That's the kind of feedback that we should like. The three kinds of plums I have that acted like yours in poor weather and always set fruit are Nadia, Sugar Twist, Owen T, and Shiro. But my information isn't as valuable as yours because of the heavier washing you get in the hills as mine.

What I think this has value is helping understand the ancestral origins of these that do well and from where the possibilities exist of a common trait from the family tree. The contributor might be Santa Rosa for Sugar Twist, Owen T and Splash. The less likelihood for Nadia and Shiro I think is because I know too little about that pedigree than the others. The others grew out of California when Santa Rosa plums were very common. I have never isolated pollinators away from Shiro the way I have with the other subjects either. Shiro may gain that attribute from myrobalan (ie. wild wet stock) which derive from native British stock as part of it's cool spring ancestry.

All of these I just listed are self setting similars (ie. santarosa), limited to zero pollen producers, and that's why I wanted to see how Splash as another zero pollen producer performs in this test for 2023.

So is somebody able to vouch for the ability to set crops from Santa Rosa around here?

My unproductive plums are Lydecker and Emerald Beaut for what ever reason. Those will not even set with known good plum pollen. however the Lydecker is prolific after hand pollinations only when sheltered. In 2022 it was prolific even with prunus microcarpa pollen. 

I couldn't find that Santa Rosa. It would have been nice to have included that in the manual pollination experiment I already finished today.

Here's what I did. I put a green flag tape on Splash and put Beauty pollen on 10 flowers that were 10 inches or less air distance away from the tape. Then I moved over to the most heavily blooming tree which you know is Nadia. I put green tape on Nadia and put prunus pensylvanica pollen, otherwise known as from native pin cherry. Which I did 6 times to 6 flowers less than 10 inches from the tape. 

It was mild and dry conditions and the only insects seen were a few gnats that were resting among the Nadia hybrid. I have no idea if any of this is going to set fruit because of what I did. 

If that's a picture of Saint Julien A then it's the same thing I have and today is starting to flower as well. The only way the clone I'm talking about will fruit at my place is with pollen from another plum other than the same clone of itself. There's an older post of about 2 years ago which Reinettes and myself weighed in on the self unfruitful state of Saint Julien A.

Jafar: Keep an eye and an open mind that your clone of hybrid Nadia might be different than average. The problem with this is that Nadia is half sweet cherry and half plum, and whenever I have seen DNA clonal siblings in and hybrids within cherry I have seen variations. One might call them a sport of Nadia, for example one of possible improved quality. I have been talking about maxima-14 cherry rootstock hybrids now for a while so I won't talk more. Other than to say these two pictures here and here are exactly the same clones but different trees in the same yard today.