Iris fans have a curve to them, even it is slight on some, curving towards the rhizome. As iris grow, they grow towards the inside of the curve made by the fan If you hold up a rhizome with the fan of leaves attached, the fan is at the back of the rhizome and curves slightly towards the side where the rhizome is. So you want to plant them with the outside of the curved fans facing each other, the rhizomes away from each other, so they won't grow into each other. As said before bearded iris can be separated without digging them all up as the tops of the rhizome should be at or above soil level and easiily visible, and thus easy to spot a dividng spot between rhizome clumps. Trim the leaves back, sever the rhizome and dig under the rhizome lifting as many roots as possible. While the rhizome is up it's a good time to look for any rotted parts or evidence of borere damage. Cut off or cut out the damged areas before replanting. I actually only dig the whole clump and reset all as often the rest could use the extra room afforded by thinning and resetting. Here in NJ the timne to do this is late Aug and early Sept as it gives the plant a chance to re-establish before frost. At least that's how I was taught!
