I find that trying to over-winter daylilies in pots is hazardous to their well-being. I had many cultivars die one year when I tried sinking the pots in the ground. I have had a number of daylilies in pots above ground over many winters but each year the number of survivors has dwindled by about 10 to 15 percent. Winter kill may not be the only reason they have dwindled. I do think that for a plant to survive winter in a large pot it needs to be substantially hardier than the zone in which it is growing.
