Apple growers try many ways to coerce their trees to produce more and better fruit earlier, and torture is right there on the list. While there are no reports of waterboarding, growers will score or girdle trunks or cut the roots off recalcitrant trees.
While these are old practices, root pruning seems to be trending. A number of growers in the fruit-growing region near Grand Rapids, Michigan, are using it. Some are dusting off machines they used 25 years ago; others are ordering new ones.
Extreme situations call for extreme stresses. If dwarfing rootstocks and applications of Apogee won’t keep the trees small and productive, chopping off a third of their roots may help.
Phil Brown, at Phil Brown Welding in Conklin, Michigan, built and sold machines back in the early 1990s. Now, he’s building new and bigger ones that make two cuts at once, cutting off roots on trees on both sides of an alley in one pass. His older machines do one side only.
Basic research on the practice was done 30 years ago. Growers essentially follow guidelines developed by Dr. Dave Ferree and Dr. Jim Schupp, both then at Ohio State University, who recommended the practice be done at bloom. There’s a long list of reasons why they do it.
Chuck Rasch grows 220 acres of apples but uses root pruning mostly on one variety, Jonagold. “I didn’t want them to go to processing because they get too big,” he said.
There’s a 5-cent-a-pound price difference between a Jonagold that’s 3.25 inches or smaller, he said, and one that’s 3.5 inches or larger. The smaller apple is ideal for making fresh slices; the larger ones go to less lucrative processing uses. Schupp and Ferree’s studies showed a 20 percent reduction in fruit size.
“I first used root pruning 20 years ago on an IdaRed block on a 15-by-20 (foot) spacing,” he said. “I did it one time and it really shocked them.” By “shocked” he means they quit growing so vegetatively and got down to the business of producing apples.
After that, he didn’t use it again—until three years ago. After using it once, he took his long-idle machine back to Phil Brown and had it expanded to do two rows at a time instead of one.
“The size of Jonagold is a big issue,” Rasch said, and some growers are using it on Honeycrisp for the same reason. Many planted their Honeycrisp on M.26, thinking the scion was a weak-grower and that Malling 9 or Budagovsky 9 would produce too small a tree. You can definitely see the difference in root-pruned trees, he said. “They look paler,” he said.
Chris Kropf grows 133 acres of apples. “We have used it regularly for the last few years,” he said. “Some of our varieties are way too vigorous, and it seems to help reduce bitter pit. Bitter pit is associated with high vigor, and vigor also makes it hard to set fruit.”
Kropf, who is Michigan’s sales representative for Valent USA as well as a grower, says he and many other Fruit Ridge growers have strong soils. “We planted too big a rootstock, M.26, on some virgin soil,” he said. So he root-pruned Gala, Jonagold, Gingergold, and Golden Delicious.
“We definitely set more fruit on the treated trees and reduced bitter pit,” he said. “We got smaller fruit, firmer fruit, and better color.”
A problem for Fruit Ridge area growers is variable soils, with weaker soils on eroded hilltops and silty, high-organic matter soils in the valleys.
Dan Dietrich at Ridgeview Orchards in Conklin talked to growers who visited the family’s farm during RidgeFest in July. Their rolling orchard, operated by Joe, Al, Dan, and Ryan, can become a mixture of large and small, productive and nonproductive, trees. They’ve used trunk scoring—even removing wedges of trunk wood—to control vigor and this year used root pruning extensively.
They try to target the problem areas, but that’s not easy.
Chuck Rasch noted the same problem—uneven soils and uneven trees. But he finds it hard to prune selectively. The arms and the blades must reach under the limbs, close to the trunks, and there’s not much clearance to raise the pruner, which is running 14 inches into the ground.
Tom Rasch, a grower from Belding, east of Grand Rapids, uses root pruning almost every year. “I’ve used it for over 25 years, and I use it every year on all my Macs,” he said.
McIntosh are quite vigorous. “You want a weaker tree or else you get big, soft fruit,” he said. “We want it smaller and firm.”
Paulared is another early, soft variety that was popular on the Ridge 25 years ago and prompted the use of root pruning.
Rasch also uses Apogee (prohexadione calcium) to control shoot growth, and says root pruning is “a last resort for correcting a mistake. We chose too big a rootstock for the soil and the variety, so it was growing and not bearing, and we couldn’t get it to crop.”
Twenty years ago, he said, growers like himself were choosing M.26 and Malling-Merton 106 rootstocks. “We were scared of the 9s,” he said. Trees were also spaced more widely apart.
Besides improving color, root pruning promotes fruit buds for the next year and reduces biennial bearing. “With apples selling for $30 to $60 a bushel, we don’t need a lot going to waste because they are too big or too green,” he said.
“Be sure you prune that tree and thin it well, or you’ll get a bunch of itty-bitty apples,” Tom Rasch cautioned.
Chuck Rasch said the trees need good support and irrigation should be available. A dry season after root pruning can really hurt apple size.
The Phil Brown machine has blades at the end of arms that can move in or out hydraulically. The “rule of thumb” seems to be to set the blades to cut at full depth, 14 inches, at a distance from the tree of about three times the trunk diameter. That would put the blade 18 inches away from a six-inch tree. A skid plate behind the blade closes the slot in the ground.
Brown, who builds the root pruner and has sold units in Michigan and New York, summarized its effects this way:
“The main thing, it controls the size of the tree. It’ll enhance color and make a firmer apple. It also downsizes the fruit a little bit, and some fruit is just too big. It slows tree growth and saves a lot of pruning. On over-vigorous ground, it’s like going to a smaller rootstock.
“If you have a freezeout year and no fruit to hold back the vigor that year, root pruning can help,” he said.
“It’s hard to pull,” Tom Rasch said. “It sometimes takes two tractors.” Fruit growers often don’t have the large tractors field crop growers have, however mainly it is stability more than power that’s needed. The lead tractor serves to keep everything in line.
Rasch’s one-row pruner’s blade runs under the tree, to the right side of the tractor, so the tractor wants to pull right. The two-row pruner runs straighter, but it takes more power.
Chuck Rasch uses a 100-horsepower four-wheel-drive tractor to pull his.
Michigan State University horticulturist Ron Perry used a root pruner last year to try to restrict the size of Montmorency tart cherry trees to make them harvestable with an over-the-row berry harvester. In searching the literature, he noted there was a lot of research and interest in the practice several years ago.
“The practice became popular when apple growers were using semi-dwarfing rootstocks and finding that trees were shading neighboring trees due to tight spacing,” he said. “Studies have demonstrated that restricting root extension and volume by pruning with a sub-soiling knife can reduce canopy volume and vigor of fruit trees by nearly 30 percent.”
Tart cherries are not currently grown on dwarfing rootstocks, so trying root pruning seemed like something worthwhile. A problem is the practice reduces the Montmorency fruit size about 20 percent, just as it does in apples, but that’s undesirable in cherries.
“We are seeing excessive stress for tart cherries when pruning is accomplished in bloom compared to during the pit hardening period,” Perry said. He is currently determining the optimum time to root prune for high-density tart cherries.
Perry noted that interest in root pruning began in the early 1990s, just as growers were making other changes.
“Several Michigan growers purchased an implement and in fact hardly used it as they were making the transition to vertical axe and slender spindle on M.9 stocks,” he said. “Usage went dormant except for guys like Tom Rasch, who loaned me his machine for my cherry work in southwest and northwest Michigan. Tom still has significant acreage on MM.106 in multirow beds, which is why he uses his annually.”
Why the resurgence? “I believe that most of the interest lies in growers, especially on the ridge, transitioning to tall spindle systems at 3-by-10 (foot) spacings and wanting to develop wall systems with summer pruning,” Perry said. “The challenge is that some of the soils are a little fertile and some varieties like Jonagold, Fuji, and Gala can be fairly vigorous and can challenge these spacings.
“Secondly, one of the results of root pruning is a 15 to 20 percent fruit size reduction with increased color. For Honeycrisp, this is a benefit.” •
– by Richard Lehnert