Bakersfield.com

No scientific trials have been conducted to give the pistachio growers guidance on the best way to prune pistachio trees.  So what is a pistachio grower to do?

Observation made in some older orchards on the west side of the San Joaquin Valley suggests that this lack of information is translating into a reluctance of some growers to control tree height for fear of losing yield.  Standard industry practice for pruning branches in the upper canopy of pistachio trees is to make the pruning cut at a point on the branch located two or three vegetative buds above the last flower bud.  These vegetative buds will provide locations for new growth the following year.

Of course, continuing this practice indefinitely will result in ever larger pistachio trees.  In some orchards, excessive tree height is resulting in excessive shading of the lower tree canopy resulting in loss of fruit wood and leaf area.  Shading results in increased humidity in the tree canopy and increased problems with fungal pathogens, like Alternaria species.  Tall canopies make harvest less efficient.  Long limbs dampen the energy that the harvest shaker is able to transfer to nut clusters, and results in nuts being slung outside the catching screen.

Although no comprehensive pruning studies have been conducted, related scientific information does exist.  Research indicates that pistachio can compensate, to a degree, for bud removal in one part of the tree by producing more nuts or heavier nuts in the remaining clusters in other parts of the tree.  In addition, since pistachio is alternate bearing, yield loss as a result of pruning that occurs in one year, has been compensated for by increased yield the following year.  Huge trees are not required for huge yields.  Relatively small 10th leaf trees in the southern San Joaquin Valley are capable of producing 5000 pounds of dry yield in a single season.

Dr. Louise Ferguson, University of California Extension Specialist, in cooperation with others, has demonstrated that two successive years of mechanical topping mature Kerman trees, on P. atlantica rootstock, mitigated alternate bearing without affecting cumulative yields for the following 5 years.  She also noted that cutting into 3 and 4-year old wood during the mechanical hedging of both sides of a tree decreased cumulative yield over the following 6-year period.

University of California Farm Advisor Bob Beede has experimentally pruned an orchard that was initially 16 years old and composed of large, crowded trees on P. integerrima rootstock (planted 17 ft between trees and 17 feet between rows).  Farm Advisor Beede found that while hedging every-other row in alternate years did not significantly decrease cumulative yields, moderate mechanical topping (removal of approximately 50% of the fruit buds at the top of tree) prior to the on-year in successive cycles, did.  He also reported that biennial hedging both sides of the tree was detrimental to cumulative yield.

Clearly, attempting to control tree size, once trees are mature, may result in significant yield loss. With the above described studies in mind, factors are examined that can influence the rate at which trees grow and influence how trees grow.  Later, possible ways to mitigate these factors are discussed in relation to management of tree size, structure and yield.

FACTORS CONTRIBUTING TO OVERSIZED TREES

Factor One: Rapid Tree Growth

Oversize pistachio trees are generally a developing problem on the west side of the southern San Joaquin Valley.  Conditions here appear to be perfect for pistachio growth.  The soils are deep, boric, and calcareous and heat units accumulate quickly during the summer.  As a result of problems with Verticillium wilt fungus, the trees are planted on rootstocks with P. integerrima heritage.  Pistachio trees on P. integerrima rootstock or its hybrids are more vigorous than those on P. atlantica rootstock.  The result is a bigger tree that produces good yields of nuts more quickly.

Factor Two:  Close Tree Spacings

Pistachio trees, historically, have been planted fairly close together in California.  There are good reasons for this.  Close plantings (usually less than 18 feet between trees and 20 feet between rows) will give higher yields per acre when young, because there are more trees per acre.  Also, since trees will eventually be harvested with mechanical shakers and nut catching frames, tree size needs to be managed so nuts do not fall outside of the frame.  Since the standard frame is something of the order of 20 feet in length, problems arise in catching the nuts if trees are more than 20 feet apart within the row.  However, if trees are planted too close together, mutual shading and crowding occurs more rapidly.  Once trees begin to shade each other, the only direction left to grow is up.  Shaded branches tend to grow thinner and extend more rapidly toward sunlight in response to shading than unshaded branches.

Factor three:  tree training

Branches attempting to grow within trees that are already densely packed with branches will compete for light and productivity.  The dense canopies of older trees are often the result of the way young trees were trained initially.  Often trees of older orchards are characterized by long, thin interior scaffolds arising low in the tree and that don’t branch.  This lack of branching on a scaffold is the result of too many scaffold branches being left in the tree during training within the first few years of planting.  Crowded scaffolds have no room to branch as they grow due to competition with neighboring scaffolds.  The eventual result is a large tree composed of narrow, weak scaffolds.  These scaffolds will tend to sag into the middles once they bear leaves and nuts.

Factor four:  high cost of hand pruning

Sending pruning crews through the orchard with pole pruners and other hand tools can be an expensive operation.  Skimping on hand pruning can be a temptation, especially in an off-year when income is reduced.  Difficulty in finding trained labor for this task is also increasing.  The further bad news is that the cost of hand pruning can only go up.

DEVELOPING AN ORCHARD WITH MANAGEABLE TREE SIZE

As discussed previously, no scientifically designed experiments have been conducted comparing possible strategies for maximizing yield while maintaining manageable tree size from planting to advanced maturity.  However, based on the identification of factors that have promoted oversize trees above, perhaps, strategies are suggested for limiting tree size of trees in future plantings.

Factor one: rapid tree growth

Rapid tree growth, at least early in the life of the tree, is probably worth promoting.  Pistachio trees appear to have to reach a certain size before producing enough nuts to be harvested economically.  On the west side of the southern San Joaquin Valley, some pistachio blocks reach full maturity by the 9th leaf.  Once the orchard reaches full production, it may be advisable to reduce fertilizer and irrigation inputs.  Excess fertilizer and irrigation may stimulate unnecessary and unproductive vegetative growth that will cause competition for light among branches within a tree canopy and between canopies of neighboring trees.  The objective of this strategy is to find levels of irrigation and fertilization that will support high yield and reduce pruning requirements.  Regulated deficit irrigation in conjunction with leaf nutrient analysis may be useful tools to apply experimentally toward meeting this objective.

Factor two:  tree spacing

Because trees grow very rapidly on the west side of the southern San Joaquin Valley, growers have less time to enjoy the early yield benefits of closely spaced trees and more time to pay for the pruning required to keep older trees separated for efficient nut harvesting.  This observation suggests that pistachio trees could be planted further apart in this area of the pistachio production area.

Although tree spacing is confined by the size of the harvesting equipment, spacing trees 20 feet between rows and 20 feet between trees within the row appear to be economical.  Wider in-row spacing could have several advantages.  Trees can be hedged not only down the rows, but the wider spacing would provide space for hedging across the rows (called cross hedging).  Hedging the four sides of tree could be accomplished on some alternating multi-year schedule.  The wider tree spacing, in combination with intervals of light hedging in both directions, would reduce shading between neighboring trees.  Less shading would further decrease inter-tree competition for light and the associated production of branches stretching toward light.  Moderate mechanical topping, perhaps once every three or four years, would limit tree height, and fruit wood would be encouraged around the entire circumference to the canopy and not just on top, as currently occurs in some crowded orchards.  Perhaps a 12 or 13 foot tree height would be the objective instead of 16 feet or higher now seen in some orchards.

Mechanical hedging and topping should probably begin relatively early in the life cycle of the tree, focusing on pruning relatively less plant material more often.  The trees would end up being shorter and wider than the tall closely planted trees currently seen in many orchards.  Shorter trees would result in similar nut production to taller trees since the productive area of the tree remains the same.  With shorter trees the productive area is closer to the ground and a greater surface of the canopy is exposed to high light levels.  Shorter trees would be more efficient to shake for harvest and to spray for pest control.

A more frequent mechanical pruning schedule should moderate alternate bearing as well.  Trying to play catch-up topping on older trees will only result in the huge yield loss associated with trying to remove 3 or 4 feet of large scaffold branches from the top of the tree in one swoop or 2 feet in each of 2 swoops.  Pruning operations of this kind will destroy tree structure, leaving very little intermediate size wood for rapid replacement of lost fruit wood.  Growers with excessively large trees do not have attractive options.

Factor three:  tree training

Mechanical pruning can reduce the amount of hand-pruning required, but some interior pruning would still be required.  Relying more heavily on mechanical pruning has the attendant concern of producing a tree that is nothing more than a cellulose box.  The amount of interior pruning required could be reduced by stringent tree training in the first few years after planting.  Tree training generally begins during the first dormant season after planting.  Training usually involves a combination of thinning cuts (i.e. branch removal) and heading cuts (i.e. branch shortening).  Wherever a heading cut is made, two or three vegetative buds below the cut will push, producing two or three new branches.  If heading cuts are favored over thinning cuts, a geometric increase in new branches occurs low in the tree during the first few training years.  In contrast, by emphasizing thinning cuts during the first few years, and heading cuts later, more of the tree arises higher in the tree.  By reducing the number of branches lower in the tree, long, leggy branches that have no space to subdivide are minimized as is competition for light among branches.  A more open tree will be produced that is more amenable to mechanical pruning in that fruit wood will be produced at greater depth in the tree canopy, and mechanical pruning operations will not be as destructive to overall yield.

Factor four:  high cost of hand pruning

Dependence on outside labor, and at what rate its cost will increase, is a major grower concern and one that is often outside of the control of the grower.  Watching trees get taller and taller, and orchards shadier and shadier, is another concern, that appears to have no easy solution once it exists. Greater reliance on mechanical pruning should reduce overall pruning costs compared to the heavy reliance on hand-pruning that currently exists.

IN CONCLUSION

Based on current observations of some orchards on the west side of the San Joaquin Valley, increasing tree height is a problem that facing some growers.  Eventually the grower will have to risk losing some yield to maintain trees at a manageable height.  The question posed is whether it is better to spend some money and risk some yield loss now or postpone concerns to a much later date.  This article suggests that it may be of economic benefit to the grower to begin to think about managing tree size even before trees are planted and to address tree size annually there after.  The pistachio tree has demonstrated an excellent ability to compensate for relatively modest annual pruning activities.  Waiting until closely-spaced trees are twenty years old before a problem with excessive tree height is first addressed will certainly limit a grower’s options and eventually result in a less productive and more expensive orchard to operate.