I love mixing lots of other perennials among Irises too, but have to really control my inclination to over crowd my Irises with other things. I've just spent the last couple of days digging creeping perennials out from among Iris clumps- and that was a real pain. Once they've formed clumps, it's easy to just mulch right up to the clump, but for newly planted rhizomes I find the mounding method sound. Then you can mulch all around the mound. I pot my new rhizomes when I receive them, and then start planting them out about 3 weeks later (when I see roots growing from the drainage holes). When I planted them, I left the upper inch of soil above ground to create a little mound, and mulched right up to it.
