Tanlu, depending on the size you're looking for, nursery hornbeams will come with a very large rootball, straight trunk, and not much for lower branching. Last year I planted (in the landscape) a nursery American hornbeam with a 3.5" trunk and it's rootball was massive (probably 500+ lbs). Looks great in the landscape but if you're looking for one with character in the lower 12 inches of the tree, I would suggest getting a collected one or one field grown as bonsai stock. Last I checked, I notice a couple at bonsai-south.com.