Posted: 29/06/2015 at 18:26
That sounds wrong to me. I've always understood that over the years the original rhizome eventually gets too old to flower well, but by then it's produced loads of new rhizomes - that's why we lift and divide every three to four years and discard the old central rhizome http://www.bhg.com/gardening/flowers/perennials/how-to-grow-maintain-and-divide-bearded-iris/
The rhizomes certainly flower more than once. I'm sure Wonky would agree with me - she was taught about Bearded Irises by the daughter of Ellis Carpenter, the who was the breeder of the beautiful iris Boxford Spice.