Fall through winter would be my choice. Cut it back, then dig it up.

i imagine that fall would work ok in your zone. i moved one once i saw spring growth (late) and cut the plant back. it is doing fine and i put it in an area that is not cultivated beyond being mulched with compost (no watering, though i watered it for about a week after moving it).

It's been my experience that you would have a hard time killing the stuff. I vote either for the fall after it has finished blooming and needs to be cut back, or in the EARLY spring. We have moved it while it was blooming, before it bloomed and in the dead of winter. Nothing seemed to bother it. It may be that it just likes us and our yard. Good Luck!
Froggy