The only western Agastache likely to survive zone 6a winters is rupestris and maybe its hybrid Firebird although one must keep in mind that there are winters where temperatures stay above average and then all bets are off. Rupestris also survives summer droughts much better than other western Agastaches. Some of my plants are 10 years old now. Size varies according to conditions. I have some in extremely dry barren sand that don't reach 2 feet and a few hundred yards away other plants that are shaded part of the day in richer sandy soil that reach almost 4 feet. Rupestris throws off lots of seedlings, often crossing with other species. Those can be fun to leave where they are since some may also turn out to be really hardy too.
