UCCE Pistachio Specialist Louise Ferguson, with UC Davis Department of Plant Sciences, said managing a mature pistachio canopy has a direct impact on quality harvestable yield of an orchard, or how much money a grower receives per square foot of orchard floor

In a November presentation during the UC Cooperative Extension’s virtual Pistachio Short Course, Ferguson said young pistachios are pruned to develop the structure of the tree, while pruning mature trees, or canopy management, focuses on production. Decisions must be made on tree height, volume, accessible trunk for shaking, branch and shoot orientation, light penetration and net profit.

Pruning a mature pistachio tree can also help with orchard operations including spraying and harvesting, and can also help with alternate bearing. Pruning a mature tree will result in more branching, more leaves and more bearing points- buds to produce more nuts. Proper management will help achieve light interception of 80 percent of the orchard floor, she said.

Optimum time for canopy management is December and January before the auxin hormone begins to suppress bud break of lateral buds. Auxin, Ferguson explained, is produced in the stem, buds and root tips to maintain apical dominance. Removing apical bud stops auxin distribution that inhibits lateral vegetative bud break.

Ferguson, who has been involved in pruning and mechanical hedging and topping research trials, said main methods of canopy management are hand pruning, mechanical pruning and a combination of both. Hand pruning has the advantage of being precise in cuts, but is also more expensive and time consuming. It is also requires skilled labor. Mechanical pruning is less expensive and can decrease alternate bearing. Topping, to control height, is done annually. Trials showed that mechanical hedging is best done every other year on alternate rows. Tree training must be done during the first years of a tree’s life before mechanical hedging is possible.

Hand pruning, Ferguson said, is better for the tree horticulturally, but the decision for mechanical pruning should be made based on sustainability and economics.

With either method, the grower and pruning crew must know the difference between fruiting and vegetative buds, recognize bearing habits of pistachio trees, and recognize the desired shape of the tree.