Well, they spread by rhizome, right? So if you leave them too many years, they kind of crowd each other out and none of them are quite as hardy because they're competing with each other. So when they go dormant in the cold season (yes, they won't grow well until Spring, you had that right), you can dig up the massive entangled mess of roots, separate them out, and then replant them further apart. Then a few years later you can do the same again... .and gradually have the whole place taken over with pretty flowers instead of frustrating, needs-mowing grass. Plus, it means you can just get 2 or 3 plants of any given color or variety, and a few years later they're ready to split so you can swap the neat colors with other people who are also ready to split theirs, and soon you have a profusion of types of irises, a whole panoply of heritage varieties and variegated colors and tiny half-wild ones and huge double-petaled and everything in between. I love it so much
