It should be hardy, but there are so many micro-climates in these zones. I don't mulch mine and it does fine down to the lower teens. If I lived in Springfield, I would heavily much my plants and if you only have a few rhizomes or have it only in pots, I would dig it up and store it in a dry place, with temperatures above 20F. Living in zone 8b, it is really hard for me to give definitive answers for those in zones or sub-zones other than mine.
