been a subject of controversy and differences of opinion everywhere even among experts. In many cases the trend in concepts has gone from one extreme to the other. The question of severe pruning, casual pruning, or no pruning at all has been debated, and is still a point for argument in relation to much older tree crops. The proper time to prune has also been debated. Obviously then, if such wide differences of opinion exist with respect to older tree crops, it is not surprising that questions relating to Macadamia culture should be hard to answer with complete assurance.
After nearly seven years of experimentation at Keaau Orchard we have developed our thesis and ideas on pruning and how an "ideal" Macadamia nut tree should look at maturity. Our pruning efforts have been directed toward the realization of such a tree. Frankly, we do not know for sure whether our "ideal" tree is the thing to have. From the information we now have we think we are correct--only time and more experience will give a truly complete story.
To generalize, Macadamia nut trees at Keaau Orchard are pruned and trained to
a central leader growth. Structural lateral branches are conveniently placed
about six to eighteen inches apart along the whole length of this central leader.
Off these laterals, whenever necessary and possible, we put on our fruiting
twigs. Our trees are also pruned to minimize the formation of bad Y-crotches.
Branches not conforming to set standards, branches not serving a definite purpose
or branches, which are interfering with the proper development of a dominant
leader, are also attended to.
The Macadamia, strictly speaking, is not a natural central leader tree. If permitted to develop unattended some trees will develop a true leader, others will develop twin-leaders with bad Y-crotches, other will develop multiple leaders, and still others a bushy dome. Training a tree with such inclinations to a central leader is an effort and hazardous if not done properly. However, in spite of the difficulties attributable to getting a central leader, we feel the benefits of a leader growth far out-weigh the demerits under our conditions because of two important reasons.
First, under our high rainfall (125 inches per year) and liberal fertilization program the Macadamia trees grow very fast. Although the Macadamia wood is hard, the tendency to split at Y-crotches is severe--especially when the Y-crotches are close to the ground level. If a split occurs when the tree is mature and in full bearing the loss is considerable and irreplaceable.
Second, with a central leader we feel the yield per actual square footage of the land area covered and cultivated is increased. In a central leader tree we would have the yielding surfaces stacked one on top of the other, simulating essentially, a multi-storied building where occupancy is increased by as many stories as the building has. This thesis is especially applicable to Keaau Orchard since we have an "intercrop" of macadamias planted at twenty-five foot centers on a square. This spacing is entirely too close for our growth conditions. Tree thinning-out time can be considerably delayed by having the trees, volume for volume, go upwards rather than sideward, thereby tending to increase and hold maximum yield over a unit area.
Leader training is begun within the first year after the nursery-grafted trees are field planted. During the first year a normally well-grown grafted Macadamia nut tree puts on new growth flushes three to four times. Since we "top" the trees at planting time to restore the balance between root and top, usually three new equally strong stem growth will develop from the buds in the axial of each of the three uppermost leaves.
If these three are permitted to develop normally one or all three could develop structural lateral branches on the next growth or soon thereafter. If this happens one or two alternatives can be taken.
A. Two of the three original branches can be completely severed, or, one of these two can be cut to a short stub, which on the next growth flush might grow into a desirable lateral. Sometimes the short stub may grow and develop water sprouts which may "compete" with the leader for nutrients and growth space. If this happens the water sprouts should be kept trimmed back sufficiently to subordinate its development to that of the leader. If the leader branch that is left unpruned develops natural lateral branches the tree essentially may be left alone provided the number of laterals formed do not interfere with the continued development of the dominant leader.
B. If none of the three new terminals sends out lateral branches, induction of laterals at this junction may be done by the method outlined in (A). The third terminal should be "topped" about a foot or so above this junction. On its next growth the "topped" terminal will again develop three new stem growths. One of these should again be retained as the leader and the other two handled in the manner suggested above.
Occasionally, when pruning is severe, six and sometimes nine branches will develop
at the "topped" terminal. If this happens one of the three dominant
(size-wise) buds is again retained as the leader and the other two completely
severed. One or two of the remaining "lower" buds may be advantageously
retained as laterals. Development of such buds tends to develop wide-arched
lateral branches. These wide-arched laterals are desirable on macadamias. They
should be put on whenever possible. However, when it is necessary to have narrow-arched
laterals the bad effects of such a state can be reduced by keeping the growth
and development of these definitely subdued and subordinate to that of the leader.
Through a series of "toppings" and "subordination’s"
we finally attain a leader growth with lateral branches throughout its whole
length.
The formation and laying down of fruiting twigs are done throughout the tree training period. Fruiting twigs are any and all small branches, no matter how small, within the crown of the tree.
The yielding potential of a Macadamia nut tree seems to be well related to the amount of these small fruiting twigs on the tree. The formation of fruiting twigs in some varieties is natural. In others however, these have to be induced by pruning and proper handling. The importance of these fruiting twigs on a Macadamia nut tree cannot be over-emphasized. Once on the tree they should not be taken out for any reason. This writer has seen trees growing healthily in the same location and condition as others and yet yield twice as many nuts simply because they have more of these fruiting twigs.
Induction of fruiting twigs may be done in any one of several ways. Occasionally,
in pruning, we lay down more laterals than necessary. Some of these necessarily
have to be taken out. Whenever this is done a short stub should be left. Subsequent
development from these stubs, if kept subdued, is in the form of many fruiting
twigs. Severe pruning which tends to "open up" the crown seems also
to encourage the formation of these twigs.
As with other tree crop, branches crossing into the pie-section of another branch
and interfering with its development are taken out as they become apparent and
damaging. These branches, whenever possible, are trimmed back to encourage the
formation of fruiting twigs. Otherwise they are trimmed completely back. Low
hanging branches which begin to impede proper leader development or which begin
to interfere with other field operations like herbicide spraying or harvesting
are also taken out. Dead and other non-essential twigs and branches are also
pruned out.
In the Islands we do not have definitely clear-cut growing and dormant seasons as one would expect in the Temperate Zone. Dormancy, or the lack of it, seems to be more closely related with the plant itself rather than the season. Consequently, we prune our trees throughout the year, more or less fitting this operation in among the other important field operations.