You should be ok. If the irises look like they might heave, just put a brick on the rhizome (not on the end with the leaves) to hold it down.
That said, we have many cycles of frost and thaw in winter and I never had a rhizome heave out of the ground, maybe it is never cold enough?
Edited to add: I think some of the iris people here use mulch (@irisarian?) but are careful to remove it as soon as it warms up?
