They just didn't get dug up last fall. The mulch was heavy, so the 18" frost rule of thumb for around here probably didn't apply, and it wasn't too bad of a winter anyhow. They were Youkon Gold, and they were only about 6-8 inches below ground level. The mulch was about 6" when we put it on, but not much left by spring, so I tilled around the plants and put on more.