Around here the iris society digs in July for their annual sale. I looked up on line to see the proper time & there is completely different information, maybe it depends on the area of the country. I've read 2 weeks after blooming, late summer, early fall. The site I tend to go with said any time after blooming, it works for me. I know I have divided even during bloom up til November here. There are some iris that just don't like to be moved, & I know which ones they are now, but generally they all have bloomed the following spring. The ones I got the least amount of bloom on were the November ones.

You can do this as soon as they are done flowering. They will do great.

The rule is 6 weeks after blooming, but you can divide any time, may sacrifice bloom next year though.

Thanks