I would use the mower, too, but it makes too many "small pieces", that are too hard to rake up. I "weed whack", instead. I really don't want tree leaves, trash, or even the dead, collapsed iris leaves left in my beds, over the winter. IMHO, the iris are hardy enough to withstand any cold the Illinois winter will ever throw at them.....but they WON'T stand being under a layer of "leaf mulch". If the "mulch "could go on AFTER the ground freezes, and come OFF in early spring, it might be "OK", but, going on before freeze up is inviting rot, vermin, insects, fungus, and mold......and I don't want any of those ! Over the years I have tried various methods of "winter culture"......from" doing nothing", to 'doing everything', and for me, weed whacking the fans, raking out the big pieces, using a leaf blower to get the "small pieces, and then burning over the beds, gives the best results. We all grow the "same' iris, but our climates, soils, and time constraints are all different, so we have to find out what works in our garden!