Well this one is said to be hardy to Minnesota and was selected by a Minnesota nursery. Flowers are pink or blue depending on soil ph and can get 8-10 inch clusters. Its success to its ability to bloom on new wood as well as old.
 
Many Hydrangeas are perfectly hardy in the north, but late frosts freeze out the buds and they never come into bloom. By repeating over such a long period, this one avoids this much like the Japanese mountain hydrangeas H. serrata.
 
Zones 4-9.