I'm not sure, to be honest.
I have only ever cut the dead leaves off of the irises - some winters they stay green all through, but last month they were mostly killed. When I refreshed the compost and mulch on them yesterday, I found a lot of rotted rhizomes.
Blooms seem to be cyclical - some years, they bloom like crazy and some years, only a few. This year, I'm going to feed the heck out of them all summer to build the rhizomes for next year, then I'll divide them in the fall and refresh the whole pot.