Dear PBS friends, I am surprised that no one has mentioned H. x. johnsonii. I have always read and considered this the hardiest species (actually an old hybrid). I have grown it here in my Zone 5/6 garden in the ground for years. It is planted quite deep (for a Hipp) and in not very protected spots. It is not a great bloomer or vigorous, but it does survive, grow and bloom. I recall seeing beds of this in Dallas where it was larger, more floriferous and beautiful. Have not seen this sold in local garden centers, but it is fairly common in Wichita KS garden centers where it is modestly priced. Best Jim W.