Yesterday I visited Arnis in his greenhouse. His Onco irises stay in quiet small clay pots with rhizomes only half in soil. Only just just moist, more to dry side. He still didn't finished replanting and is very busy now.
I'm growing Onco's on raised beds in soil (not in pots) under greenhouse cover. When I'm harvesting some clump, I'm dividing it and keeping in cardboard box covered with (burried in) dry sand. Regarding stoloniferous species I'm never cutting of stolones, but I'm allowing them to make rosette and only then saparate from mother plant (in following year).
Janis