As the North American NC-140 Sweet Cherry Canopy Architecture and Rootstock Trials near their planned completion — 2017 is Year 8 of 10 — it is time to envision some of the “next generation” intensive canopy training systems that will build on what has been learned thus far. Since the trial was conceived, the need for increased labor efficiency, partial mechanization, and simplified fruiting units for easier management of leaf-to-fruit ratios, fruit quality and yield targets have only grown stronger. This column will summarize some of the conclusions to date and potential continued evolutions regarding each system in the trial.
The Traditional System
The Tall Spindle Axe (TSA) is the most traditional canopy architecture, having already evolved over several decades from central leader, Zahn spindle, Vogel spindle, and other variations. It provides very good yields of high-quality fruit, borne as a mix of around 55% to 70% spur and 30% to 45% non-spur fruiting sites.
It is well-suited to dwarfing and semi-dwarfing rootstocks, though semi-vigorous rootstocks can be used as well. The more vigorous the tree, the more complicated the canopy and lower the labor efficiency for pruning and harvest.
The greatest challenge is to maintain light and vigor, and retain fruiting spurs, lower in the canopy. As a three-dimensional canopy, its potential continued evolution is likely to move towards a more narrow profile (favoring use of more dwarfing rootstocks), possibly by incorporating some summer hedging.
Tweaking the SSA
The Super Slender Axe (SSA) represents the big leap forward in central leader canopy architectures, absolutely requiring dwarfing to semi-dwarfing rootstocks and very high tree densities. Such a combination provides the highest early yield potential, and very high-quality fruit borne mostly of non-spur fruiting sites at the base of year-old lateral shoots. Semi-vigorous to vigorous rootstocks are absolutely unsuitable for traditional SSA production.
The SSA creates a narrow fruiting wall but requires extensive labor for annual dormant pruning, and has the highest initial tree cost. Evolution of the SSA canopy may include some summer or fall hedging to reduce pruning labor. Adoption of bi-axis trees, oriented either as a single wall or a V-trellis to increase light interception per orchard, would maintain or increase the number of leaders while reducing tree density.
Production with SSA is highly dependent on a variety’s propensity for non-spur fruit bud development. Research needs to continue to explore the potential use of growth regulators to modify non-spur flowering (increasing or decreasing it, depending on natural propensity) for certain varieties.
Similarly, the short-pruning critical to SSA production might be modified on some varieties to include a little higher proportion of spur fruiting sites to improve on currently modest yields at maturity, at the expense of creating a slightly less narrow fruiting wall.
The Planar Canopy
The Upright Fruiting Offshoots (UFO) system represents the big leap forward in fruiting wall canopies, being the most narrow, most suitable for hedging, and most labor efficient to harvest. It provides very good yields of high-quality fruit, borne primarily on fruiting spurs. It is well-suited to rootstocks of any vigor level (as long as they promote fruiting precocity), since the number of upright fruiting offshoots (mini-leaders) can be increased or decreased proportionally to the vigor of the rootstock to achieve the desired amount and fruitfulness of annual growth.
Evolution of the UFO canopy may include use of bi-axis trees to create two “cordons” at planting (rather than planting the nursery tree at a 45° angle or spending the first year heading the nursery tree to grow two leaders), and orientation of the upright fruiting offshoots as dual fruiting walls with a V- or U-trellis (see diagram and picture) to increase light interception and yield per orchard. As with all fruiting walls, precise leader orientation and the adoption of narrow tractors to facilitate close-row spacing will help to optimize yields and labor efficiencies.
The Low Input Canopy
The Kym Green Bush (KGB) creates the lowest input, most simplified, three-dimensional sweet cherry canopy that has relatively high labor efficiency for pruning and harvest. It provides very good yields of high-quality fruit, albeit not as precociously as the other systems. It is well-suited to semi-vigorous to vigorous rootstocks, though semi-dwarfing rootstocks could be used at higher tree densities; a critical rootstock consideration is for promotion of strongly upright leader growth, as we have learned that a significant proportion of the multiple leaders formed on Gisela (Gi3, Gi5, and Gi6) rootstocks tend to “flop,” requiring tying to remain upright.
Similarly, KGB is best suited to strongly apically dominant varieties like ‘Lapins’ since varieties that readily form lateral shoots (like ‘Sweetheart’) increase hand pruning costs. This is also true of UFO, but the latter can be hedged while KGB upright leaders cannot.
Hence, the next generation KGB canopy may evolve toward adoption of a V- or U-trellis system to orient the many multiple leaders into two distinct fruiting planes, allowing some hedging as well as the use of semi-dwarfing rootstocks, like Gisela 5, that may need the support of a trellis.
However, any tying of leaders for support negates one of the advantages of KGB: being able to harvest free-standing leaders from the ground by pulling them down. A dual-plane KGB canopy oriented and/or supported by a trellis would be conducive to mobile orchard platforms for harvest.