could the "new" USDA zone map be optimistic?
The new USDA zone map that came out a few years back re-zoned large areas, mostly to a half zone warmer, esp in parts of the Midwest & West.
However, I'm not fully convinced it is accurate long-term. This isn't intended to be an argument about climate change, as yes, there is a warming trend (regardless of why it is happening).
Here in the Mid-Atlantic it doesn't appear all that much has changed-my zip code comes up as 7a on both the old and new map (although 6b was, and still is, less than 10 miles west).
My hometown of Akron, Ohio though is different - it went from 5b to 6a, and a small spot downtown is now rated 6b!
The 1990 map was based on temperatures from 1974 to 1986, the new map from 1976 to 2005.
Now, I realize zones are based on AVERAGE lowest winter temperature, meaning some years will be higher, and some lower. However, using Akron as an example, the ALL TIME RECORD LOW as well as the second and third place records all occured DURING the 1976 - 2005 time period! In '94 it was -25F, in '85 -24F, and I think it was -19F in '77 as well as a couple other similar temps in '84.
However, since the other winter lows were that much milder, the average lowest temp for the winter at Akron-Canton airport in that time period was about -8F or so (I actually tried to calculate it). So hence the zone 6a rating.
It seems that while average overall temps might be a few degrees warmer, the coldest it could possibly get didn't change much during the 1976 - 2005 timeframe, so I would proceed with caution.
Of course, hardiness isn't just a straight number at which the plant will die, it depends on duration of the cold, the temperatures before and after the cold, etc, etc.
Still, I think planting according to the new map is an "at your own risk" proposition at this point.