Why You Should Consider Whole Tree Renewal Pruning for Cherries
My last two articles have discussed the importance of renewal pruning to sustain the optimal “sweet spot” balance between good yields and high-quality fruit. This continuous recycling of new, temporary fruiting growth can promote productive and profitable orchards for as long as the tree’s fundamental, permanent structure stays healthy.
Renewal pruning usually is imposed selectively to remove a small portion of secondary (non-permanent), structural fruit-bearing growth so that the regrowth creates new, young, strong fruit-bearing sites.
In many high-density orchards, this would remove one to three of the largest branches, mini-leaders, or upright offshoots on a spindle (TSA), multiple leader bush (KGB), or multiple leader planar (UFO) tree. Therefore, this percentage of the canopy that is removed to be re-grown tends to be around one out of every eight or so major fruit-bearing structures, or 12.5% more or less, depending on the orchard, orchard manager, and canopy architecture.
A Radical Concept
Now, let’s consider that concept of annual renewal from a somewhat radical point of view: Whole-Tree Renewal (WTR), that is, the removal of every major fruit-bearing portion of the canopy such that the entire secondary structure of the canopy is regrown. In sweet cherries, this idea germinated in 2013 when Washington State University (WSU) Professor Matthew Whiting and I visited the sweet cherry germplasm collection of a breeding program in Spain.
There, decades-old trees on seedling rootstocks had been chain-sawed back to stumps and were vigorously regrowing, giving the impression of a KGB orchard in its development years (but taller and with massive trunks). In 2014, Matt imposed WTR on some UFO trees at WSU’s Roza research orchards, and in 2015, I imposed WTR on some very small, very productive UFO-trained ‘Rainier’/Gi.3 trees that had been suffering from poor selective renewal due to the dwarfing rootstock and competitive crop load.
Both preliminary trials gave remarkable results. The well-established root systems of the trees, and the removal of the competitive crop load, promoted extensive and vigorous replacement of the canopy. In the ‘Rainier’/Gi.3 trial, timing of WTR was examined, from dormant to green-tip, to full bloom, to petal fall. All timings resulted in full canopy renewal, although the greatest number of new shoots resulted from imposing WTR at bloom.
The regrowth of WTR trees resulted in a relatively uniform canopy (compared to that of standard selectively-renewed trees, which have branches of many different ages), since WTR synchronizes the age of the regrowth, though some new shoots are more vigorous than others. This led to expanded studies in the coordinated NC-140 cherry training systems/rootstocks trial across North America. Excellent canopy replacement occurred across training systems and rootstocks of varying vigor.
The rapid canopy replacement for the KGB and UFO canopies (see Figures 1 and 2) was expected, since WTR removes apical dominance, and the fruiting canopy regrows vertically from a relatively low trunk or cordon. However, even regrowth of the TSA canopies was relatively uniform and complete along the central leader, although apical dominance became re-established the following year.
Nevertheless, WTR re-invigorated lower portions of the central leader canopy, which can be difficult to achieve in selectively renewed trees, though growth in the upper canopy reverted to its previous vigor.
Nuts and Bolts of WTR
So, the next question is how often might WTR be imposed to synchronize renewal growth? Considering the relatively standard 12.5% renewal rate across entire orchards, one can envision imposing WTR annually to one tree in the row, skipping 7 trees, and repeating, thereby achieving a 12.5% renewal of fruiting wood across the orchard. Even better, an entire row of trees can be subjected to WTR, the next seven rows should be skipped that year, and, thus, is repeated.
Again, 12.5% of the orchard is being renewed annually, but the renewed rows can be managed differently from the rest of the orchard, improving management efficiency for spraying, harvesting, etc. while improving individual tree uniformity. Each renewed row would require two years for spurs to form, then would be fruited for six years before again being renewed (an eight-year rotation).
Research in the NC-140 trial has shown that WTR-pruned canopies in high-density orchard systems can refill their orchard space within two years and, depending on cultivar productivity, return to “full” yields in the third year. In British Columbia, self-fertile ‘Skeena’ yields in the third season after WTR equaled or exceeded those of selectively renewed trees, while in Michigan, modest-bearing ‘Benton’ yields fell just below those of selectively renewed trees in the third season after WTR.
Too Much Vigor?
An unexpected consequence of WTR was that some of the regrowth was so vigorous that sylleptic (secondary) shoots formed on the primary shoots during the year of renewal. This can be an advantage for the branching canopy of TSA trees but is a disadvantage for the development of spur-bearing, narrow upright leaders in KGB and UFO canopies. Not only must labor be expended to remove sylleptic shoots in these training systems, but their formation precludes development of a fruiting spur at the node where the sylleptic arose.
Consequently, future research must determine how to modulate the vigor of the WTR response, perhaps by using specifically-timed growth inhibitors like prohexadione-Ca (e.g., Apogee, BASF) or imposing a late spring heading cut to the more vigorous WTR shoots to diffuse their vigor into two to three new primary shoots (which can be thinned out as necessary during the dormant season).
Also, yet to be determined is the potential bacterial canker infection risk of large WTR cuts imposed at budbreak vs. perhaps in late summer when warmer temperatures and drier conditions reduce susceptibility to canker.