Renewal cuts; L: showing branch renewal after 1 year; R: newly cut.
The primary goals of pruning in a high density orchard system are to maintain even light distribution throughout the narrow tree canopy, to facilitate movement of air and crop protectant sprays within the canopy, and to balance vegetative and reproductive growth. These goals are met by thinning out large-size primary branches so as to maintain the thin spindle canopy shape, leaving an appropriate number of smaller side branches growing from a single vertical trunk, and spaced out at an appropriate branch density. Branches that have an upright angle (under 40 degrees of vertical), are too strong; and those branches that are too pendant (over 140 degrees from vertical) are too weak. Branches <40 or >140 degrees are pruned off, retaining more horizontal branches that will possess a better balance between growth and fruitfulness. Finally, each side branch is thinned to maintain a single axis with minimal branch complexity. Thus, we propose that there are four prioritized categories or "orders" of pruning rules that should be followed:
Excess side branches are removed, leaving a short "duck bill" shaped stub at the base to stimulate renewal growth. These renewal branches will develop over time, and will eventually replace the current side branches when these have grown too large and must be pruned off. Renewal pruning is key to obtaining large attractive fruits growing on healthy spurs in a favorable light environment. Apple bears fruit from mixed buds and on different ages of wood, with the primary bearing surface comprised of spurs on wood that is two years old or older. In the past, the volume and branching complexity of the apple tree canopy made it extremely difficult to create a simple, predictable and repeatable set of rules for pruning. Horticulturists often cited the "art and science of pruning." With the advent of uniform narrow canopy training systems on dwarfed trees with a simple branching structure that is enforced by renewal pruning, the "artistry" can be phased out in favor of pruning to a few scientifically sound principals. A prioritized set of apple pruning rules based upon these principals follows.