It could be a sucker indeed, I can't be sure. I think I'll remove it because I'm not a super fan of Japanese cherry anyway, the bloom is so short. Thanks for the reply.

If it's grafted on sweet cherry, as is often the case suckering and often superficial roots are to be expected after some time has elapsed.

The tree is very old, my father planted it and he died in 1982. So I don't know if it's grafted but I certainly want to keep the tree as long as possible. Google seemed to suggest they are propagated by seed as well, but I was suspicious about how true to form it would be. I'll dig around the roots to see what's going on.

They can certainly be propagated by seed but there's no guarantee the babies will be the same as the parent. If the tree is the straight species the offspring should be similar to the parent. If it's a cultivar they'll be more variable. But since seed is produced from sexual reproduction rather than cloning there's always going to be some degree of genetic variability.

The double hybrid "serrulata" such as 'Kanzan' aren't likely to fruit.

I assumed, perhaps mistakenly, that Groucho had seen fruit, since they believed that the tree had produced a seedling.

I didn't realize Kanzan was sterile... So most likely a sucker of wild cherry rootstock.

Aha. If I'd known it was Kanzan I wouldn't have wittered on about seedlings ;-)

And in which case we then know the tree is not Prunus serrulata either - the correct name is Prunus 'Kanzan', without the name of any one wild species being included in the combination.