Sorry to join this disscussion so late.
My understanding is that I. japonica does best in areas with no
freezing weather. It will tollerate some cold, but the more cold the
less likely it is to bloom.
My personal experience is limited. My I. japonica only made it
through three seasons. I'm on the 7/8 border and always covered mine
in freezing weather. We had several nights in the teens last winter
and the plants survived but did not bloom. It had bloomed the
previous spring after a less "severe" winter. I lost it this summer,
I believe, due to stress from our prolonged drought. My new plant is
starting life in Texas as a pot plant, the better to avoid both the
cold and the heat.
BTW - Although it's named "the japanese iris" and is widly grown
there. I. japonica is believed to have been introduced into Japan
Hickory Creek, (North Central) Texas, USA
Zone 7/8, typical temp range 15 - 105 F (-9 - 41 C)
AIS, SIGNA, SPCNI, SLI, FWIS, Iris-L