I have 50+ cherry trees of various types inc: Bing, Rainier, Bk Tartarians and three way type trees. This spring we had 3 bee hives to help pollinate the trees. All the trees had a great flowering, and what appeared to be a good fruit set. When the fruit grew to about the size of green peas about 60 to80% fell off the trees. The same thing happened last year. Two years ago we had a very heavy fruit set and harvest. What could have happened?
Jackson County Oregon cherry trees
It's difficult to say for sure. Things that can affect pollination on sweet cherries include the following:
Poor pollination weather including too cold, too wet, too windy and too hot. When the temperatures are too warm the ovules die very quickly and the pollen tube doesn't have time to grow down the style. Not much that can be done here. The use of AVG or Retain can help as it is an chlorophyll inhibitor and will keep the ovules alive longer.
Frost during bloom: temperatures in the 20's as bloom approaches can kill the bloom.
Too much shade. Fruit will drop for lack of enough photosynthesis taking place. The obvious solution is to prune harder to open the orchard to more light. Make thinning cuts as opposed to heading cuts to prevent a lot of rampant growth after pruning. Even so the vigorous response may delay better fruit set for a year or so.
Low boron. Boron is essential for good fruit set. Most orchardists will apply B either in the fall along with urea or at delayed dormant. This is an annual application and can improve fruit set, even when B levels are adequate. It is not too unusual for B levels to be less than normal unless this annual application is made.
Here is more information about B applications: https://catalog.extension.oregonstate.edu/sites/catalog.extension.oregonstate.edu/files/project/pdf/...
Please refer to pp. 78-79 of the book to learn more about boron applications.