Mechanical pruning of 'Improved French' prune trees

The California prune industry relies on hand pruning to thin fruitwood, improve fruit size, reduce alternate bearing, and control tree size and shape. Both the high cost and limited availability of labor have increased the interest in mechanical pruning. An experiment was established to evaluate timing and severity of mechanical hedging compared to a hand pruned control. The severe mechanical pruning treatment consisted of hedging 4 sides of the canopy and topping (boxing) every year and the moderate treatment was hedged 2 sides of the canopy every year. These treatments were conducted either in early spring (May) or postharvest (October). Over two years of the study, no significant yield differences were found among the treatments compared to the hand pruned control. Early spring or fall are equally effective, indicating a wide seasonal window in which to mechanically prune 'Improved French' trees. Hedging and boxing provided $ 1071 and 894 savings ha^‑1, respectively, compared to costs of hand pruning. Significantly more wood (larger branches) was removed in the hand vs. the mechanical pruning treatments. Pruning weight of the hedged and boxed trees were about 70 and 50% lower than the hand prune trees, respectively. Canopy volumes were also significantly larger in the hand pruned trees. Mechanical pruning resulted in a compact tree which may affect spur longevity. The long-term effects of mechanical pruning on disease incidence and orchard longevity still need to be investigated.