Is it the cold that concerns you? I have a Persian Black mulberry from Burnt Ridge which I kept in a pot for a few years. I planted it into the ground in fall 2014 or spring 2015. Probably spring, but I’m not sure which- I just see it in my notes that fall as slated to be planted. That means that it has survived at least 2 winters (15-16, 16-17, which went down to -8F and ~0F). I took cutting from it recently and it looked good, though that’s no guarantee. The thing is still tiny, so I suppose that helps support the idea that it is a true nigra, as BR claims.