I like how you can shift back and forth between the 2012 and 2023 maps on your linked NPR page. As they note, half of the locations have changed. A 30 year minimum average (1 January 1991 through 2020) ...
The basis for the zone designation is the average annual minimum but if you decide to go with it and the information on a particular species/variety, does that mean that your "hardy to zone 6" plant will not survive if the temperature drops below -10°f ? Or, that it has a 50% chance of survival every Winter in zone 6 ? I think that there was the word "thrive" in one of the pages that I read. Okay, good.
Averages ... Our zone changed in 2012 but not this year. But, what does it mean when our lowest temperature, 1991 to 2020, was
-24°f and the warmest minimum was
+13°f if we compare that to:
Portland Maine with -16°f to +1°f ? How about
St Paul, Alaska (in the Bering Sea) with it's -16°f and warmest minimum of +11°f ?
Equivalent?
Steve
Perhaps for a few days yet, the National Gardening Association has the 2012 map and the 1990 map
LINK.