Take a Close Look at Super Spindle Axe

‘Benton’ sweet cherry SSA trees shown in bloom in 2016. (Photo credit: Greg Lang)

The next intensive training system to be examined in the North American NC-140 Sweet Cherry Canopy Architecture and Rootstock Trial is the Super Slender Axe (SSA), which is a central leader-based “fruiting wall” alternative to the Upright Fruiting Offshoots (UFO) I discussed in the November 2016 issue.

The SSA incorporates several extreme innovations for cherry production: ultra-high-density plant spacing (20 to 40 inches between trees, depending on rootstock vigor); annual renewal of nearly 100% of the fruiting wood (instead of 10% to 15% in the other canopy systems); and fruiting sites that are limited primarily to basal flower buds on previous season shoot growth, rather than fruiting spurs on older wood.

Starting this System off Right
The year-by-year development of SSA cherry canopies is described in detail in Cherry Training Systems, the free 63-page guide downloadable in English or Spanish at Hrt.MSU.edu/Greg-Lang. Rootstock selection and horticultural techniques to induce extensive lateral branching are critical to the success of this production system. The use of plant growth regulators, such as Promalin from Valent, and/or scoring of vegetative buds on the leader, is essential to promote lateral shoot formation during the planting/establishment years.

Precocious rootstocks must be used to promote rapid initiation of flower buds at the base of the new shoots (as described in the August 2016 issue). These basal fruiting sites not only provide significant yield potential in Years 2 and 3, but also help moderate vigor by distributing some growth resources to fruit even as the canopies are filling their allotted space.

In addition to the vigor-moderating effect of early cropping, the close spacing of SSA trees creates root competition that can significantly reduce vigor. This was detectable by the end of the first growing season in the Michigan NC-140 trial with ‘Benton’ sweet cherry.

After six years, the SSA central leader trunk diameters were half the size of comparable Tall Spindle Axe (TSA) trees. In fact, SSA trees on vigorous Gisela 6 (Gi6) rootstocks were comparable in trunk size to TSA trees on dwarfing Gi3. The extreme pruning has a further dwarfing impact on tree vigor, since a high proportion of the canopy is removed and re-grown annually.

Extreme annual pruning promotes large basal (non-spur) fruit and renewal shoots when done properly. (Photo credit: Greg Lang)

Pruning in SSA
Annual maintenance pruning of the SSA canopy further sets it apart from other cherry training systems. The solitary flower buds at the base of previous season shoot growth tend to produce the largest fruit in the canopy. This flower bud population is developed preferentially by annual pruning of every new shoot back to one to three vegetative buds just above the solitary basal flower buds.

It can be tricky to differentiate between the flower buds prior to budswell, so winter pruning of SSA trees is often delayed until early spring when the difference between round fat flower buds and narrow pointed vegetative buds is more evident.

Pruned shoots lower in the canopy retain two to three vegetative buds to fill available canopy space, while higher shoots are pruned to one to two vegetative buds to maintain a narrow top and reduce the potential for shading. If the renewal pruning fails to include a vegetative bud, the resulting stub will bear fruit, then die. The goal for SSA development is simple: The greater the number of lateral shoots that are formed in Years 1 and 2, the more balanced the vigor among those laterals and the greater the yield potential in Years 2 and 3.

Properly-pruned annual shoots will have short sections of “blind” wood (where fruit were borne), and the terminal vegetative bud(s) from which new shoots arise. Eventually, the basal fruiting sites move out from the central leader, necessitating cutting back into the “blind” nodes in hopes of forcing latent buds (see below) to erupt, usually at the “collar” where a pruned shoot arose from an older shoot. The extensive annual pruning helps stimulate more frequent outgrowth of these latent buds, but sometimes the blind stubs produce no new shoots.

Emergence of a new shoot from a latent bud not previously visible. (Photo credit: Greg Lang)

Yields in SSA NC-140 Trial
As might be expected from their precocity and high tree densities, the SSA Benton/Gi3 trees had the highest yields per acre from Years 2 to 4 of the canopy architecture trial in Michigan. However, SSA/Gi3 yields were surpassed in Year 5 by the TSA, UFO, and KGB (Kym Green Bush) trees on Gi3 (as well as TSA and UFO trees on Gi5). SSA/Gi6 became the lowest yielding of all canopy/rootstock combinations, and these trends have continued through Year 7. In coordinated NC-140 trials with ‘Regina’ in New York, the SSA cumulative yield per acre through Year 7 slightly lagged that of the TSA and UFO (but surpassed the KGB) in Geneva, while SSA trees on Gi3 and Gi5 have had the highest cumulative yields in the Hudson Valley (followed by the TSA and UFO, and the KGB a distant third).

This illustrates the important cherry canopy training system caveat that cultivars and rootstock/site interactions can differ in propensity for forming basal flower buds, lateral shoots, and/or fruiting spurs. SSA will be less successful if too few lateral shoots or basal flowers form, resulting in poor yields, or too many basal flowers form, resulting in extensive weak, blind wood.

Also, SSA pruning is the most labor intensive of the canopy architectures in the NC-140 trial, requiring not only a high number of pruning cuts per tree, but also judgment decisions for differentiating vegetative from reproductive buds. The integration of mechanical hedging strategies into future SSA management may be a requisite step to reduce the current high labor cost.

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