UndertheSun said: Once you remove the weeds and start digging, you may be surprised to find more rhizomes than you can currently see. .
Yes, something that I discovered for myself... irises can sink themselves down, or pull themselves in. (Daylilies can do that, too.)
This last summer I went on a rescue mission for 'Black Phantom'. In the case of 'Black Phantom', I thought that the small remaining patch (which should have been ginormous by now, even with the part shade) was due to gophers. Well, possibly some rhizomes were lost due to gophers, but apparently other rhizomes had just sunk down, and then stalled growth/propagation. After I had pulled out maybe 5 small rhizomes (all that I thought there were, and more than there were at first glance), my garden helper found another 3-4 (he was doing some work in that spot; presumably these are also BP), and now there is another rhizome coming up right next to a sprinkler head there (that one is deep and is definitely BP).
I also dug up most of my 'Busy Being Blue' this summer (no other irises were anywhere near there), and after the fact, I found a few more irises coming up.
In addition to these, some irises which I had planted a couple of years ago (in a freshly dug/made bed area) were definitely sinking down and getting buried. I've rescued two of them thus far, but I have another few to go.
I guess the lesson here is to not just weed, but carefully dig up the soil around your iris patch, and you might find more rhizomes than you first thought.
(Along with that lesson... make sure the soil is fairly firm when you replant, and keep an eye on your iris patch, and make sure that the replanted irises aren't pulling themselves down.)