Elena,
Hello and welcome. Just because you are seeing bloom on the irises now that did not bloom in the Spring does not mean they are re-bloomers. Since your new iris are "new" vs established, the more likely explanation is that the iris cultivars were immature and did not have enough energy to bloom in the Spring. Iris bloom is based on the amount of "energy" stored in the rhizome. Since next years bloom is this years "offspring"; they need time to mature. It is likely you will get no bloom from this plant next year or you might see the same situation next year as the plant will have a much shorter chance to mature after it blooms this Fall.
I have had bloom on re-bloomers as late as Thanksgiving (actually later; but that sounds better). Even with snow on the ground! But this bloom so late meant a sacrifice of not bloom the following year.
My experience with one particular cultivar is most enlightening. Again and Again. This remarkable re-bloomer by Sterling Innerst is somewhat "ugly" compared to today's "bright" re-bloomers, but this large flowered cultivar is a blooming machine. I had replanted some of them to a new location at my normal time (replant for me on the Maryland/Pennsylvania border starts July 4th through October) in mid July. I usually do not water or pamper them (to make them re-bloom) but I did get a re-bloom that Fall. I figured since it bloomed late that it would not bloom the next year. I was wrong! It bloomed "ok" in the Spring (ok meaning not large/humungous blooms as normal but more like all the other irises. This cultivar had pushed out a lot of "babies" that Fall and it bloomed in May and just into June. It then bloomed around July 4th. Then in late August, and all through September and October. Amazing. You just never know.
Having said this, the late bloom I got that year was not true re-bloom but really immature rhizomes from the previous fall that did not have enough energy to bloom during the "normal" Spring bloom. This is the most likely explanation for your current bloom on newly transplanted irises from last year. My rhizomes eventually straighten themselves out and started blooming in the Spring on schedule.
