You can do it any time, BUT, the best time would be after the hottest part of your summer is over. Iris seem to go into a somewhat dormant stage after bloom season. They are re-generating, and adding increases, mostly under ground. If you divide and replant after that period, they will grow new leaves and start some new increases for you so you have bloom possible for the next year. I like to do that up here in the "Northwoods" in Mid to late July. I would guess for you it would be later. Maybe some of the "Southern" people will have more specific suggestions for your area.
