Hey Jeanr - after they have bloomed for two years is when I divide mine. Some people wait 3 years - I guess it depends on the iris and how fast it makes increases.

I divide mine immediately after they are finished blooming. In a cold winter area, it's best to get them divided and back in the ground early in the season to give them as much time as possible to root in again before winter. They can even be divided and replanted before bloom in the spring, but that will usually abort that year's bloom.

The only irises that should be moved to fresh soil after division are Japanese irises. All other iris types can be replanted in the same locations from which they were originally dug, though it's a good idea to amend the soil with compost and any necessary nutrients before replanting.

Laurie