Miniature iris are relatively easy to grow, but are not as hardy as the standard varieties. This guide is about growing miniature iris.
Can bare root miniature iris tubers be held in a cool place over winter and replanted in spring?
Hardiness Zone: 5a
By jowanee from Madison, WI
Unless you're moving and want to take them with you just leave them in the ground and they'll be just fine :-) I lived through 15 Michigan Winters and had dozens of Iris' both regular and miniature and never had a problem. Just had to divide them every three or four years in the Spring ;-)
In your zone after the ground has frozen, I would lay down mulch over the iris area. Iris are notorious for frost heaving because they are so shallowly planted. Frost heaving happens when there are freeze/thaw cycles over the course of the winter season. Even in my zone 6a the Iris will heave out of the soil.
If you lay your mulch down AFTER the ground has frozen it will help keep the ground frozen until you remove it. Just don't lay it down over the iris before the ground has frozen to avoid rot problems (that goes for any plants that are susceptible to crown rot from excessive and prolonged moisture. You will also want to remove that mulch/rake it aside, ect. as soon as possible in spring for that very reason so moisture does not get trapped and the ground can warm up faster. BTW I use wheat straw often with a mix of fall leaves for winter mulch.