MSU Extension

Any tree fruit training system will provide a framework with the potential to produce large quantities of high-quality fruit. However, many traditional training systems create complex canopies, take years to fully develop, lack a systematic plan to renew fruiting wood, and are difficult to clearly and specifically explain. Due to the rising cost of production, an increasingly limited skilled labor pool, and escalating competition on the world market, modern training systems must offer growers results such as these:

• Precocity or early fruiting for high early yields and a more rapid return on investment
• Production on well-exposed wood of moderate vigor to provide high-quality fruit
• A systematic process to renew fruiting wood
• Repeated canopy units for simplicity in training and pruning

Cherry growers around the world use many training systems, both supported and freestanding. This publication describes eight commercially successful systems: Kym Green Bush (KGB), Spanish Bush (SB), Steep Leader (SL), Super Slender Axe (SSA), Tall Spindle Axe (TSA), Upright Fruiting Offshoots (UFO), Upright Fruiting Offshoots “Y” Trellis (UFO-Y) and Vogel Central Leader (VCL), all of which can be achieved with a whip (vs. feathered) nursery tree. Each system has benefits and limitations. Choosing the right system depends on growing conditions, variety, rootstock, labor availability, and management skills. This training manual will provide readers with the information needed to choose a training system for new plantings. You will learn how to implement the chosen system to meet your production goals.

Understanding the Natural Tree

Undisturbed in its native environment, the young sweet cherry (Prunus avium) will grow as a central leader tree. Growth is rapid and apical dominance is strong. Thus, the tree tends to branch only just below the terminal bud of annual growth, can grow to 60 feet, and is non-precocious so it can establish a competitive “footprint” in the forest before shifting resources to fruiting. All growers face the challenge of managing sweet cherry so that the tree remains small, generates branches at the desired locations, and begins producing fruit by the second or third leaf.

• Excessively vigorous growth – Cherry trees are inherently large and vigorous. Without intervention when young, they produce long shoots with few lateral branches or fruiting spurs. This trait makes orchard canopy development challenging and limits fruit production.
• Narrow crotch angles – Cherry trees tend to produce branches with narrow crotch angles. Such angles often are weak and prone to bark inclusion, a condition in which bark is trapped between the trunk and the branch, preventing layers of annual wood from growing together. Splitting and/or bacterial canker infections (caused by the freezing of water trapped in a narrow crotch) can occur at these locations. Narrow crotch angles are associated with upright vigorous growth that competes with the leader and is less productive than lateral growth of moderate vigor and wider crotch angles.
• Delayed fruiting – On their own roots, cherry trees can take 5 to 6 years or more to produce fruit. Pruning can modify tree vigor and increase branch production. However, pruning—and especially dormant heading cuts—tends to delay fruit production in young trees by stimulating vegetative growth. Other factors, such as deep, productive soils or over-fertilization, increase tree vigor and delay the onset of floral initiation.

Dwarfing and precocious rootstocks have made it possible to harvest a crop in the second or third leaf, more easily control tree size, and encourage wider branch angles at desired locations. Proper management and training of trees on dwarfing rootstocks controls fruit set, balances leaf area with crop load, and promotes larger fruit.

Annual shoot elongation is imperative for maximum fruit quality. The largest and highest quality cherries are produced at the base of the previous season’s growth and on young spurs. The orchard manager’s goal should be to produce an abundance of new shoots about 24 inches in length in targeted zones throughout the tree, depending on the training system, while removing old spurs.

Training System Options

Consider carefully before selecting a training system. Make the right choice for your orchard by understanding the intensity of required management and how training factors interact to promote flower bud initiation, vegetative growth, and renewal wood when and where it is advantageous. Realize that the recommended tree and row spacings, target shoot/ branch/scaffold numbers, etc. in this guide usually are ranges that will vary according to the specific combination of factors associated with each orchard site, such as soil fertility, terrain, growing season length and climate, rootstock vigor, cultivar growth habit, orchard equipment size, and management skill.

Kym Green Bush (KGB)

The Kym Green Bush (KGB) is a free-standing tree that consists of multiple temporary vertical fruiting units to moderate tree vigor, enabling the system to be grown with full-size or semi-dwarfing rootstocks. The vertical fruiting units are renewed regularly to keep spurs young and productive. The KGB is the only system covered in this manual that creates a fully pedestrian orchard—one that can be harvested without ladders or platforms. Tree formation is easy and requires minimal labor; mature pruning is simple and follows a repeatable plan.

Spanish Bush (SB)

The Spanish Bush (SB) is a free-standing tree that consists of multiple permanent leaders to moderate tree vigor, enabling the system to be grown with fullsize or semi-dwarfing rootstocks. Lateral fruiting units are developed and renewed on the leaders. The SB produces a semi-pedestrian orchard that can be harvested from small ladders. Tree formation is easy and requires minimal labor; mature pruning consists of renewing horizontal fruiting wood on the permanent vertical scaffolds.

Steep Leader (SL)

The Steep Leader (SL) can best be described as a free-standing tri- or quad-axe multiple leader tree with horizontal scaffold branches projecting from the base of each leader. This canopy architecture creates a pyramidal shape that facilitates good light distribution throughout the tree. Due to the limited number of leaders, SL trees on full-size rootstocks can grow from 18 to 20 feet or more. Fruiting wood is developed on the permanent vertical and horizontal scaffolds, and each scaffold is treated as an individual one-sided spindle tree. Mature pruning consists of regular renewal of a portion of all fruiting wood.

Super Slender Axe (SSA)

The Super Slender Axe (SSA) is a very high density, semi-pedestrian system of up to 2,000 semifree-standing single leader trees per acre, usually requiring dwarfing rootstocks and a top-wire trellis. The production habit of SSA trees is significantly different from other systems, with the majority of the fruit grown on axillary flower buds produced at the base of 1-year-old branches. This aspect, combined with the fact that fruiting is located in the proximity of the axis, has a positive effect on fruit size. Mature pruning consists of annual renewal of all lateral branches on the axis, thereby requiring simplified but extensive annual labor.

Tall Spindle Axe (TSA)

The Tall Spindle Axe (TSA) is an evolution of the Vogel Central Leader (see below) and Zahn Spindle systems, incorporating new concepts to these earlier free-standing, single leader systems. The TSA tree canopy is characterized by a continuous whorl of moderately vigorous lateral branches developed by activation of selected buds (via bud scoring, bud removal, or growth regulator application), usually on semi-dwarfing rootstocks for a semi-pedestrian orchard. The TSA is further differentiated by two key concepts: 1) annual heading of lateral shoots to balance leaf-to-fruit ratios and future crop loads, and 2) annual renewal of the oldest fruiting branches so that the only permanent structure is the central leader.

Upright Fruiting Offshoots (UFO) and “Y” Trellis (UFO-Y)

The Upright Fruiting Offshoots (UFO) is a fullytrellised system that optimizes labor efficiency and fruit quality by creating a narrow fruiting wall that is precocious and easy to harvest and prune. Like the KGB system, it produces fruit on renewable vertical fruiting units, which arise from a low horizontal trunk or cordon. Mature pruning is reduced to a simple two-step process. The advantages of this semi-pedestrian system are high early and mature yields, and good air movement and light distribution to reduce disease incidence and promote uniform, high-quality fruit throughout the canopy. Although the system is easy to prune and maintain at maturity, establishment of the trellised canopy uprights is more intensive, time-consuming, and costly than the other systems.

The UFO can be trained to a dual-plane “Y” canopy architecture (UFO-Y) that is developed and managed similarly to the single-plane UFO vertical trellis; however, the renewable fruiting units are trained alternately to grow about 30 degrees off vertical, forming both sides of a “Y” trellis. The UFO-Y offers the advantage of higher yields due to greater light interception, as well as the potential for mechanical or mechanical-assist harvesting.

Vogel Central Leader (VCL)

The Vogel Central Leader (VCL) is a freestanding, single leader tree with renewable fruiting scaffolds that promotes early yields in moderately high-density orchards. The VCL fosters precocity by minimal early pruning and establishing flat branch angles, thereby requiring relatively high labor inputs in the establishment years, but low maintenance levels at maturity. The pyramidal tree shape promotes good light distribution throughout the canopy. Due to the single leader nature of this system, a dwarfing rootstock is necessary to help maintain tree height at 10 to 12 feet at maturity, creating a semi-pedestrian orchard.

Rootstock Options

Just as training systems modify the natural growth habit of the tree, the choice of rootstock impacts tree growth and response to training system techniques. Therefore, training system and rootstock must be considered together, along with vigor of the orchard site (i.e., soil quality and growing climate); some training systems are more or less suitable for trees that exhibit greater or reduced vigor, precocious or delayed cropping, etc. For this guide, rootstocks are categorized into four levels of growth promotion:

Vigorous: e.g., Mazzard, Mahaleb, Colt
Semi-vigorous: e.g., Gisela®6, Gisela®12, Krymsk®5, MaxMa®14, CAB6P
Semi-dwarfing: e.g., Gisela®5, Krymsk®6
Dwarfing: e.g., Gisela®3

Pruning and Training Techniques

The development of these canopy systems uses the following pruning and training techniques.

Thinning cut

A thinning cut removes a branch at its point of origin or back to a lateral branch that is large enough to assume the terminal function. The thinning cut often is used to reduce the height of a tree or to reduce the size of a branch system that is shading a lower branch. Thinning cuts permit better light penetration and do not stimulate as much re-growth as heading cuts, thereby reducing the risk of delayed fruiting in young trees.

Heading cut into 1-year-old wood

This cut stimulates the growth of lateral branches and often is used in the early developmental stages of cherry training systems to force branching. Since heading into young wood invigorates the area around the cut, this type of cut tends to delay fruiting on young trees that have not yet flowered. On bearing trees, heading cuts into 1-year-old wood (often called tipping) are used to reduce the subsequent season’s crop on highly productive varieties or rootstocks. Tipping removes future flowering sites before they develop on 2-year-old wood and helps maintain a balanced leaf-to-fruit ratio.

Figure 1. The stub cut or renewal cut

Stub or renewal cut

A stub cut is used to renew fruiting wood to keep it young and productive. The term "stub cut" usually describes a heading cut that removes all but a few inches of growth on a branch, although it is possible for a stub of 1 foot or longer to be left. If the wood is relatively young, visible buds below the cut may be stimulated to grow and replace the older branch. When older wood is stubbed back, visible buds may no longer be present. In this case, adventitious buds located beneath the bark may begin to grow and develop into a replacement branch.

Another form of a stub cut can be made when a branch is too upright or is simply becoming too mature. If a lateral branch is present, head the primary branch so that the cut is slightly below the terminal point of the secondary lateral (Figure 1). It is important that the secondary branch terminal (a) be higher than the remaining stub (b). This discourages vigorous vertical wood from growing out of the stub, and it keeps the secondary branch more horizontal and less vigorous.

Making stub cuts late in the dormant season or at bloom will provide a higher percentage of regrowth than during other seasons.

Sectorial double pruning cut

A sectorial double pruning cut is made during the dormant season at an upright-oriented (top) bud. As the buds break and begin to grow, the top-oriented bud will be very vigorous and grow vertically. One or two subtending buds, located on the sides or bottom of the cut branch, also will break and grow, though more slowly than the primary bud and thus more horizontally. The vertical branch is then removed later that season, or in some cases, the following dormant season, leaving the less vigorous and horizontal branch to produce fruit.

Limb manipulation and spreading

Spreading or manipulating tree limbs improves light penetration, reduces branch growth, encourages precocity, and avoids bark inclusion and subsequent weak limb attachments.

Branch angles are best manipulated while the tissue is still green but after new shoots have grown to 3 or 4 inches in length. To establish wide crotch angles, place a toothpick between the trunk and a young, 3- to 4-inch shoot growing from the trunk.

In moist climates where toothpicks may increase the risk of bacterial canker infection, a clamp-type clothespin can be used as a spreader. Attach the clamp to the trunk just above the developing branch and force the shoot to a 90-degree angle.

One of the easiest ways to spread more mature branches is to use 1 in. x 2 ft. spreader sticks with notches cut into the ends. These force a branch to grow more horizontally, although they may blow out in windy conditions. A more stable method is to tie branches to the ground using twine and a “W”-shaped clip (used in hop production) pressed into the soil.

Summer pruning

Summer pruning reduces tree vigor and the potential for pruning cut disease infection. It is commonly used with the Spanish Bush and KGB systems to quickly establish a target number of multiple leaders for the tree framework. Summer pruning done by mid- to late-June (prior to the summer solstice) allows time for regrowth. Pruning later in mid-summer can result in narrow crotch angles and weak growth that may not cold-acclimate well in the fall. By late summer and early fall, the chance of regrowth from summer pruning is much reduced.

Bud selection and activation to create lateral or upright shoots

The selection and activation of buds to form shoots on a whip nursery tree (or on previous season leader growth) can be key for early formation of canopy structure in some training systems. This increased precision in promoting branching can be accomplished in a number of ways:

1. Application of a cytokinin+gibberellin-based plant growth regulator, e.g., Promalin®, to selected buds at the green tip stage of bud swell. Note that this bud activation gives the best results when temperatures after application are relatively warm; extended cool temperatures may result in a lack of activity and poor shoot outgrowth.
2. Scoring (cutting through the bark and green cambial layer with a moderately coarse saw blade about 3/32-inch wide) above the selected buds where shoot formation is desired. This bud activation has a bit longer window, from initial bud swell through budbreak, than the use of growth regulators, and it is not sensitive to temperature.
3. As buds are selected by position for future shoot formation, removal of the intervening buds between those selected will promote activation of the selected buds by removing competition for stored energy and resources. This bud activation has the longest window, from the dormant bud stage to post-budbreak (though buds are easiest to remove when swelling has begun), and it is not sensitive to temperature.

Note that scoring or, to a lesser extent, bud removal may increase the risk of bacterial canker infection; if practiced, treatment should be timed to forecasts of several days of dry weather, and pre-and post-wounding application of antibiotics like copper may be valuable to reduce bacteria populations. Also, bud activation techniques tend to be more successful on newly-planted trees when transplant stress is minimized and on trees with more-established root systems (such as fall-planted trees or second-year trees that failed to form the desired shoots in the year of planting).

Sweet cherry growth and fruiting habit

To be successful with any training system for sweet cherry, it is helpful to understand the inherent growth and fruiting pattern of the tree. In nature, sweet cherry is a forest tree, genetically programmed to grow a tall trunk and annually form a tier of strongly upright, vigorous branches positioned where the previous season’s growth terminated. In the forest, this is essential to successfully compete for light with other forest trees before eventually shifting into fruiting mode. In the orchard, we generally want short trees with branches of weak-to-moderate vigor that are well-distributed along the trunk, rather than in vigorous annual tiers separated by gaps that lack branching. In a commercial orchard, one prunes annually, from Year 1, to modify tree structure and improve light interception so the tree doesn’t need to compete with other trees.

Training systems were created to manage tree vigor and fruitfulness, as well as increase orchard operational efficiencies, in various strategic ways. Vigorous tree growth takes longer to begin forming flower buds and spurs. Except for developing the essential primary structure of the tree, strongly vigorous growth is not conducive to precocious, productive fruit yields. Therefore, orchardists moderate growth and increase earlier and higher levels of fruiting by the use of precocious or dwarfing rootstocks; shoot and limb bending, tying, or positioning to a less upright orientation; and/or providing moderate levels of water and nutrients (primarily nitrogen) during the growth cycle, especially postharvest.

The basic fruiting unit of a sweet cherry tree on precocious rootstocks, or managed for weak-to-moderate vigor, takes three seasons to form a complete complement of fruiting components, which include three types of leaves and two types of fruiting sites. A shoot that forms in Year 1 has large single leaves at each node (Figure 2). In Year 2, those nodes will become non-fruiting spurs with six to eight leaves each, except for the most basal nodes on the shoot which can have solitary flower buds (with no vegetative bud) (Figure 3). So, some basal (non-spur) flowering may occur in Year 2; these nodes become “blind” after flowering and fruiting since there are no vegetative buds present. During the Year 2 growing season, flower buds begin forming in leaf axils on the non-fruiting spurs. Thus, in Year 3, each node now has the potential to be a fruiting spur, with 6 to 8 leaves and one to 10 flower buds, depending on variety, vigor, and spur location (Figure 4). Spurs near the shoot terminus tend to have more flower buds per spur than more basal spurs. These spurs may remain fruitful for many years, if managed correctly for light and nutrition. However, the best quality fruit usually is borne on younger spurs and the basal (non-spur) flowers within a fruiting unit. Thus, many training systems are designed to manage and renew these fruiting units for optimal fruit quality.

Figure 2. Year 1—new shoot growth with single leaves at each node.

Figure 3. Year 2—first season growth forms non-fruiting spurs, with greater spur density in the terminal portion and a few basal non-spur fruit buds.

Figure 4. Year 3—first season growth forms fruiting spurs, with more flower buds per spur (and greater spur density) in the terminal portion.

Table 1. Appropriate Plant Material Considerations and Uses of Various Training Systems