A tale of two iris beds.
Meanwhile, the irises that are flowering in the foreground of the photo sprouted from some left-over irises that we dug up many years ago. I left the rhizomes in a pile on the ground right where you see them and have done nothing to them. They thrive. The ones in the bed that I weeded and thinned out are just standing there. No flowers.
My conclusion: I must dig up that bed of irises, divide them, then leave them on the ground, right where they are, if I want them to look better. I guess irises like to be disturbed and divided periodically, then left for dead in order to thrive.