Apple Chemical Thinning – Facts for Fancy Fruit

Apple Chemical Thinning

The pollination season was challenging this year and it appears that chemical thinning won’t be simple either. I think in most cases, pollination turned out to be adequate to set full crops. If growers are in doubt they should cut a few fruit in half and look for normal seed development. Before we talk about chemical thinning, let’s review the basic process and what controls the effectiveness of a chemical thinner application.

At this time of the year, various organs on the tree are developing very quickly. There could be over 800 developing fruit on the tree all requiring food and energy (carbohydrate) to grow. Also, developing shoots are not net contributors until they have at least 5 leaves, so that also increases the demand. So here we have a high demand for carbohydrates and a limited supply because the leaf area of the tree is still developing. So when demand is greater than supply, there is a shortfall of carbohydrate and fruit drop off. This is an entirely natural process that is often called “June Drop”. Thinners act by increasing this deficit causing more fruit to drop. Other factors that influence the carbohydrate balance are also likely to affect fruit drop and thinner response. One of the important factors here is sunshine, or lack of it. In cloudy weather, the light is lower therefore the rate of photosynthesis by the leaves is also lower. This increases the carbohydrate shortfall and increases fruit drop.

Ok, with this in mind, what should be the approach to chemical thinning this year? Given that:

  1. Pollination weather certainly wasn’t perfect so we may be slightly iffy on seed set
  2. There was a lot of cloudy weather over the early fruit growth period

I would expect fruit drop to be heavier than normal this year. Growers may want to be a little more conservative than usual with their thinning approach. In almost every situation, the absolute worst decision is thinking that there aren’t that many fruit on the trees and deciding not to thin. Come mid-summer, growers invariably regret this decision as fruit grow and become more visible, but by then it’s too late to act. So here’s the recommended strategy for this year:

  1. Review your chemical thinning records from previous years. Consider the weather and how well those thinning applications worked.
  2. Since we expect heavy fruit drop and a lot of natural thinning this year, for each block of trees, take the most conservative thinning approach that you have tried the last few years that has still been effective.
  3. With varieties that are prone to be biennial, such as Honeycrisp, Fuji and Golden Delicious, don’t be too conservative with your thinning. Inadequate thinning of these varieties will cost you, both in terms of this year’s crop and next.
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