Prior to the annual Mid-Atlantic Fruit and Vegetable Convention, the Young Grower Alliance hosted a pruning workshop at the Penn State Fruit Research and Extension Center in Biglerville, PA. The event featured Dr. Jim Schupp, Penn State professor of pomology.
Growers observed the pruning process in an apple orchard established in 2019 with six-foot Premier Honeycrisp on G-11 rootstock. Trees are trained to what Schupp referred to as his version of a tall spindle: a tree with a tall, narrow conical shape. “Spindle refers to the main trunk that’s a spindle and the appendages on the sides are bearing branches,” he said. “It’s a tall, skinny central leader tree.”
Schupp said height is for productivity and spindle growth encourages trees to maintain a thin, short side structure that allows good sunlight to penetrate thoroughly. “That’s where pruning comes in,” he said. “If we were talking about pruning from a ‘why’ standpoint, the shortest, simplest answer is fruit quality – big, red, sweet apples. We aren’t pruning for yield or longevity of the tree.”
With the goal of pruning to improve fruit quality, the objective is reducing the amount of cropping by removing fruiting branches – spurs and flower buds – from the tree, leaving fewer that have less competition. The fruits borne by the remaining branches can be larger and sweeter.
“We’re opening up and removing branches in such a way that we get sunshine all the way into the center of the canopy,” said Schupp, “to illuminate leaves and buds so we get quality fruit.”
The trees arrived at the orchard feathered with abundant side branches. They were planted three feet apart. “Because they already had plenty of nice side branches, we didn’t have to head the trees,” said Schupp, recounting first year management. “Instead, we headed the feathers. That keeps the side branches small. Let the leader go – this results in a tall spindle tree – a tall leader with short spindles.”
Schupp explained that feathers more than one-third the size of the leader were stubbed all the way back. “Anything less than one-third the size of the trunk we headed back to four inches,” he said. “The idea is to keep side branches weak and the leader strong – fill out the top.”
He added that this can be challenging with size-controlling rootstocks. If too many strong branches are left, it’s difficult to achieve height, which affects the long-term orchard productivity. “We want a 12-foot-tall tree,” said Schupp, “and in order to get that, we have to keep side branches from taking over. It’s a way of manipulating the tree to keep a tall, narrow canopy.”
An important aspect of pruning is cropping. “The sooner you crop a tree, the calmer the tree will be, and the harder it will be to get it to grow tall,” said Schupp. “A heading cut stimulates vegetative growth at the expense of reproductive growth.”
Proper pruning at planting helps prevent cropping the following year, with the possibility of hand-width cuts to prevent cropping in year four.
Blocks are managed for cropping, which Schupp said should be possible in year three. “Getting the tree to finish out with three more feet [in height] will be difficult,” he said. “It’s important to do that early.”
Pruning decisions aren’t always textbook. The pruner might look at a tree and see four pruning options. “The important thing is that you do one of them,” said Schupp. “You can’t leave it all. Sometimes it isn’t important which one is left and which is cut, as long as you leave one to singulate.”
As a guideline, he suggested looking at the diameter of the branch being removed – if it’s more than one-third the diameter of the trunk, it goes. The other guideline is if it’s more than the diameter of an index finger.
Thinning cuts remove a branch at its point of origin to thin the canopy. Heading cuts are used when a portion of a branch different from its point of origin is cut. Leaving that portion results in a local stimulation of growth that fills out the canopy.
In general, the first rule of thumb is to remove the biggest branches. “Two, four or six branches that are too big have to go,” said Schupp. “These systems are designed to grow apples on small branches, and if you do that, you have a nice narrow, compact, well-lit canopy.”
If more than one-third of the canopy needs to be removed to achieve good light penetration, Schupp said the grower should probably change rootstock because it’s producing too much wood.
The second rule is dealing with hangers and risers: “If it’s drooping down, it’s a shade-maker,” said Schupp. “If it’s sticking straight up, it’s too vigorous and is also a shade-maker.”
Rule number three is allowing spacing so there aren’t too many branches, and rule four is singulating remaining branches. Following pruning, about 20% of limbs are removed, which should open the canopy.
Schupp predicted making more renewal cuts on the trees he’s pruning this year since they’re coming into their sixth bearing season and a strong crop is expected. “It’s Honeycrisp so I don’t want to prune too hard,” he said. “There can be problems with bitter pit if the fruit is too big, and it’s a large-fruited variety.”
What about the top, when branches have reached the desired height and the growth pattern looks like a whorl of branches? “How to decide what to leave or take goes back to leader management,” said Schupp. “You’re managing the height of the tree as you maintain the narrow, cone-shaped tree. The danger is having nice branches but some are getting too big. You can end up with an upside-down pyramid shape that shades the lower portion of the tree. If it’s the proper shape, the lower limbs aren’t shaded. When you get to the top, be sure to maintain a cone shape. Branches should flop – remove some of the top to keep it tall and narrow.”
One pruning measure for the future involves knowing the bearing potential of the tree.
“Growers have to know how many fruits they want on a tree,” said Schupp. “You can count buds and get the tree in the right ballpark with pruning. The concept goes back to sizing the bearing surface of the tree so the tree has an appropriate number of buds for the number of fruits to carry to maturity,” he said. “Thinning programs are more efficient if the tree is the correct size from the start.”
by Sally Colby