Answer: Butterfly bushes are not always reliably winter hardy in zone 5, and in many cases will die back all the way to the ground during the winter even in zone 6. Usually, this is not a problem because they will regrow and then bloom on the new wood. And if they don't make it, most gardeners like them well enough that they are willing to replace their butterfly bush each spring. But I can understand wanting to try to keep it.
You will have to try to protect the "trunk" and the stems at the top. You might be able to do it by moving it into a shed or garage or unheated basement for the winter where it can be kept quite cold (45 maximum) but above freezing -- if it is in a container this would work the best as it also allows you to protect the roots from excessive cold as well as from oscillating temperatures. Water as needed to keep the soil from drying out completely, it should be just barely damp, never wet. If you do not have a storage place, you might try sinking the container in the ground, applying a generous layer of mulch over the roots, and then wrapping as described below.
If it is planted in the ground, you might be able to wrap the branches and trunk in burlap and stuff that with dry straw or dry oak leaves for insulation, then cover the top with plastic sheeting or tarp to keep it dry. Leave gaps in the plastic so it can breathe and prevent condensation building up inside. Also apply a generous mulch over the roots.
Be patient with it next spring, these usually begin to grow fairly late once the soil has begun to warm up. Best of luck with your butterfly bush!
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