Tonight there is a freeze warning and later this week there may be another. The advice I have gotten from a member of the Greater Rochester Iris Society is that as long as there is no heave, iris rhizomes will be fine. It's still hard to look at the some of the damage already done.
This is "Splashcata" ordered from Shreiners last year. It clearly shows the growth that has occurred since planting the single very healthy rhizome was planted last fall. Some of that happened last fall, but most occurred in March when the weather was unseasonably warm. Then the frost hit.