The best time is in the dead of winter, when the plant is dormant, and doesn't know that anything is happening.

I was told early spring. Late Feb., early March.

Dead of winter in a zone 7 bog garden would be a bit impractical. I do it when we have a good long thaw here on Long Island. This year I waited 'til late April and they are all doing fine. You can probably do it any time in spring it will still be nice and cool for a while. Just make sure you get a nice clean cut or break in the rhizome between growth points and that each growth point has plenty of its own roots intact. Make sure the roots are long enough to reach well into the water table in your bog garden.