A rule of thumb, that I use, Liz...is two weeks after the last flower. That's about the length of time it would take for a seed pod to form (if you left it, to form, of course !), and, once the pod has inflated, the roots on the spent" mother" rhizome start to die, and the new roots are beginning to start growth on the increases.....essentially , the plant is 'summer dormant" at that time, and, the way I see it, that's the time to "move things around"...BEFORE the increase roots have grown very long, and begun to grow the "root hairs".....after that happens, no matter how carefully I dig, I'll damage those new roots.
