Mechanical Pruning Shows Promise for Sweet Cherry Trained to the UFO Architecture

The planar, upright fruiting offshoots (UFO) architecture for sweet cherry (Prunus aviumL.) was developed at WSU, in part, to facilitate the incorporation of mechanization and automation technologies. In the current study we investigate the potential to mechanically prune sweet cherry trees trained to the UFO system.  This trial was established in a 6-year-old block of ‘Selah’ on ‘Gisela®6’ rootstock at the WSU Roza experimental farm.  Each of three rows was divided into 3 equal blocks of 16 trees and assigned one of three treatments: hand pruning, full mechanical pruning, and mechanical pruning + hand pruning follow-up. Mechanical pruning was accomplished in February 2012 using a hydraulic, tractor-mounted pruner containing three adjustable rotary disc blades for cutting. This machine pruned only one side of the trees in each pass.  The pruner was set to cut all wood extending in the row beyond about 4 inches on both the east and west sides. First, the blades were oriented vertically to remove lateral wood.  Then, the blades were oriented horizontally, topping the trees at 11 feet. Hand pruning was carried out to remove all lateral wood and top the uprights to a height of 11 feet.  In each replicate block, the pruning time and the weight of prunings were measured.  The average time for hand pruning was about 42 min/section (~158 s/tree) and the average weight of cutting wood was 59.87 kg/section (3.74 kg/tree). In contrast, the average pruning time for full mechanical pruning, for both sides of the trees, was 2 min 40 s (~9 s/tree) and for the tops was 1 min 7 s (~4 s/tree); a total 3 min 47 s per section (~13 s/tree). Full mechanical pruning therefore took about 8% of the time of manual pruning.  The average weight of prunings was ~24kg/section (~1.5 kg/tree, -60% less than hand pruning). In the combined mechanical + hand treatment, the average time for pruning was almost 34 min/section (~127 s/tree, -20% lower than hand pruning alone) and the average weight of prunings was almost 58 kg/section (~3.6 kg/tree).  We will also report on the effects of these pruning strategies on fruit yield and quality, canopy light interception and distribution, and tree growth. This preliminary evaluation shows great potential to reduce pruning time/costs mechanical systems.