There are many varieties of prune plus that vary in their performance, especially region to region. Probably the best plum for prunes here would be Empress of varieties I’ve tried, simply for it’s reliable production and the common ability of prune plums to get up very high sugar when left to fully ripen. You may be in a cold enough spot to make Mt. Royal a good choice. It takes a pretty long season to properly ripen Empress. Castleton is also very reliable. Both of the latter are relatively small compared to Empress. Japanese plums just don’t tend to get high enough sugar to be good prunes I suspect.
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