I have kept iris over winter here in my Utah 5b garden. I do not bury the pots but do not keep them where they will heat up during the day and freeze at night. Keep them on a eastern or northern exposure. A mulch of loose leaves or pine straw might help. I use potting soil as my clay based soil here turns to concrete in a pot. I use the black plastic 1 gallon pots that I have saved for years. Good luck......make a little money to buy more iris.......
