Apple thinning has been a major problem for farmers seeking to transition from conventional to organic production. Consumers expect organic apples to have comparable size and quality to non-organic fruit, and thinning is needed to achieve that goal. Also, many apple varieties will bear heavily in some years and less heavily in the following year in an alternate bearing pattern. Thinning in the heavy years will increase yield in the light years. Organic farmers are not able to use most plant hormones and instead rely on time-consuming and expensive hand labor to thin their trees. In Europe, organic farmers also use mechanical methods, as well as lime-sulfur and molasses. These other methods are not as effective as hand thinning and can damage trees.
Now, scientists at the Research Institute of Organic Agriculture (FiBL) have found a way to thin trees using an ingredient commonly found in kitchens. By applying potassium bicarbonate, a substance used in baking, to trees at a key times in the growing season, organic farmers can efficiently regulate crop load better than hand thinning and the other alternatives currently used in organic production. The results were consistent for 11 varieties treated over a three year period. Potassium bicarbonate has the advantage of being very environmentally friendly. In 2012, Swiss authorities approved the commercial product, Armicarb©, for use as an apple thinner based on FiBL’s research.
Weibel, F. P.; Lemcke, B.; Monzelio, U.; Giordano, I. und Kloss, B. (2012) Successful Blossom Thinning and Crop Load Regulation for Organic Apple Growing with Potassium-bi-carbonate (Armicarb®): Results of Field Experiments over 3 Years with 11 Cultivars. European Journal of Horticultural Science, 77 (4), S. 145-153. Available at http://orgprints.org/21157/