Bumper Apple Crop Requires Thinning Strategy For Consistent Crop

"Nibble" thinning may be necessary to chemically thin multiple times throughout the bloom and fruit set window.

Published on: May 21, 2013

By Phil Schwallier & Amy Irish-Brown

Following a season where apple growers were robbed of their entire harvest, it may be a bit painful, but very necessary, to thin the apple crop this year.

This 2013 apple thinning season factors are unique, as always, and will need consideration before performing thinning. Michigan State University Extension Educators suggest that growers plan on making multiple thinning treatments this year to achieve targeted crop load. We currently have the potential for a very large apple crop and starting your thinning early, at petal fall, is highly recommended for 2013.  The extremely low crop set in 2012 has triggered the biennial bearing cycle that apples are notorious for (on 2013, off 2014, on 2015 and on).  We need to break this potential biennial cycle by thinning this crop down and then use summer naphthaleneacetic acid (NAA) to promote return bloom for 2014.  

Bumper Apple Crop Requires Thinning Strategy For Consistent Crop
Bumper Apple Crop Requires Thinning Strategy For Consistent Crop

We had no crop last year, which has produced a “green” snowball bloom this year and is expected to strengthen trees and fruit set for the 2013 apple crop. The “green” bloom refers to the abundant amount of leaves that are present on the trees before and during bloom. Last year’s hot and very dry weather has caused some minor reduction in bloom on limbs, trees and on a few varieties such as Red Delicious and Empire. This is the exception and only in minor areas.  

Frost occurred May 13 with minimum temperatures at 24 degrees F which caused some damage to flowers. Growers will have to carefully assess frost damaged sites and adjust their thinning program accordingly. Excellent sites and frost-protected areas have almost no frost damage and need to be thinned aggressively.  Bee activity, pollination and even fertilization appear to be good.

Temperatures for the next 7 to 10 days are forecasted to be warm, ideal for excellent fruit set.

Thinning Approach: Nibble Thinning 
“Nibble” thinning is a strategy to chemically thin often and multiple times throughout the bloom and fruit set window. Technically “nibble” thinning begins with blossom thinning (Lime-Sulfur & Oil or ATS).  This treatment seems to be less successful in Michigan and thus is not practiced very much. The real first thinning period occurs at petal fall to 6 mm. At petal fall, trees are not very sensitive to thinning, and most years, no significant thinning occurs.  This year with warm temperatures forecasted for the next several days, some good thinning is predicted at the petal fall timing - perhaps perfect thinning for some sites. This first thinning will nibble off perhaps up to half of the target thinning of this excessive cropload. Usually additional thinning will be required at the 10 mm stage. The next chance to thin will occur at the 10 to 12 mm and the last will be at 18 mm.

This process of reducing the crop gradually will result in a better consistent thinning with a reduce risk of over-thinning or under-thinning.

Other Considerations:
1. Oil can be added to thinners to increase the thinning by 10%.
2. Oil is not compatible with Captan and Sulfur. Where this is a concern, use a surfactant
instead of oil.
3. Agri-Mec & Oil can cause additional thinning when mixed with thinners.
4. Cloudy, warm weather will increase fruit drop.
5. Sunny, cold weather will increase set.