Multiple Leader Training Optimizes Sweet Cherry Labor Efficiency
The final intensive training system to be examined in the North American NC-140 Sweet Cherry Canopy Architecture and Rootstock Trial is a multiple leader, bushy canopy based on training and production techniques popularized by the Australian cherry grower, Kym Green.
Unlike the secretive former Soviet security agency known as the KGB, the personable and outspoken Kym Green has openly shared his Kym Green Bush (KGB) training system ideas with cherry growers around the world who are interested in training simplicity, labor efficiency, elimination of ladders, use of vigorous standard rootstocks, and partial pruning mechanization. As with the previously discussed training systems, it has its advantages and limitations.
The development of the original KGB system was based on vigorous, non-precocious rootstocks like Mazzard, as well as ‘Lapins’ high productivity and growing habit (strongly upright, with minimal lateral branching). Multiple severe heading cuts imposed over the first two to three years of establishment — at planting, possibly again in early June, prior to second season budbreak, and possibly again in early June and/or prior to third season budbreak — delay fruit bud formation but diffuse vigor into initially 4-5, then 8-10, then 15-18, then 25-30 vertical leaders (see Young KGB Tree picture). This establishes a strong root system with many relatively uniform, balanced vertical leaders of moderate vigor that should be comprised primarily of spurs with minimal branching.
The key for the KGB establishment phase is not so much a specific target number of leaders, but rather whatever number is required to achieve the relatively uniform, balanced, moderate vertical growth that ultimately will be allowed to mature and form fruiting spurs. Vertical leaders that are too vigorous produce fewer reproductive spurs, and thus must be headed back again to diffuse vigor into two or more replacement leaders, or removed entirely to prevent domination over the population of other leaders.
Those that are too weak may never catch up with the majority and are removed to maintain a uniform population of future fruiting leaders. Once the target leader vigor and/or number is achieved, heading back ceases. Any lateral shoots are removed, and fruiting capacity can increase rapidly from minimal to extensive on productive varieties or precocious rootstocks.
Benefits To This System
Labor efficiencies are realized through simplistic Shakespearean establishment pruning decisions (“to head, or not to head, that is the question”), and the ability to work without ladders. The limber, narrow fruiting leaders can be pulled down easily by workers for pruning or harvest. When leader height exceeds that allotted for the orchard spacing, the tops can be mechanically hedged. No trellising is required, and tree costs are relatively low, depending on rootstock choice and tree density.
Original Examination of KGB System
To my knowledge, the NC-140 trial was the first to examine the adaptation of KGB training principals to precocious, vigor-limiting rootstocks (i.e., Gisela [Gi] 3, Gi 5, and Gi 6). This revealed some unexpected consequences. First, when Gisela rootstocks were evaluated in the 1990s, largely with traditional central leader or open vase training systems, a positive trait was the increased number of, and more horizontally oriented, lateral shoots that they conferred.
This trait, obviously, is undesirable for KGB canopies, leading to fewer strongly vertical (or more floppy) multiple leaders (see Mature KGB on Gisela 3 picture), and an increase in lateral shoots to be removed from those leaders. This latter trait is also influenced by scion variety; for example, ‘Sweetheart’ naturally forms more lateral shoots than ‘Lapins.’ In such cases, the lateral shoots can be short-pruned (like super spindle ax — SSA — see the February 2017 story), rather than removed entirely, if retention of the basal fruit is desired on less productive varieties like ‘Regina,’ ‘Benton,’ or ‘Early Robin.’
Second, KGB fruiting leaders are renewed after five to seven years, as fruiting spurs age and spur density declines over time. Generally, each year the largest one or two leaders are removed and regrowth is initiated for continual renewal of a fraction (approximately 15%) of each tree’s canopy. On highly productive, vigor-limiting rootstocks, achieving adequate regrowth can be problematic due to the competition of the crop load with renewal shoots for growth resources.
The Potential For KGB
In the North American NC-140 trials, the KGB was the least precocious of the four canopy architectures examined, but by Year 6 in Michigan, annual yields on KGB trees across rootstocks were relatively similar to those of upright fruiting offshoots (UFO) and tall spindle axe (TSA) trees on the same rootstocks. Harvest efficiency was similar to that for SSA trees, only slightly lower than that for UFO canopies, and much better than TSA harvest efficiency.
In summary, KGB training for sweet cherries has the lowest establishment cost, simple management decisions, and relatively high labor efficiency, with delayed but eventually competitive productivity and good fruit quality. Like UFO, canopy development can be adapted to a wide range of rootstock and site vigor by adjusting the number of fruiting leaders proportionally.