Greene, loved your story!!
I'm not a fan of bumping zone numbers up at all because all it takes is 1 unusually harsh winter to wipe out some/most/all of the marginally hardy plants. Plants don't give a hoot about averages. And don't forget, these averages are based on less than 200 years of weather data in most areas. If one looks at the dates of record low temps, there's no trend upward, many of them happened in recent years. There's no reason to bump zones except a financial one.
There is a trend for warming temps in general though, which is said to be expected to produce more wild extremes, of both heat and cold. IDK if that's true, but if it is, relying on a bumped zone number will not have good results.
Agreed, the diff between z 5/6 may be the least distinct on the scale. Moving to Z7 is where, if the ground freezes at all, it's not deeply or for very long. Going below Z5 is approaching arctic conditions. Given a choice, I would stick with being in Z5 vs. Z6 because the snow cover is more reliable. In the exact same temps, a winter with much more snow cover would be much less likely to damage plants than one during which the ground is bare most of the time.
