4 yrs ago We retired and moved from zone 5b in WI to 4 here in La Crescent MN. A Butterfly bush we moved successfully does not show any signs of life and My husband says ” You say this every year!” I am ready to replace with a Fragrant Cloud rose. Verdict ? Wait for life or remove now.
Butterfly Bush die back to the ground in our zone 4 gardens.
It can take awhile for it to grow from the roots.
As a zone 4 gardener myself, I have to say with the below normal winters, I have lost all my Butterfly Bushes.
They are quite inexpensive and readily available, so if yours does not come back plant another.
Adding some extra mulch in the winter can help.
Here are some links with more information.