There are lots of folks that successfully grow Iris in the PNW. I think as long as the drainage is good, you should be ok for the most part. If not, there are Siberians, Japanese, louisianas, and others that like wet feet.
Find a club in your area. Most have sales in the summer, and it's a good opportunity to meet and talk to folks about Iris. You've got king county Iris society in the Seattle area.. Check out their website.
