I agree with Greene - the USDA Hardiness Zone is a very general guideline and your micro-climate does vary a lot.
Also, the USDA Hardiness Zone only states the average coldest temperature reached in the last (??) 50 or 100 years for some weather station somewhat near you. I bet the WAY they do the average and the weather station's exposure make a big difference.
Statistics is a way of making fuzzy things sound much more precise than they are.
The thing that I think makes Zones almost useless is just the fact that they ARE an average. Even if there were no microclimates and every spot in a ZIP Code zone had IDENTICAL temperatures and wind, what does it really mean to me that HALF of the time, the temperature WILL go lower than the Zone's "limit" says? If that were the whole story, would I plan to grow things that I would EXPECT to die in half of all winters?
If the Zones WERE precise, and predicted a low temperature that was only exceeded on 10% or 5% of years, it might be a more useful guide. But it isn't even that.
Another drawback is that it records the very coldest temperature reached, even if it was only reached for 30 minutes one night that year. I don't think most plants are killed instantly by few minutes of temperature! Three cold nights and days in a row might do the deed.
A hard cold snap after a deep blanket of snow is not a big deal. Snow insulates. The same cold in a bare yard will freeze soil much deeper and kill lots more plants.
Second to last, cold still air is "meh" ... maybe it'll kill something and maybe it won't. Cold air howling around a plant at 30 MPH will freeze it and blow its shards away (so to speak).
Last, for most plants it matters HUGELY whether the cold snap comes after a gradual fall in temperatures over several weeks in mid-winter, or if the mercury drops like a rock in the middle of an otherwise mild fall. Plants that aren't ready for winter are probably 5 times as vulnerable to cold as fully vernalized plants that have made every adaptation to winter they know how to make.
Long story short, nature is almost always more complicated than scientific theories can easily express or predict. And anything that has been boiled down to just one little number has NOT captured the whole story.
At least you have a pretty good idea that plants "listed for" Zone 6 are more likely to survive in any given spot than a plant "listed for" Zone 5. If you almost never lose a Zone 5 plant, try some Zone 6s.
And you may find some genus of plants that "most people" can only grow down to Zone 7, but YOU have the mulch or shelter or drainage or TLC that THAT one genus needs to survive YOUR Zone 5.
Although gardening is all about experimenting, it isn't likely to become a science. There are too many variables, and science is only good at generalizations. Every garden and every gardener is unique, beyond science's ability to reduce to a number.
And that's GOOD!
