Assuming the trees were on the property, when you bought it. Why not contact the previous owner and find out more about the tree and how they took care of it.

Excellent idea! Thank you for your response.

Yes, they were here when we bought the house last summer. But the folks we bought from were flippers, who bought it after it was foreclosed. The neighbors (who haven't had any ideas about the plum trees) have been a wealth of info about this house's very dodgy past. My best guess is that the owners who lost the house to foreclosure are incarcerated these days.

They look like 'Mirabelle de Metz', a small sweet plum that is excellent for making jam. I have one of these trees in my orchard, they are a true treat! You are very lucky! Mrs. G

Oh that is good news! I hear nothing but good about Mirabelles! Thank you Mrs. G. Any ideas why they are hardly producing this year?

It would be pretty normal that after a heavy crop they rest a little
for one season.

The two most common reasons I've seen for this happening are biennial bearing and late frosts. The pattern of heavy crop alternating with light crop is likely to continue. One thing you can try is thinning your heavy crop early in the season which will not only improve the quality of fruit that year but should increase the numbers of fruit the following year. Late frosts can merely thin or can entirely eliminate your crop. I've found that Japanese plums are more susceptible than European to this damage. Some research on low temperatures in your area early this season during the prebloom and bloom period may give you a better idea if this was the cause.

Plums usually don't bear biannually. Your smaller crop was probably due to a late frost/freeze. that zapped the earlier blooms.

Konrad is very right, the tree is taking a break after giving you so many last year. If your plums were sprinkled with a bit more red color they would be 'Mirabelles de Nancy', I have two of this varieity. They take five years to start fruiting so you are in great shape with these trees. The "Metz' variety is best for cooking, and the 'Nancy' variety is better for fresh eating. Late frosts probably hindered more blooms as well, as stated by rayrose. You are lucky, the plums are gems! Mrs. G