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Here, I'm told to divide my irises at the end of July or beginning of August. I think the idea is to give them some time to recover and grow after blooming (each rhizome blooms once, so you want to give the "mother" rhizomes plenty of time to form "daughter" rhizomes). Also, and probably not a consideration in your mild zone, you want to plant the divisions at least 6 weeks before frost/cold, if possible. I know this isn't a direct answer to your question, but I'd say your window for dividing irises would be between 7 or 8 weeks after they bloom and 6 weeks before freezing fall weather. I hope that helps!
For more expert advice, and to find other gardeners in your zone &/or region, I hope you'll consider subscribing to DG. A trial subscription would let you look around... The number of forums keeps growing, so its easy to feel overwhelmed, but pick a couple and jump in! Meanwhile, if you're not ready to subscribe, check out some of the forums that are open to both members and subscribers. And welcome to DG! :-)
I have read a number of articles that advocate dividing and transplanting Iris a few weeks after they bloom and that's when my mother usually divided and transplanted iris. But we live in Central Virginia, (planting zone 7a) and I have begun to doubt this conventional wisdom in our area. It seems to me that in our area, newly transplanted iris are in more danger from a late summer/early fall drought than they are from winter. So, I am wondering if it might make more sense to divide and transplant in April, knowing that the iris roots will be somewhat established before the summer gets too dry. Just a theory. I have tested it only to a limited extent.
I also would like to give one of those testimonials to how hardy iris are. I have found them thriving at house places that have been abandoned at least 60 years. And my mother used to give away a lot of rhizomes when she thinned the iris beds, but sometimes she would also give some to my father and ask him to plant them in the cow pasture in case she wanted them in a couple of years. Now, 10-15 years later, they are choked with fescue and don't bloom, but have spread widely. If you did up a rhisome out of the fescue and put it in a flower bed without competition, it blooms in a year or two. Now, I am experimenting with whether they can bloom where they are. They have well established roots and good moisture under the grass. But they have too much competition for light and fertility from the fescue and may have danger of rot because they are under all that grass. I am experimenting with pulling the grass around them and putting down a little bone meal to see if they will bloom where they are, so I will know what colors I am transplanting.
Yours, Overton McGehee
I'm glad you found us, Overton! I hope you'll decide to subscribe... we had an active MidAtlantic Gardening forum, and with the care you're putting into those irises I think you'd fit right in! :-)
I've never tried dividing irises before they bloom, but I've also never had them die of drought. I'm more likely to lose a few rhizomes to rot over the winter, so I'd be concerned about spring/summer rains rotting new transplants... your soil may be less heavy than mine, though, so perhaps that's not an issue for you.
As with everything about plants, different things work for different people -- and experimenting is good!
