High density is tricky in pears
Controlling tree growth without a dwarfing rootstock is challenging.
OSU horticulturist Dr. Todd Einhorn is experimenting to find out the best angle to train branches of d'Anjou pear trees to discourage suckering and promote early fruiting.
Agricultural economist Clark Seavert devised the Competitive Orchard System several years ago to help pear growers lower costs and improve returns.
Seavert, former superintendent at Oregon State University's Mid-Columbia Agricultural Research and Extension Center, said that as costs increased, growers would have to switch from the standard orchard with three-dimensional trees and plant more efficient and profitable systems.
As an example, the center planted a pear trial in 2006 with trees spaced 4 feet apart and 12 feet between rows (907 trees per acre). The trees are trained on a vertical trellis, with limbs tied flat to the wires, to create a fruiting wall.
Seavert calculated that a competitive pear orchard must produce 15 bins per acre of target fruit in the third year and reach full production of 50 bins per acre by year six. If that is achieved, the establishment costs will be recouped by year five, and the venture will be profitable. However, he foresaw that finding a precocious, dwarfing rootstock for pears would be critical for the complete success of the COS system.
Varieties in the center's trial are Bartlett, Bosc, d'Anjou, Comice, Black's Pride, and an unnamed variety. Seventeen rootstocks were used, none of which are truly dwarfing.
OSU horticulturist Dr. Todd Einhorn, said the planting has been a battle to manage because there is no dwarfing rootstock for pears. The goal was to develop the canopy rapidly by doing minimal pruning during establishment and tying the limbs horizontally to the wires. Limbs would be renewed later in the life of the orchard to maintain production.
On d'Anjou, bending the limbs down flat stimulated prolific growth of upright suckers, and the planting has not yet been productive.
"You're fighting the inherent vigor and horsepower of the rootstocks," Einhorn said. "It takes a lot of management up-front. D'Anjou is not precocious. You get into this battle where you're waiting and waiting and you can't prune. We've ripped shoots off, but any time you do a horticultural technique, you typically stimulate vigor. I think dwarfing rootstocks are entirely necessary for the industry."
Einhorn is working to find potential dwarfing rootstocks for pears. The COS experience also prompted him to begin research to find out if there's a better angle to train the limbs, rather than totally flat, to improve productivity and avoid sucker growth.
This season, he removed all the horizontal laterals on a number of COS Bartlett and d'Anjou trees at the station, leaving a small stub from which a new limb grew. Using bamboo poles, he is training the new limbs at a 30-degree angle on some trees and a 45-degree angle on others, and has kept some trees with the horizontal limbs for comparison. He hopes to learn the ideal branch angle for fruiting and vegetative growth so that training systems can be designed in the future to take advantage of that.
OSU Extension educator Dr. Steve Castagnoli, who is based in Hood River, said the idea behind the COS was that with a moderately high tree density and fairly intensive management, the canopy could be established in two years. Early cropping is critical in a high-density planting because of the high cost of the trees.
The system was designed to have different fruiting levels, along the trellis wires, with spaces in between the layers to avoid shading and allow light into the canopy to stimulate fruiting.
The planting has highlighted the challenges of growing high-density pears without an ideal rootstock or growth regulators to keep the tree in balance, he said.
"Clark's COS concept was not a horticultural model. It was really an economic model, and one of the points of the model was orchards should generate at least a 15 percent return on investment."
Castagnoli said the 15 percent return is arbitrary. What is important is to ensure a positive return on investment by producing target fruit (fruit that generates a return in the marketplace) and lowering costs.
"It's not really about high-density orchards per se," Castagnoli explained to growers during a summer field tour in Hood River. "It's about profitability."
Castagnoli said when Seavert devised the COS system, there were concerns about a potential lack of labor. The system was designed to be adaptable to mechanization and, ultimately, robotics in order to reduce dependence on labor.
The expected labor shortage did not materialize, partly because of the recession and a downturn in the construction industry. However, in the long term, growers still need to think about making their orchards more efficient, Castagnoli said.