Perry, R. 1998. Suggested training strategies for dwarf sweet cherries. Great Lakes Fruit Growers News. Vol. 37(2): 45-46.
Suggested Training Strategies for Dwarf Sweet Cherry
Dr. Ron Perry
Department. Of Horticulture
Michigan State University
East Lansing, MI 48824

The 1998 MSU Fruit School was successful in getting many of us researchers, extension agents and growers together to get updated on sweet and tart cherries. There is great excitement, world wide, for the new dwarfing rootstocks from Europe, such as Gisela 5, 6 and 12, Edabriz and the Weiroot series. Unfortunately, we still have limited experience in North America with these rootstocks. Growers have already planted many trees on these rootstocks, with little guidance as to how to space, train and prune them. The school gave us the opportunity to learn how and to develop some suggested strategies until we are further along in the learning-curve on how to train sweet cherries on the new dwarfing rootstocks for high density plantings. Fortunately, Mr. Martin Balmer, from the Rheinland / Phalz Research Station in Germany, reviewed with us the techniques that are getting wide usage in the different cherry growing areas in Europe. In the north, near Hamburg, Mr. Fritz Zahn, has developed a unique system that is specifically designed for growing cherries under their cool humid climate. The" Zahn" system was developed to help growers contend with the bacterial canker problems that are so prevalent there. The system is fairly complex and calls for heading cuts made on branches, especially those more than one-half the diameter of the leader. This practice leaves a stub anywhere from 6 inches to as long as 12 inches long. The recommended length of the stub is dependent on a mathematical formula based on branch diameter to leader diameter ratio. The consensus, by many, is that the system is fairly complex and needs to be fully understood before application. The other more popular system being used in Europe for training and pruning sweet cherries on dwarfing rootstocks is known as the "Spindle". There are different versions of the Spindle, as I learned this past summer in Europe, but most are similar to the Slender Spindle, as applied to apples. One version of the Spindle is being promoted in Germany by Mr. Tobias Vogel in Franconia. Generally, this version differs little from the Slender Spindle system used in apples. At this time, I am encouraging growers when planting trees on dwarfing rootstocks to follow the Spindle protocol. The primary reason is that the system is relatively simple and because many of our sweet cherry growers also have developed experience and expertise in the apple Slender Spindle system. Mr. Vogel recommends the application of two techniques for sweet cherry not here-to-fore used in the Slender Spindle apple system. They are; de-budding of upper-most buds in late winter, prior to bloom and the use of stubbing of excessively vigorous branches (Zahn system) in the Lower Table Zone. De-budding is beginning to get some attention in application on apples and other crops in Europe. The strategy in the strongly apically dominant sweet cherry, is that the upper-most 4-6 buds on the leader possess excessive amounts of the growth hormone, Auxin, which prevents laterals lower in the tree hierarchy from breaking. In apples, we normally suppress the branches that develop from these buds by pinching soon after growth starts in the spring and once they approach 2-3 inches in length. Instead, this technique with sweet cherries calls for their removal before flowering. We have not tried this technique out to determine its value in our high density fruit crop systems yet, but hope to this year.

General Training Strategy

The general strategy in training trees for high density systems does not differ. As in the apple Slender Spindle and Vertical Axe systems, the grower needs to divide the tree into two zones, the Lower Table of Production Zone (Lower Table) and the Upper Fruiting Zone (Upper Zone)( Fig 1).

Lower Table

The Lower Table Zone constitutes branches which develop a permanent structure in the first 2 ft to 4.5 ft of canopy. Here, the 7-8 branches are spread to slightly above horizontal (never below, unless trees are spaced less than 3 ft. apart as in apple). After developmental years, the branches are pruned by making thinning cuts to avoid encroachment in the alleyway and with adjacent trees.

Upper Zone

Treatment of the leader and branches that develop above the 4.5 to 5 ft. canopy height area differs according to tree vigor, expected tree height, variety, spacing, and crop. In the case of the apple, growers have 4 options in handling the leader. It can be tied up to the vertical for tall Axe trees, snaked for shorter statured trees and the leader replaced after 2-3 years for shorter trees and in limited cases, headed to cause branching in hard to branch and weak varieties. In the apically dominant cherry, the leader should be headed each winter for the first 3 winters by 50 %. The first 6 buds (below the first allowed to force and become the new leader) are removed prior to bloom (just below the heading cut)( Fig. 2).

Treatment of laterals in the upper zone for apples depends on the height of the tree. For short trees, the grower must strive to gain weakness and productivity as soon as possible in this zone. The most effective technique to accomplish this goal is to bend or weight new developing branches below the horizontal after branches are 10 inches long and are still succulent (late June - early July in Michigan). Laterals in the taller Vertical Axe system require little attention. For cherries, the recommendation is to clothes-pin branches (to form a horizontal angle perpendicular to the leader) when laterals are 4 inches in length (same length as the clothes pin) in May (Fig. 3). Once branches are 10-12 inches in length and are still succulent (before lignin or woodiness sets in), tie or weight laterals down below the horizontal using clothes pins, elastics or cotton string. This practice, much like treatment of apples in the shorter Slender Spindle, is continued for the first 3 - 4 years (Fig. 4).

After the first 4 years, trees are pruned to maintain canopy vigor and productivity.

Spacing and Support

The dwarfing rootstocks for sweet cherry are more vigorous than the equivalent apple dwarfing rootstocks, such as the M.9 clones. Depending on site, soil and variety combination, most European orchardists are planting trees about 8 feet by 15 feet on Gisela 5 and 9-10 feet by 16-17 feet on Gisela 6. This spacing will apply to orchards in eastern North America. Gi. 12 is too new to know how far apart to plant but is expected to be slightly more vigorous. Our experience in the NC-140 1987 cherry rootstock trial indicated that tree vigor was highly variable according to site and variety. For example, trees on Gi. 6 were similar to mazzard in Washington and 38 % of mazzard in our sandy soil of Northern Michigan. Secondly, it depends on level of cropping in the initial years. Unfortunately, we still are low on the learning curve regarding spacing on these rootstocks. Expect for canopy vigor in supported sweet cherry trees to be greater by 20-30 % than non-supported. Since the dwarfing stocks appear to be well anchored, support for these trees for improved anchorage is not a concern. However, the precocious nature of these rootstocks does not rule out the need for support of the leader to keep the leader erect and to keep it vertical and maintain good canopy volume.

 

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