It also depends on the type of iris and the particular cultivar. My MDB's in pots froze into big balls of ice. I was told they needed a period of winter to thrive so I put them northside of the house, in the shade and on top of crow ridge. The drop off does not get sun in the winter as it is steep. I had to beat the ice off of them this spring and they all bloomed. If last winter was any indication, irises in pots can survive a pretty cold winter. Don't know about sub 0 temps. I could not survive that so what can a poor little iris be expected to do. Sinking the pots even half way will help a great deal. I am hearing that we are going to have a cold winter..... maybe that means we will not.
