What is intereeting to me (and this is in no way a āclimate change denialā thing), is the REALLY OLD zone maps were warmer than the 1990 map. At least in Ohio where I grew up.
I recall a very old map in the 80sā¦they had Zone 7 almost touching Portsmouth, Ohio. Then the 1990 map put most of Ohio in 5 or 6. The 2005 map now puts almost all of Ohio in 6. So they were warmer, then colder, then warmer.
Ok. So I donāt know about the āreally oldā map. But per the USDA, the 1990 map only included data from ā74-ā86. They were some very cold winters during that time frame in the mid western and eastern US. I believe out West had some doozies during that time as well. In fact, save for a couple years in the ā60s, that was probably the coldest set of years in the 20th century you couldāve chosen as far as ultimate winter lows.
The ā05 map uses ā74 - 2005.
https://planthardiness.ars.usda.gov/PHZMWeb/AboutWhatsNew.aspx