I've read different things, from 3 weeks after they finish blooming up through August. I've divided immediately after they finish blooming up until the first of november here in zone 6 & they have all done well, coming back to bloom the next year, although the ones in November didn't bloom that well the following year.

Irises need to be back in the ground atleast 6 weeks before your 1st hard freeze..I am in Zone 7 (W TN) and we always dig about the middle of July..( seems like its always the hottest time of the summer)

I was just about to post about this. My bearded irises bloom around May-June. They vary a bit and they're starting to look pretty ugly after 8 years or so in the ground and lots of rain this year. Oh and my husband buried them in the mulch this year despite my warnings. They're overcrowded and I've been chicken about splitting them in the past since I never have before.