I do this and it works good for me. I don't have to dig them just pop out the old rhizomes or the ones that are to thick. I try and stay on top of this every year so I still get blooms every year from the undisturbed iris. I usually use the fork used for weeding weeds like dandilions, but get a good one the cheap ones bend easy and I keep it sharp. With the fork on the end I can find the spot where the rhizomes join and just push it down through cutting them apart then pop out the ones I want gone. This pic is a flimsy one but I couldn't find a pic of the stainless one I use.
