Having looked at over 70 gardening sites, I am beginning to wonder if all you so called experts even know the various growing ZONES. I was checking various sites to find some small shrubs or perennial flowers to plant under our fir trees where the soil is very acid. I found only one site with one flower (Columbine) for our zone 2a-3. Sure makes one wonder about the intelligence of the gardening EXPERTS.
Perhaps you will find some useful information in one of the references listed in this article; https://www.ag.ndsu.edu/plantsciences/undergraduate/courses/docs365/ReferenceList.pdf