I cannot answer regarding it's vigor, as my soil is acid and sandy and some plants do not act typically here, but I am unsure as to it's hardyness in zone 6. It almost died here with our last Winter, and I am in Zone 7b. I took it out of the exposed pot it was in, gave it fresh soil in a new pot. It has achieved 6 feet since Spring. I am planning to Winter it in a sheltered place against the South side of the house.