Extracted from:

https://en.wikipedia.org/wiki/Plum#Species:~:text=A%20plum%20is,and%20juicy%20flesh.

Plum is a fruit of a number of species in Prunus, subgenus Prunus. [1][2] There are many known species of plum and several that are grown commercially. Prunus domestica has been traced to East European and Caucasian mountains and is now referred to as European Plums. While Prunus salicina and Prunus simonii originated in China and varieties of these species are referred now to as Chinese, Japanese, or Asian Plums.

Plums may have been one of the first fruits domesticated by humans, with origins in East European and Caucasian mountains and China. They were brought to Britain from Asia, and their cultivation has been documented in Andalusia, southern Spain. Plums are a diverse group of species, with trees reaching a height of 5-6 meters when pruned. The fruit is a drupe, with a firm and juicy flesh.

Ploidy

Prunus simonii and Prunus salicina (Chinese or Japanese plums) have a diploid number of chromosomes (n=16) whereas Prunus domestica (European plums) have a hexaploid chromosome number (n=48). Chromosome number, as well as other factors, affect the ability of these cultivars to cross-pollinate one another. European and Asian plums do not cross-pollinate.

Many plums (European or Asian) are partially or fully self-fertile. Cross-pollination between cultivars is also quite dependent upon the dates of blooming such that early and late blooming varieties are unlikely to effectively cross-pollinate. In general, Asian plums bloom early in the spring and European varieties bloom in late spring.

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Letter From Longwood Avenue
By: Al Cosnow

from Grapevine, September 2004

Hello Everyone,

I keep complaining about how I don’t understand plum pollination and about how I have 3 mature plum trees that bloom well and how none of them will pollinize any of the others. I started by trying to find something that would pollinize ‘Purple Heart,’ which I love. It is said to be a Japanese plum by some, and a Japanese-American hybrid by others. I dunno. The one and only year it bore a crop (2001) my Shiro (Japanese) tree was too young to bloom. Could be it was pollinized that year by the wild American plums on my property—I never paid any attention to whether they were blooming or not, and after that I cut down the American trees to make room for other things I wanted, like persimmons.

Maybe that was a bad move. Since then the Shiro tree has bloomed copiously each year, but neither it nor the Purple Heart have borne anything, nor has ‘Alderman,’ another Jap.-Am. hybrid, but Alderman has an excuse: it blooms a week later. So I have been doing what one is supposed to do—read up on it, and now I’m more confused than ever. Here are a couple of items. First from the Fruit, Berry and Nut Inventory under the write-up for Purple Heart “Easily pollinated by Toka or Underwood”. Sounds like my answer. But then from Modern Fruit Science by Norman F. Childers “Some hybrids between American and Japanese species [have] non-viable pollen; they include…Underwood… obviously of no value as [a pollinizing variety]”.

Well, one of those sources is right, and one is wrong, but I don’t feel like taking a few years to experiment and find out, so Underwood is out for me. Now what? Try Toka. I went over to Oriana Kruszewski’s place in Skokie and got some budwood from her too-young-to-bloom-yet Toka to graft onto my tree. And while I was there I tasted the Shiro from her tree. Boy, are those good! I asked her what was pollinizing it, and she said she didn’t know; she has various trees multiple-grafted to many different things, and at least one of them is doing the job on her Shiro.

So that is what I’m going to do too—have a “zoo” of plum varieties grafted a branch here and a branch there, and hope that something will do for the Purple Heart and something for the Shiro. I even found a sprout from one of my American wildlings I missed, and I will let that grow back to blooming size as well. With a diversity like that, something will have to work. At this point, I don’t even have to know—I just want to eat Purple Heart and Shiro.

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