Answer: According to your zip code you are gardening in zone 5A or the coldest part of zone 5. Depending on your microclimate it could be as cold as zone 4.
It is difficult to overwinter perennials in containers. They need to be chilled, but the roots are not well insulated the way they would be in the ground. The plants also seem to be subject to fungal problems when stored. You will need a place that is above freezing but below about 40 degrees. Water just enough to keep the soil from drying out. Wait to bring them into storage until they have gone dormant.
In my experience, it would be better to plant them in a holding bed than try to store them. It is best to plant them 8 to 10 weeks before the ground is expected to freeze up. This allows them time to root so they are less likely to frost heave. Then transplant them to their permanent locations in the spring. Or, it's probably even better to wait and buy them in the spring when you have the planting area prepared.
I hope this helps!
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