What you cut, depends on what you want to do. If you want to thin out that clump of iris, dig it out, cut off the old, diseased, bug infested, knarly roots. Cut the top about 6-8 inches tall in an inverted V and then soak in a bleach/water bath for about 1/2 hour to kill any disease/ bacteria that could be on the roots. Then replant however you want and enjoy. This should be done about every 5-7 years with iris to keep down the borers and bacteria.