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[mauch1@aol.com]

Just so we're all clear

 

The statistic used by the USDA is the average annual minimum temperature

the USDA means the average, over ten or more years, of the very lowest temperature (the absolute minimum) observed for each year for each meteorological station. 

 

Megrette is correct that you can have a winter that exceeds your zone, but most of us would take a

2.5% bet.  Of course, if you want to be safe, you can always plant stuff that is hardy to a zone colder, but that gets more difficult the lower your zone. 

 

And those proponents of the the Sunset zones are right too, I can think of two plants (getting away from fruit) namely delphiniums and Primulus viallii, that are USDA zone hardy, but don't like the warm humid summers here on the east coast(EC) so don't thrive for long (neither has ever survived more than two summers for me).

 

Of course there's the famous Cox Orange Pippin.  Wonderful apple, but to truly thrive and produce great apples, it prefers a maritime climate (i.e. England)  (though I did know a amateur grower who said it did fine in my area when grafted on the North side of a tree).

 

But to be honest, at least here on the EC, most plants are not rated for the Sunset zones and it seems complicated, and most plants will do well enough if you attend to their needs based on the USDA zones. 

 

I haven't found the heat zone map useful at all yet.  Not enought plants are rated by it for it to be very useful.

 

Chris Mauchline
SE PA, zone 6
40° 5' N 75° 51'W
~650 ft elevation