I have been thinning stone fruit trees and spring pruning them at several sites a day and all day long as my helper mostly installs squirrel-coon baffles since June 1st when we finished the second round of insecticide-fungicide sprays in our low-spray regime. I got most of the thinning done in my own orchard and a few lucky customers by mid-May
Peaches are all overloaded with set fruit and so are many plums, but plums are very variable from site to site, variety to variety. I observed events on my own property very closely and here is what I saw and see.
When my apricots were in bloom in early spring almost the only pollinators were my carpenter bees. Many of them nest in the wood trim on the stucco cement walls of my home (so not structural at all) which are the walls my apricot trees are trained against. All 4 varieties set crops with only the very youngest one setting a small one, the rest required aggressive thinning. Apricots in the east usually require cross pollination to set heavy crops.
For some reason the carpenter bees vanished for the first week of J. plum and pluot bloom (not because it became cool) and nothing stepped in to replace them. I attempted to cross pollinate them with a dusting brush, but pluots were very mixed, certainly Flavor Queen and Flavor Supreme, mature trees (maybe, but 7 years old and vigorous), are total duds, with Spring Satin not far ahead with only a few set fruit. Early Magic plum set a very light crop, and it blooms as early as my earliest pluots. The early bloomers were still in bloom when the pollinators finally showed up in force and later blossoms were plentiful and well tended on these trees that failed to set much fruit. Dapple Dandy, Flavor Grenade and a small graft of Flavor King, which I believe bloom a little later, all set fruit very well. I have to assume the possibility that a blossom can be attractive to bees while lacking a vital ovary. However, given that Elephant Heart can take 8 years to settle into reasonable productivity I will stick with my shy bearers for a few more years. They are the ones that most resemble Elephant Heart, with its sweet, deep red flesh, the unsurpassed queen of J. plums.
All of my other J. plums set quite well, although my most reliable and mediocre Shiro set relatively lightly, for which I’m grateful. Ruby Queen was the most overloaded of my J’s and the last to bloom.
All of my E. plums set quite well except De Montfort, which usually is reliable. It has well established branches in two large trees in separate parts of the property and I have no idea why this is the first year it stood out for light cropping among my Euro plums. The Green gage and Valor to which De Montfort is grafted set heavy crops as did Castleton, Autumn Sweet and especially ,my heaviest consistent bearer, Empress (probably tied with Castleton).
On other orchards I manage sometimes only the E. plums set particularly well and at others it is the J’s. Why the late bloomers did better at some sites and early bloomers at others is a mystery to me.
Unexplainable events are a major feature of managing home (estate) orchards and a bearing age fruit tree nursery for a living. If you require the comfort of certainty you probably need to find another hobby, but I enjoy unanswered questions. They make things more interesting, which is useful when you are in the midst of the drudgery of endless fruit thinning.