In my August column, I began exploring some of the intensive training systems in the North American NC140 Sweet Cherry Canopy Architecture and Rootstock Trials, focusing on the relatively traditional, three-dimensional tall spindle axe (TSA). This month will focus on the potentially transformative upright fruiting offshoots (UFO) canopy architecture, which has demonstrated good early and sustained yields that, thus far, are comparable to the TSA.
The UFO arranges and orients the classic “fruiting unit” of sweet cherry trees (described in the August column; see diagram) as multiple vertical leaders in a single plane to create a very narrow, trellised fruiting wall.
The UFO architecture can be established as a single or double “cordon,” like grapevines. This can be achieved by planting the nursery tree at a 45-degree angle (as illustrated in the diagram) or by planting the nursery tree upright, heading it low, and growing two dual leaders in the first year, which are then bent to horizontal at the end of the growing season.
The advantage of the single cordon UFO is the potentially earlier development of the upright fruiting units and thus, earlier yields. If dual leader nursery trees can be obtained, this earlier yield advantage can be realized for dual cordon UFO trees as well.
Once the single or dual cordon(s) is/are established, the buds on the bottom of the cordon are rubbed off prior to budbreak. One or two top or upward-oriented side buds are selected about every 8 inches for retention as future fruiting unit shoots and the remaining non-selected buds are removed.
This thinning of the vegetative bud population during budswell helps promote the remaining buds to emerge as shoots in relatively precise locations for optimal light interception. Maintaining the cordon at a slightly upward angle just above horizontal, and especially the tip of the cordon in an upward position, is important for maintaining vegetative vigor across the length of the cordon.
Goals Of UFO System
The goal for upright shoot development in the year of emergence is uniform spacing, relatively uniform vigor, and filling the foundational positions of the future fruiting wall as completely as possible. Any new vertical shoot that is excessively vigorous (usually near the base of the cordon) at 5 to 6 weeks after budbreak should be cut back at that time to a few inches above the cordon.
This will promote two new shoots to regrow, diffusing the excessive vigor across the two replacement shoots, achieving a better balance with the remaining single shoots on the cordon. If, at the end of the season, any upright shoot is still excessively vigorous, it may need to be removed entirely to prevent it from outcompeting the more balanced upright shoots around it.
In Year 2 after the upright shoots form, UFO trees planted on precocious rootstocks usually produce flowers and fruits at the base of each shoot, just above the cordon (see diagram). The remaining nodes on these shoots will form spurs with flower buds during Year 2, which will then fruit significantly in Year 3.
The spur fruiting sites comprise the majority of the fruit population in UFO canopies. Consequently, the more complete the development of the fruiting wall foundational shoots in Year 1, the greater the first significant crop potential will be in Year 3.
Likewise, the sooner the vertical space is filled in Years 2 and 3 by extension growth of the foundation shoots, the more quickly the entire fruiting wall will reach its full yield potential, possibly as early as Year 4 or 5, depending on variety and annual growth rate (see photo). Any lateral shoots that emerge from spurs should be removed, thereby maintaining the narrow wall and promoting the filling of vertical space.
The UFO canopy architecture can be developed with rootstocks that impart a wide range of vigor levels. Since the UFO deconstructs the typically complex sweet cherry canopy into very simplified fundamental fruiting units that are repeated about every 8 inches, UFO trees on a dwarfing rootstock may be planted, for example, 40 inches apart with the goal of developing 5 upright shoots (5 x 8 = 40).
Trees on a semi-dwarfing rootstock might be planted 64 inches apart to develop 8 upright shoots, on a semi-vigorous rootstock 88 inches apart with 11 upright shoots, or on a vigorous rootstock 120 inches apart with 15 upright shoots. The most important rootstock trait for UFO canopy development is precocity rather than vigor control, so that flowering spurs begin forming in Year 2 and significant cropping begins in Year 3, which then helps moderate vigor as the canopies fill their vertical space in Years 3 to 5.
The Potential Of This System
This radical canopy architecture has the potential to be transformative in several ways. First, as a narrow fruiting wall, light distribution to all fruiting sites is optimized, improving fruit quality and uniformity. Second, the fruiting wall is suitable for hedging to reduce hand-pruning costs. Third, the canopy is so narrow that it can be picked entirely from one side, facilitating highly efficient use of harvest labor on self-propelled orchard platforms, eliminating inefficient and dangerous ladder use. Fifth, the deconstruction of the canopy into simplified, repeated fruiting units is very suitable for precision management of fruit-to-leaf ratios to optimize yield and fruit quality.
Through 7 years, the cumulative yields of UFO trees have been the highest of all canopy architectures for the Michigan (‘Benton’) and British Columbia (‘Skeena’) sweet cherry trials when projected on an appropriate orchard tree and row spacing.