This seems normal to me for plum trees that age.

Really? That is what I was hoping. What age are they considered mature?

JJ, Japanese plums often have started fruiting some by four years. Both of those plums are super hardy types, and there are some puzzling pollination issues in the very hardy plums. I don't know a lot on the topic but have noticed a similar problem on my Gracious plum, it sets a ridiculous number of blossoms but they nearly all drop. Generally I wouldn't recommend getting such plums unless you are so far north that nothing else would work. I would consider contacting the nursery you got them from and see if they have any advice. Maybe those types just take longer to start fruiting. My Gracious has given me zero until this year when it finally decided to set a couple fruits.

Scott

My Italian Plum (prune) is going on eight years old. It finally had 10 plums on it. I hope they make it. Very long wait!

I've had a Rosy Gage plum that has bloomed for the last 2 or 3 years. It's getting to be a big tree. This year it was really loaded with blooms, but only set two fruits. All the rest turned yellow and fell off. I've noticed one of the two fruit that set is gone. Down to one fruit.

Same thing with Coe's. Lots of blooms, all the fruitlets fell off.

Green Gage set quite a few plums, as did Kirk's blue.

I grew Italian plum once (Early Italian). It was productive. As I recall it started producing at year 3. I had a President plum right next to it.

I found this paper...here ... sounds like pollination may be the biggest problem. I've got a decent crop out there on my trees (6 year old Superior/Alderman) and I wonder if it isn't because I live near a thicket of wild plums (around 100 yards from the trees). I should really get some bud wood and chip bud some wild plum onto one of my trees.

"I maintain a rather regimented spray schedule..."

I know the arguments for and against spraying, so I won't beat on that drum. But if any of your spraying is at a bloom time in your locale, or when pollinators are active, they may be staying away or worse. If that is not the case, I would recommend planting flowering herbs, veggies or ornamentals that your local pollinators enjoy. If you keep them around and happy they will do all the work for you. For this reason I have potato bushes (for the big black bumbles), allow cilantro, arugula and nasturtiums to freely/wildly reseed (for the hover and honey bees). Oh, and the awesome blooms, fragrance and greens for salads & salsas are off the hook too.