Distinguish training (before bearing) from pruning (done annually on mature trees).

Pruning is one of the cultural practices in tree nut orchards that has evolved over the years. Annual hand pruning was a standard practice in almond orchards for many years until research proved it decreased yields. Early training of walnut trees is still standard, but many growers now do only minimal pruning to eliminate dead wood and for disease control. Pistachio trees, the relatively ‘new kid on the block,’ were trained and pruned to achieve an upright tree shape, decrease alternate bearing and facilitate harvest.

No pruning or minimal pruning after initial tree training became a more accepted practice in all three tree nuts in the last decade. Now, a primary reason for pruning in many almond, walnut and pistachio orchards is for harvest and equipment access. Aside from removing branches that block access or interfere with harvest, there is no shortage of people who firmly believe hand pruning after trees have been in the ground for more than two years is a waste of time and money. Mechanical pruning in orchards at appropriate times was one solution shown to achieve a return on investment in orchard health.

Training a hand pruning crew to do minimal cuts where needed is another consideration in orchard pruning. Growers also recognize pruning can play a part in disease control in an orchard.

1. Harvestability

Louise Ferguson, a cooperative extension pomologist known for her work in pistachio rootstocks and salinity tolerance, has done research trials in pistachio pruning. Harvestability is the No. 1 item on Ferguson’s list of pruning considerations in pistachio orchards.

“You have to be able to get it off the tree and put it in the bin,” Ferguson said.

Large pistachio tree canopies make it difficult to catch all nuts in the catch frame of the harvest equipment. Nuts that fly over and hit the ground when trees are shaken cannot be recovered and yield is lost. Ferguson said harvestable yield should drive canopy management. She noted an ideal canopy is under 15 feet in height, with upright scaffolds. Pruning with thinning cuts to direct branch growth upward keeps the crop close to the tree axis and improves harvest efficiency. Lower-cost mechanical pruning with a few selective hand thinning cuts  can achieve this, she said. Hand pruning is done to remove flat branches or branches that interfere with harvest.

Sebastian Saa, director of agricultural research for Almond Board of California, said pruning for orchard access and worker safety may be the only reasons for pruning almond trees. He said a survey of growers showed almost half prune their almond trees for that reason alone. If more off-ground harvesting is done in almond, canopy size could then be a harvest issue.

Escalon-area walnut grower Robert Longstreth said early training is important for balanced trees.

2. Sustained Production

This consideration can apply to walnut and almond as well as pistachio.

Since pistachio trees bear fruit on one-year-old wood, selective pruning can yield a better mix of fruit and non-branches and therefore mitigate alternate bearing.

“The major reason for pruning a mature pistachio tree is to produce harvestable, predictable yields,” Ferguson said. Topping mature orchards can decrease alternate bearing. She noted alternate bearing in trees on UCB-1 and Platinum® I rootstocks can be mechanically pruned entering an ‘on’ or ‘off’ crop year and remains about 0.3 on a scale of 0 to 1. However, alternate bearing in trees on Atlantica or PGI rootstocks is better mitigated by pruning entering a low crop year.

Pruning decisions in walnuts are more likely to be based on economics. Skipping pruning every other year keeps production costs down, and research by former UCCE Farm Advisor Bill Olsen showed little impact on crop yield or quality. Escalon walnut grower Brent Barton said initial tree training is an investment in the long-term production of the orchard. In mature orchards, left unpruned, there may be issues with shading out fruiting wood in the lower parts of the trees. Barton said the fruiting area migrates higher in the tree and deadwood can accumulate.

“Hedging encourages vegetative growth, and you don’t want that,” Longstreth said.

3. Costs (Mechanical and Hand)

The advantage of hand pruning in a tree nut orchard is precision. Hedging and topping mechanically can be less expensive, but all trees are pruned the same, regardless of need. If canopy management is desired, mechanical is more cost-effective.

Ferguson said that hand pruning for canopy management requires knowledge of the bearing habit, they bear the crop on last year’s shoot growth and the strong apical dominance that results long drooping shoots when the crop matures. The growth resulting from heading and thinning cuts must be recognized. Thinning cuts (removing a branch where it originates) produce better canopy light interception. A heading cut precipitates branching. Both remove crop, but the thinning cut does not produce a strong vegetative response like the heading cut does when done distal to a vegetative bud.

There may be a need for pruning in almond production, Saa said, but it is difficult to justify the cost. One example would be tree canopy extending over the middles, causing a longer drying time for nuts on the ground. Trees planted closer down the row tend to stay smaller, he said.

Ferguson’s work showed the cost differential between hand and a combination of hand and mechanical pruning is about 50%.

4. Training Hand Crews

The advantage of hand pruning is precision, Ferguson said. This requires training of hand crews to know the difference between fruiting and vegetative buds and to recognize the transition on a branch. Hand pruning crews need to know the difference between a heading and a thinning cut and the response from each of these cuts. Heading cuts are when the shoot is cut in half and the lower portion has few buds. A heading cut also removes stored carbohydrates. Thinning cuts remove the entire shoot at the base, opening the canopy and allowing more light penetration. Thinning cuts also force the canopy upward.

5. Disease Control

Prune out diseased limbs, cutting back into healthy wood (where the wood is no longer discolored); at least 2 to 3 inches beyond the lower canker is sufficient to remove the disease pathogens. Timing is important as fresh cuts can become infected if a rain event closely follows pruning. In walnut production, research recommends removal of dead wood from the orchard when there is a high level of Botryosphaeria inoculum.