This question has been asked many times on different forums and mailing lists, and the general consensus seems to be that there really is no ideal time to divide rebloomers. Whenever you divide, you're going to interrupt the growth/bloom cycle; it's inevitable. You may as well just divide whenever you're ready to do so, as long as you get them back in the ground at least 6 weeks before first frost.

Laurie

If a rebloom bud has already formed, it will go ahead and bloom anyway, although on a shorter stalk with a smaller flower.

In a Tom Craig iris order one time, I had one bloom in transit in the box. Another bloomed while drying out in the bedroom unpotted as these were big, hunky CA rhizomes. Also I used to get proliferations on Craig irises. I have heard people tell similar tales on irises from this garden, one being the late Lloyd Zurbrigg.

I have had some from Sutton's bloom in Oct., after having been planted in August or September. The bud was formed in CA before the plant was dug.

You can transplant now if you don't live in a hot/damp climate like I do. I pot all incoming irises and hide them under shrubs to keep the sun from hitting them and to deflect some of the rainfall. Planting a huge iris out in the open here in July is an open invitation for rot.

Walter Moores