seedling morus alba would be my guess. I've never heard of any marketed mulberry rootstocks.

I have tried rooting mulberry hard-wood cuttings with no success, maybe semi-green cuttings. Grafting is usually the recommended way to go (though I have not done it myself)

~Chills

Some mulberries root very easily from hardwood cuttings (just put them where you want them to grow) and some are very hard to root. I suspect the grafted ones are the latter (they certainly were many years ago). I have had a mulberry with a different leaf shape arise as a root-sprout from a purchased selection. People used to graft a piece of easy-to-root mulberry to a hard-to-root cutting, and then treat the combination as a cutting (they would use rot-able twine to tie at the graft). This is similar to grafting a piece of root to a cutting and using that combination basically as a dormant cutting for rooting.

Thanks guys, I think I will order some cuttings from UC Davis for next year.

Super popular in Canada rootstock is Morus Alba Tatarica because of its' hardiness to zone 3 (in Canada). However, I've heard Maclura Pomifera "can be used as rootstock for Morus genus" so I am going to try!