My experience with irises is that you can not kill them. My mom use to clean out her beds and just throw the extra out back in the hedge. Well they would grow the next year in the hedge.No dirt on top just laying there. So yes I woyld say that you could just put mulch on top of them and they will be fine till spring.

One year I purchased some dry iris rhizomes on a fall trip and didn't get them planted until mid- to late November, and they were just fine, somehow managing to catch up with their neigbors and boom the first summer in my zone 7 garden.

Based on that experience, I'd expect that yours should do fine, especially since you've retained some of the root ball and we've had relatively mild temperatures for December.

Good luck with the irises and best wishes to you in your new home :-)

Dunno how it works with different varieties but where I live (6b) they're weeds, huge areas where the rhizomes are almost on top of the ground get covered with snow and nothing seems to bother them.