The USDA hardiness zones are based on temperature information compiled over an extended period, in most cases, at least the last 30 years. And that is based on the average annual extreme minimum temperature. So that is taking into consideration the coldest weather you might experience in the winter - those 20's or even teens (or lower!) that come around every so often. That is not to say this is a routine occurrence - only that plants listed to that zone designation should be able to withstand the colder conditions that may come around only periodically. It is the same here in the Puget Sound area :-)

But the USDA zones are intended only as guidelines. Every garden has individual microclimates, both colder and warmer, that can affect plant hardiness. Because I am at sea level and within a quarter mile of the Sound, my microclimate tends to be even milder than the typical zone 8 designation, as a much as a full zone higher! So I routinely push the limits and grow a lot of zone 9 (and sometimes higher) plants with the understanding that these can be adversely affected by more 'typical' winter weather.

btw, if you doubt global warming, just look at historical winter lows over the years. Winters in the 50's and 60's presented much colder average temps than do more recent winters :-) At least here in the PNW.

Thank you. Your bolded 'extreme' helped, as the other sources specified only minimum averages. So 8a is right for me.

I am another zone pusher, though at a much colder level. I've found that I can push zones more for perennials and some shrubs which are safely hidden under snow during the coldest part of most winters, while I have to be more careful about larger woody plants. Half-dead trees after a particularly cold winter are pretty ugly.