In any zone:
For bulb iris (sometimes called "Dutch" iris), divide when the foliage
For rhizome iris (e.g., bearded iris), check with a LOCAL nursery and
ask them when they will have bearded iris for sale as bare root
rhizomes. (This must be a "real" nursery, not a lumber yard or hardware
store -- not Home Depot.) Go back at that time to verify they really
are available. That's the time to divide them. Work on one clump at a
time, replanting the same day they are dug.
See my <http://www.rossde.com/garden/garden_divide_iris.html .
David E. Ross
Climate: California Mediterranean
I do it right after they quit blooming. I take a shovel and cut the plant
in half. I leave half where it is and remove the other half, and then cut
it in half again. Then you will have two to plant, or give, or throw away.
The reason I do it that way is so the half that was left where it was, will
bloom again the following year. Sometimes the transplanted ones will need
an additional year to make blooms.
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