Pruning for high-density systems

Dr. Jim Schupp, Penn State University Fruit Research Center, demonstrates proper pruning methods to growers. PHOTOS BY SALLY COLBY.

Most fruit growers who are planning and planting new orchards are switching to high-density systems to maximize profit and take advantage of technology such as mechanical thinning, platforms and robot-controlled devices.

Dr. Jim Schupp, associate professor of pomology at Penn State University’s Fruit Research and Extension Center in Biglerville, Pa., says that growers should understand the importance of renewal pruning for intensive systems.

“Pruning is defined as the cutting away of a portion of a plant for horticultural purposes,” said Schupp. “Renewal pruning is annual dormant pruning. We’ve always used renewal pruning, but now it plays a more important role. One reason to prune is to modify the canopy shape – it’s getting too wide at the top and should be narrowed, or made shorter to maximize spray coverage and make it easier for workers to access fruit.”

Pruning also renews the bearing surface. “As fruit trees get older, particularly apples and pears, as that wood gets to be 10 to 15 years old, the size and quality of the fruit starts to decline,” said Schupp. “We can renew that canopy by pruning a portion of it away and stimulating the growth of new tissue. The main reason to prune is to improve light penetration in the canopy and improve fruit distribution, and to get better spray distribution.”

Schupp says that in 2005, when new planting systems were becoming more popular, he compiled a list of characteristics of high-density plantings. “An intensive planting uses dwarfing rootstock, which means from 691 trees per acre (4.5 feet by 18 feet) to as many as 1,320 trees per acre (3 feet by 11 feet). It also means using quality nursery stock, supported canopies and single rows of tall (10 feet) narrow canopies.”

Maximizing productivity in single rows requires tall canopies. “Those can be narrow, and that’s what I recommend to get the most bang for the buck,” said Schupp. “We want a canopy shape that follows the natural tree form – we don’t want to fight the tree its entire life. We’re looking for trees that require minimal pruning, especially in the early years.”

One of the most commonly planted new systems is referred to as central leader. Schupp defines this as a single vertical extension of the trunk from which all secondary branches originate. The central leader system is also referred to as vertical axis or spindle. Schupp defines some other basic terminology so that growers can follow his list of eight steps for successful pruning.

New high-density plantings should start with high-quality nursery dwarfing stock that will bear fruit within a few years of establishment.

“A secondary limb is any branch that originates from the central leader,” he said. “The tertiary limb refers to that branch’s point of origin. Thinning cuts refers to removing wood with a cut close to the point of origin. When we make a renewal cut, we’re removing most of the branch, but leaving a short, duck-bill shaped stub to stimulate a new shoot or sprout near the base of the original branch. A renewal cut means a renewed branch that will provide young, healthy bearing wood of correct size relative to central leader.”

Schupp explains that a heading cut is somewhat different. “With this, we remove a portion of the branch,” he said. “In the mature tree, I call it a stubbing cut, which is a heading cut into older wood (two or three-year-old wood), with a wide-angled secondary branch. It has to be a wide-angle branch – I wouldn’t make that cut in a branch that has a vertical angle. What we’re trying to do with that stubbing cut is shorten the branch or make it less pendant (hanging). We might undercut a branch to bring it up to level.”

In creating the permanent framework in high-density plantings, the central leader is the only permanent structure that should remain long-term. “We might keep a secondary branch for multiple years,” said Schupp, “but at some point, it will be cut out. The key to remember is that this system can have only one central leader.”

In a medium-density system (6 feet by 14 feet), the permanent framework includes a central leader along with three to five permanent scaffolds (secondary branches) in the lower canopy. Schupp noted that this system was commonly planted in intensive systems 20 to 30 years ago.

High-density plantings allow growers to take advantage of technology including mechanical thinning, platforms and robotic devices. Shorter trees with fairly uniform branching allow workers to tie branches, prune trees and pick fruit without climbing more than several steps high on a ladder.

Schupp says that those who don’t have a lot of experience in pruning can do a good job by following his list of eight pruning steps, which begin with heading the leader. “Once the leader is above 14 feet, it’s headed,” he said. “Cut back to maintain tree height at about 11 or 12 feet. Schupp added that it’s important to wait for a crop before heading the leader. “When you have a crop at the top of the tree, you’re cutting into bearing surface, and it’s ok to lower tree height.”

Next, maintain the narrow cone shape by thinning long shoots at the top using renewal cuts. Third, remove any secondary limb when the limb diameter becomes half as large as the diameter of the leader. Fourth, remove two to four secondary branches using renewal cuts.

The fifth step involves removing any damaged, diseased or drooping limbs. Sixth, look at the tree and cut otherwise good branches to space them out. “I suggest around 36 branches coming off the central leader,” said Schupp, “but figure that number out based on your tree density per acre and yield expectation.” Schupp says that after doing several trees, it’s no longer necessary to count because it becomes obvious what the tree should look like.

Next, remove vertical shoots – anything less than 40 degrees from vertical. The final step is to prune each remaining secondary branch to a single axis by thinning tertiary branches or by stubbing the limb back to a new axis.

Schupp says that these steps can altered slightly for older trees that were planted about 6 feet apart in the 1980s. “Because we have a wider spacing,” he said, “we have to leave more permanent structure. We have three to five secondary limbs, or scaffolds, in the bottom of the tree. The framework is that of an upside-down umbrella but the same rules apply.”

The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania.