I'm in coastal SC, and our lowest lows are down to the upper 30s now (with highs in the 70s/80s). We won't have a freeze here until November with winter temps through March.

Our cold season is plenty long enough for irises to bloom very well in the spring.

Mike

I have never really read anything about an iris needing a cool period to bloom, or rebloom. I'm curious as to why you are asking. Do you have in mind trying to 'force' some iris to bloom, as folks often do with spring blooming bulbs after a cooling period?

Are you maybe confusing irises with bulbs like daffodils, hyacinths, tulips, etc, that require a cooling period if they are being grown in the really warm zones?

I've just always planted irises, and then Mother Nature and her temps did her thing naturally, and thus the rhizome grew and bloomed.

Sue

Chemocurl, Need your viewpoint.... My iris bloom in June (zone3) The RHZ tend to be on the small side (Historic) I usually don't see many increases so I posted the question. Anyway, we had a bad summer with little rain. Sept we had some rain and I did some topdressing with fertilizer. I am finally seeing increases on all of them. Could it be they didn't get enough of the 6-10-10?? Are they heavy feeders??

Different irises, grown in different parts of the country, will have rhizomes of different sizes.

Could it be they didn't get enough of the 6-10-10?? Are they heavy feeders??
I cannot answer that, but hopefully someone with more knowledge will.

This is taken from the FAQ here at the forum.
Though iris gardeners often dig a bit of superphosphate into the soil to feed the rhizomes and help promote future bloom, it's actually a wiser tactic to fertilize according to the needs of your particular soil. This can be determined through a soil test performed by your county extension agent. Avoid high nitrogen fertilizers....taken from the link below.

Sue

Here is a link that might be useful: How do I dig, divide, and transplant bearded irises?