Maryl, I am in central AR so growing conditions may be similar to yours. Here we plant the rhizome like a 'duck on water'. That is to say half in and half out of the soil. The roots and lower half of the rhizome in the soil and the top and fans clearly exposed..... looking like a duck paddling on the pond. Mine are covered in ice and snow for short periods of time and the ones planted on the north side of the house for longer periods of time. None have ever frozen. Even the little new fans can get covered by snow in early spring and not flinch.
I also have a lot of potted irises. Two things I think are important 1) Do not use a dark colored pot unless you plan to bury the pot in some type of mulch. The sun on the side of the pot will bake the roots and 2) Pots will dry out more quickly than the garden does so you will need to water more frequently during dry spells. You will not have to worry about standing water if you have good drainage and therefore you can water more often without worry. The roots will love the extra water.
