Zones are such a vague guideline. One can have various zones in a small suburban yard, depending on the hardscape, mostly. There can be vast differences in soil temp as well as dry spots vs. wetter areas. Then there's factors mentioned above such as rainfall, elevation, heat/humidity that affect various plants in different ways. A plant has to not only be winter hardy to live here, but it also has to be able to bake at 100 deg & 90+% humidity for weeks at a time, and endure periods of daily rain, or no rain for 6 weeks sometimes.
Also think drainage doesn't get enough credit. If you can keep the ground from freezing *and* from being muddy muck, you can grow so many more things in Z8. Bad drainage + cold temps = death via rot. Good drainage + cold = lots of survivors from Z9/10 plants. A heavy leaf cover bumps the chances up significantly.
There also seems to be some confusion as one gets into the warmer zones (9/10) that I've noticed after moving to where reading such discussions is relevant, then reading 7 years' worth of discussions from people in FL, in particular. Plants that would be evergreen without frost aren't considered hardy by a lot of gardeners down there, they call them 'root hardy,' 'almost killed,' 'comes back after dying for winter,' and stuff like that. Being from OH, anything that's not truly dead is welcome in a garden, perfectly normal, but disappearing for winter is a fairly foreign concept/unacceptable to many life-long gardeners in tropical/near-tropical areas. I *like* when a lot of plants disappear for a few months. Gives me a chance to see so much more clearly, and pull any baby trees especially that were hidden. An easy time to pile on tons of leaves/compost/other OM.
When I first moved to AL, I asked if Persian shield was hardy and got some really weird answers. (They are, BTW, never had one killed over winter, even from the plunge into the teens and ice storm last winter. About a dozen plants, all still alive this spring)
