You want to allow the iris to grow thru the summer months so as to allow the new increases (babys) to go as larges as they can before dividing the clump .Because when you divide the clump you will cut the roots back to about 3 inches ,these roots will die and are only used to stabilize the rhizome in the new hole (this is another reason to only remove dying leave from the plant and never cut a good leaf off) the plant will grow from the energy stored in the rhizome until it produces new roots ,so the bigger the rhizome the better chance it will survive the winter.
You want to allow the iris to grow thru the summer months so as to allow the new increases (babys) to go as larges as they can before dividing the clump .Because when you divide the clump you will cut the roots back to about 3 inches ,these roots will die and are only used to stabilize the rhizome in the new hole (this is another reason to only remove dying leave from the plant and never cut a good leaf off) the plant will grow from the energy stored in the rhizome until it produces new roots ,so the bigger the rhizome the better chance it will survive the winter.