Nov 26, 2014
Pruning is more science than art these days

FGN recently asked Jim Schupp, an associate professor of pomology at Penn State University, and Peter Hirst, a professor of horticulture at Purdue University, to answer a few questions via email about winter pruning.

You’ll be holding a pruning research update at the Mid-Atlantic Fruit & Vegetable Convention. Can you talk about some of the topics you’ll cover – especially as it pertains to pruning in winter?

Pruning of tree and vine crops is typically done manually and accounts for the second largest labor costs, after harvesting. Availability of labor is also a major concern. We have been conducting an SCRI-funded project to address these concerns and investigate whether advances in fields such as machine vision and robotics could be applied to developing autonomous pruners for grape and apple. Our team includes participants in the fields of pomology, viticulture, engineering, economics and rural sociology. This multi-disciplinary team is focused on developing new technology, adapting and evaluating that technology for the purpose of automating pruning, and determining the barriers to adoption. Previous work by our commercial partner, Vision Robotics, has developed an autonomous pruner for grapevines that is currently being refined. It is undergoing field testing and should be commercially available by the end of the four-year project.

The pomologists on our team have formulated a set of “rules” that describe optimal pruning of modern intensive apple orchards. We are currently evaluating those rules in terms of the physical attributes of the canopy structure. The engineering team is developing a 3-D imaging decision system, and robot control technologies for automating dormant pruning operations. The socio-economic team will determine social and economic impacts of the proposed autonomous pruning system.

The pruning rules will be useful in the near term for training workers to prune trees in intensive orchards. Since the rules are simple, they are easily learned and this will enable the workers to work efficiently without uncertainty. A simplified set of rules will also enable orchard managers to objectively evaluate the quality of the work, and to make adjustments to the pruning their crew members are doing in simple, measurable terms. Ultimately, the rules will be used in development of 3-D imaging, decision system and robot control technologies for automating dormant pruning operations in order to construct autonomous pruning systems. The goal of these studies is to evaluate the effects of pruning severity and pruning rule complexity on growth, light distribution and fruiting of apple trees.

Currently in Pennsylvania, we are evaluating the effects of pruning severity on growth, fruit set, canopy light distribution, crop load, yield, fruit size distribution and fruit quality of Buckeye Gala/M.9 apple trees. Another set of experiments are being conducted to evaluate the effects of four levels of pruning rule complexity on growth, fruit set, canopy light distribution, crop load, yield fruit size distribution, fruit maturity and fruit quality of Fuji/M.9 and Golden Delicious/Bud.9 apple trees.

Additional experiments are being conducted in orchards in Indiana and Pennsylvania to compare the outcomes of pruning “by the rules” to standard commercial pruning. Initial experimental results, in addition to grower observations, indicate that pruning by the rules has similar effects to pruning by commercial orchard crews.

Will there be any new tips or techniques growers haven’t heard before, or will it be more of an update or refinement of things they already know?

The principles of fruit tree pruning are well established, so growers will be familiar with the concepts. I think the biggest surprise for many growers will be how much the pruning of fruit trees can be simplified. Our research suggests that pruning can be refined to a few simple steps for modern high-density orchards, because the tree architecture is simpler. When we set about to translate pruning principles into “machine language” so that our engineering colleagues could apply them in robotic pruning, we were surprised that we were able to narrow it down to four orders of pruning rules, which, when performed in prioritized order, should accomplish about 90 percent of the trimming that an apple orchard requires.

Will your research findings be relevant to growers in other climates – out West, for example?

Yes, any apple training system that develops a narrow cone or tree wall canopy shape will readily adapt to these principles. Whole chapters of books and even whole books have been written on the subject of pruning. Our initial results suggest that pruning can be simplified without any compromise in terms of productivity or fruit quality.

In the large trees of orchards of 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 principles.

An expert fruit hobbyist may still wish to practice the arts of Bonsai and topiary, but our results to date show that pruning can stand exclusively as a science in today’s commercial orchards. This is due to the advent of smaller, simpler canopies that are created and maintained with renewal pruning, which results in minimal permanent branching structure.

When it comes to winter pruning, what are the most important things a grower needs to remember?

It is critical to keep the goals of pruning clearly in mind. The primary goals of pruning in a perennial fruit cropping system are to: maintain even light distribution throughout the narrow tree canopy; facilitate movement of air and crop protectant sprays within the canopy; balance vegetative and reproductive growth through branch renewal.

To achieve these goals, we propose that there are four prioritized orders of pruning rules that should be followed:

Pruning based on branch size: Large-size primary branches are removed, leaving a short, “duck bill” shaped stub at the base to stimulate renewal growth. We will be presenting research to establish the threshold for branch size at the Mid-Atlantic Fruit and Vegetable Convention in January.

Pruning based on branch number per unit of trunk area (branch density): Leaving an appropriate number of smaller side branches growing from a single vertical trunk and spaced out at an appropriate branch density. These first two orders of pruning rules accomplish most of the beneficial effects on sunlight distribution and adjusting the balance between crop and vegetative growth.

Pruning based on branch angle: Branches that have an upright angle (within 40 degrees of vertical) are too strong; and those branches that are too pendant (over 140 degrees from vertical) are too weak. Branches less than 40 or greater than 140 degrees are pruned off, retaining more horizontal branches that will possess a better balance between growth and fruitfulness.

Pruning based on secondary branch complexity: Each side branch is thinned to maintain a single axis with minimal branch complexity.

The branches that develop from the renewal stubs will eventually replace the remaining 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.

Matt Milkovich

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