GA, I wonder that layering might not be an option OR digging a root up? I don't know, but can only speak from experience of working in a client's garden that a mahonia planted one side of a high fence, has bits of it coming up the other side, so, therefore, it may spring up from runners? I've been working for this lady for 4 years & I've always had to cut back the unwelcome new growth which obviously weren't new plants from seeds.