The white mulberry morus alba is the hardiest, but it isn't very appreciated in the US due to invasivenes. The american red mulberry morus rubra is next in line and should be readily available in many nurseries. The black mulberry morus nigra is also quite hardy. There are some good cultivars like "Illinois everbearing" out there to that might be worth consideration, it isn't as hardy as the straight white mulberry though.

Northrop is said to be, Saint Lawrence Nurseries. They wrote that it has survived -50F.

I fight them constantly. I lost the battle with one sapling four years ago, and now it's a 12-ft tall tree. I like it, but I won't allow any more to do that. I was just unable to kill it because it couldn't be pulled and it was coming up in the middle of a daylily bed and I was afraid to spray it.

I fought my Kingan for 4 years and finally won. I just needed to keep clipping the foliage off. We finally cut the main trunk down a couple months ago. I put a small slab of marble on the stump and now it's a table.