The goal of a good pruning program is to manage the canopy over the life of the orchard in such a manner as to achieve the maximum possible yield of clean open split-nuts from an efficient harvest.
In our quest for this goal, we must couple our knowledge of how pistachios grow and fruit with the research data developed over the past 30 years. One thing to remember about pruning is that we must think in terms of 2 years, rather than just next year – if we want to better manage alternate bearing.
Pruning harder prior to an on-year improves the yield during an off-year, in my opinion. A project is underway to test this hypothesis.
Let’s first briefly review what we know about the growth and fruiting habit of pistachio. This tree is very apical dominant, meaning that it does not branch readily and grows mostly from the terminal bud and one or two lateral buds behind it.
So, branching must be forced by removing the end portion of a limb, known as a heading cut. Heading cuts are performed regularly during the training years to develop the desired branching. Because of pistachio’s apically dominant nature, it also does not develop girth (enlargement of trunk and limb diameter) rapidly. Consequently, main structural limbs have to be headed shorter than desired in order to keep them upright.
The fruiting characteristics of pistachio also greatly influence pruning. Flower buds are born on one-year-old wood, typically towards the base of medium to long shoots and adjacent to the terminal vegetative bud on short shoots (spurs).
The lack of lateral branching causes the fruit-bearing wood to become increasingly distant from the central axis of the tree. Failure to contain the tree canopy to a diameter of about 17 feet results in crop falling onto the ground at harvest due to the limited size of the harvest equipment.
Eventually, the main structural limbs bend downward during the on-bearing seasons from the weight of the crop.
Without corrective pruning, the pistachio tree canopy begins to take on the appearance of an umbrella. This combination of less upright fruiting limbs and their greater distance from the tree’s center creates major problems for effective harvest.
The high energy imparted to the trunk by the shaker can no longer be sufficiently transmitted to the fruiting zone for its removal. Some growers attempt to solve this by simply shaking the tree harder. The result is more frequent equipment breakage, rapid sling wear (the thick rubber sheets draped around the shaker pads for protection), excessive removal of next year’s fruiting wood (spurs) and possibly greater tree stress from disruption of roots at the tree’s crown. Harder shaking also flings the crop past the catch frame of the harvester.
The solution to the above problem is to prune the pistachio with the objective of “pushing back” the canopy perimeter (reduce its diameter) and directing growth upward. This is accomplished principally by “thinning cuts,” which is the complete removal of a limb at its point of origin.
To achieve a more compact and upright tree, thinning cuts are made to flat limbs around the outside of the tree and within the canopy where excessive fruitwood exits.
Care should be taken to not perform too many cuts in any given sector of the canopy unless the fruitwood is unusually abundant. In addition to distributing the thinning cuts over the entire tree, avoid removing all of the lateral limbs on a specific structural branch in order to make room for adjacent branches.
Rather than creating these so-called “snakes”, it is better to leave the best structural branch minimally pruned and remove the competing branch entirely. Also avoid opening the center of pistachios. We do NOT want them to look like peach trees at the completion of pruning.
Because of the growth and fruiting habits described, pistachios will naturally open up and allow sufficient light into the canopy center for fruitwood production.
Loss of fruitwood in the middle of the tree over time is, in my opinion, more a function of apical dominance than insufficient light penetration. So, remember, prune to keep the pistachio canopy compact and upright for productivity and harvestability.
Above all, remember that we do not want mature trees to be pruned to the point that they produce lots of long whips! Although this looks good, it most likely means that the tree has been over pruned. Work by Tim Spann, shows that pistachio has “preformed shoots”. These are shoots with 7-9 bud positions set before the season begins.
Providing the tree is not excessively vigorous, these preformed shoots grow into spurs and set lots of crop. If mature trees are over pruned, these preformed shoots are “pushed” into continued growth. I believe the most productive pistachio tree is one that has hundreds of these short, preformed shoots, rather than lots of long whips.