December Pistachio Task List

By Robert Beede, U.C. Farm Advisor, Emeritus

For farmers, there never seems to be much “downtime” anymore. With harvest completed, you now have to prep the ground for planting next spring, tune up the herbicide rig for your pre-emergent berm treatments, apply soil amendments, and begin your pruning and sanitation program. It is ALSO time to purchase and install a simple temperature recorder in each orchard so that you have real-time weather data to refer to for chill accumulation and assessment of freeze events.

I just went online and found I could set up a Hobo weather station complete with a temperature and humidity sensor and software for about $500. I am sure there are less expensive systems, if one looks a little harder. Weather data is THE missing item to assist growers in determining how inadequate their chill portions were. Every grower, large and small, should have at least one in every orchard with a different elevation or microclimate!

Chill portion accumulation began November 1! Having your own weather data may also prove to be valuable for support of a crop insurance claim. The CIMIS stations also appear to be less reliable in recent years. Several of them reported erroneous or no data last season. The grass surrounding many of the stations has also died during the drought, thus affecting the accuracy of both the weather and irrigation data.

For real time updates on chilling hours by CIMIS station, snowpack and reservoir conditions, go to my website at: http://cekings.ucanr.edu/Agriculture/Grapes_Tree_Fruits_Nut_Crops/. Select “Management” in the main menu, then “Chilling hours” and “water and weather.” Those of you with concerns about what to do if chilling is again inadequate should read the preliminary work of David Doll, Merced County Farm Advisor, with Surround applications started in January. It can be found at his “Almond Doctor” website. More work is planned for this winter, since there are presently more questions than answers.

Also, UC Farm Advisor, Kurt Hembree, has lots of helpful pistachio weed information at this website: http://ucanr.edu/sites/Weed_Management/files/74887.pdf.

Winter Sanitation and Navel orangeworm (NOW): I have told you for 35 years that sanitation is the cornerstone of a good NOW management program. Destruction of residual nuts remaining in the trees and on the ground is critical to breaking the NOW life cycle. If navel orangeworm is a problem in your orchard, I would begin knocking the mummy nuts onto the ground as soon as possible, since you want them exposed to rainfall for decomposition.

It is well known that pistachios are NOT easy to clean up. We leave lots in the trees, in the crotches, and on the ground after harvest. The difficulty in sanitizing pistachio orchards is largely related to their small size, compared to almonds or walnuts. One pound of pistachios contains about 300 in-shell nuts compared to about 184 in-shell almonds and 50 walnuts. Pistachios are also harder to destroy.

Due to their lighter weight, they resist being sucked up and broken by the flail mower. The high air velocity of some equipment used to blow the tree berms free of trash and overwintering nuts can deposit some of them into the adjacent tree row just cleaned. They also get imbedded into the soil around the base of the tree where the ground cracks loose from shaking. Depending upon how worn the rubber guards are on the shaker frame, the amount of nuts left during harvest at the base of every tree can range from a few to as much as a couple handfuls.

Because of the smaller size and larger numbers left after harvest, the overwintering sites for NOW are much higher. For every successfully overwintered NOW female, 85-100 eggs are produced in the spring. So, it does not take very many pistachios left per acre to generate a lot of moths for next year.

Brad Higbee, Wonderful Farms NOW researcher, has concluded, after 100’s of hours of detailed research, that winter sanitation is STILL the cornerstone of effective NOW control. His data from large sanitation trials supports this longstanding UC IPM position. Dr. Siegel also has extensive data to support this practice. So, if you do not want NOW problems, you had best do the sanitation thing to the best of your ability. Dr. Kent Daane, research associate Glenn Yokota, and I found from a three-year study that there was greater and earlier NOW emergence on bare berms and in cover crops. Discing in the mummies greatly reduced the percent survival. Deep incorporation was no better than regular disking. Brad has data to show NOW larva can even crawl from beneath the soil.

Winter sanitation includes removal of mummy nuts remaining in the tree, blowing them out of the crotches at the head of the tree (this is where all the limbs rise from the trunk), blowing, and if necessary, raking the berms clean, and then DESTROYING the nuts by disking or flailing with a roller behind. This cultural practice breaks the developmental cycle of NOW without the use of pesticides. It is also a COMMUNITY EFFORT! This may be one reason why some areas are more prone to high damage. NOW pressure is also most likely increasing in the southern San Joaquin Valley due to more acres of almonds, pistachios, walnuts and pomegranates.

Pruning: 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 TWO 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. Dr. Ferguson and I have begun a project 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. Therefore, 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.

Finally, come to PISTACHIO DAY, Wednesday, JANUARY 20, 2016, at the Visalia Convention Center, to see your buddies and have a great learning day! Please pre-register to prevent a long line and wait at the door! Here is the signup link: http://ucanr.edu/sites/pistachioday/. It is going to be a FULL DAY this year, so come ready for a learning marathon! Happy Farming!