There are lots of things to consider as far as "Re bloom by zone", first, and foremost is "what irises will rebloom .....anywhere! There are many iris that are "registered" as Re......and many that Re that AREN'T registered that way, so, if you're 'looking " for an Re iris, 'word of mouth" is probably the way to hear about it. Anyone who is really interested in Re iris should consider a membership in the Reblooming Iris Society.....lots of information to be had, there, and the Magazine (The Recorder) is full of articles, tips, and information on what rebloomed, and where it did it. The Reblooming checklist ( I assume that is what Sherry picked up)( lists all the "reported" rebloom , by cultivar, and which USDA zones it happened in.
Another thing to remember is "zone" only tells you "how cold does it get there', NOT how LONG it stays cold (number of frost-free days), which, IMHO is much more important. And almost everyone can take advantage of "micro climates" to either " jump up" a zone, or two, or ,in some cases drop a zone. I can grow zone 6b plants easily in my 5a zone, if they are planted next to the south side house foundation, while the same plant ,out in the 'open' won't survive the winter.
As was mentioned, Winterberry has a wealth of information on Re's, and is a good place to start your research, joining the Reblooming Iris Society will only "cost' you the price of a few rhizomes....and gain you LOTS of information, and enjoyment. But the thing that will really increase your knowledge, AND increase your chances of rebloom....is growing some !! Pick an iris like "Immortality", with a proven track record of rebloom....almost everywhere....and grow it, watch it, move it around your yard to different 'micro climates', try the tips and suggestions listed in Don Spoon's catalog, and the Reblooming iris Recorder, and see what works best in YOUR garden....then, add some more, as your "skill level" increases. Rebloom is NOT a "sure thing", but it surely is fun to play with !!
