Take The Stress Out Of Thinning Apple Trees

Cornell’s Young Apple Thinning Gauge helps  growers balance crop load and vegetative growth. See how it works.

Cornell’s Young Apple Thinning Gauge helps
growers balance crop load and vegetative growth. See how it works.

Thinning is perhaps the most nerve-wracking task in tree fruit production. For the grower, chemical thinning can be a tremendously emotional activity.

“From the consultant’s perspective it’s just science, but for growers, your head tells you one thing and your heart tells you something else,” says Valent U.S.A. Corp. National Crop Specialist Byron Phillips. “And it can be difficult not to follow your heart.”


Fortunately, we have methods to help get us over that hurdle — models and other tools that take the emotion out of decision making and reduce the process purely (or as purely as possible) to math and science.

“Cropload management is still an art in many ways, but these new tools are helping us improve those processes and make more science-based decisions,” says Kevin Forney, Technical Services Manager with Fine Americas.

“We’re still in the early adoption phase, but these models have been the single biggest advancement in crop load management,” Phillips says.

Models And Methods

Here are five tools you should  know for your cropload management program.

1. The Young Apple Thinning Gauge, developed by Terrance Robinson at Cornell, is designed for young trees and helps balance crop load vs.vegetative growth so you don’t overcrop the tree and stunt it out or undercrop the tree and make it overly vigorous, Phillips says.

The tool, which looks a little like a ruler, is designed based on yield efficiencies and identifies a final number of fruit per trunk cross sectional area.

“You hold up against the trunk and it measures trunk diameter and tells you how many fruit per tree you should leave for an annual bearing variety like Gala or for an alternate bearing variety like Honeycrisp or Fuji,” he says.

2. Another useful tool is the Equilifruit disc. It’s a similar concept to the Young Apple Thinning Gauge but is best used in high density plantings like slender spindle or tall spindle, Phillips says. The disc has a series of notches you put on each individual branch. Based on the branch diameter, it will tell you the final number of fruit you should target for each branch.

Researchers have also developed a number of models that help growers design more accurate thinning spray programs and timing.

3. The Pollen Tube Growth model helps growers determine how quickly the pollen tube grows depending on the apple variety and the temperature. That helps in the chemical thinning decision-making processes. This model works well in both Eastern and Western orchards, Phillips says.

4. The Carbohydrate model helps predict in advance the effectiveness of chemical thinners based on weather and growing conditions.

“Trees under carbohydrate stress are going to tend to thin more fruit,” Forney says. “If growing conditions are great — bright sunlight and mild temperatures — there’s going to be very little stress for that tree. It’s going to be more difficult to thin. Cool or cloudy conditions will increase the stress load on the crop. The tree starts naturally dropping fruit and you’ll get more response to chemical thinners.”

This model, developed at Cornell, works very well in the East. It has proven a little more challenging in the West which has few cloudy days in the spring, Phillips says.

5. While you can use the carbohydrate model to predict what the response of your thinning program is going to be, Duane Greene’s Fruit Growth Rate model or Thinner Response model will tell you after you have made the application what the response was.

This model measures differences in fruit growth rates — growers take two measurements over the course of a set number of days, and based on the difference in fruit growth rates, it will tell you how much fruit is going to fall off the tree and how much will stay on.

“You can do this quickly enough after an application that you can make a decision that is soundly based in science as to whether you should make another application,” Phillips says.

Growers willing to use the models and trust the numbers improve their chances for a quality crop at the end of the season, Phillips says.

“These are powerful tools,” he says, “and tremendous ways to take some of the emotion out of the decision-making process and get really into the science of it.”

New Chemistry Coming

In addition to these models, new chemical thinning products are coming as well. Kevin Forney says Fine Americas will have three new NAA product formulations next year — called Refine — including a 3.5% WSG sodium salt-based formulation and two potassium-based formulations, a 6.25% liquid and a 23.2% liquid formulation.

National Crop Specialist Byron Phillips reports Valent U.S.A. has some research farther out in the pipeline that may help thin larger fruit and provide a rescue option. He also says growers should keep an eye on a material that’s getting a lot of attention in Europe — Metamitron, an aquatic herbicide that has chemical thinning properties on apples and it looks like it could perform well.