Up here in the great tundra of Minnesota, my choice of perennial plants is severally limited. Zone 4. Minimum temp -30°F.
I can see how a plant that is native to Florida couldn’t withstand the cold winters here, but what is different between a plant that is hardy to 0°F and one that is hardy to -30°F? Both are frozen all winter. Both have lost all their leaves (ok, I’m ignoring conifers). Both are currently buried in a snow drift.
I don’t know if lowest temperature reached is the only factor. Amount of sunlight during the day plus the duration of the cold weather season also come into play.
I know some plants produce a natural antifreeze that lowers the freezing point of the water within their cells, allowing them to remain unfrozen and undamaged. And while I was looking for more information I found this .PDF file that has some more answers for your question. Link
One possible answer is the concept of “degree days” for describing the accumulative effects of temperature upon germination or other stages of plant development.
This idea may be adapted to really any factor that influences habitat suitability. Extremes in temperature (acute tolerance) may actually matter very little, while the amount of time for which an unsuitable temperature threshold (whether positive or negative) is exceeded may be more important.
Each individual plant variety undoubtedly has its own specific habitat requirements for things such as sun-hours, temperature-days, and bee-encounters, plant hardiness zones may just be a practical way of simplifying this information.
It can also be a factor how deep the roots grow, and how the plant goes dormant: some plants are damaged permanently if any of their tissue freezes; some plants don’t “mind” if the above-ground parts freeze, because they “die back” to the ground when the go dormant. As long as the roots grow deeply enough to be below the freezing line, the plant will regrow the following spring. Some plants, what’s more, can survive even if their roots are above the freezing line. There’s no one answer; different plants have different survival strategies. A plant tends to adapt to the parameters of its environment; don’t forget that many cold-zone plants wouldn’t survive in the tropics, because they’ve adapted an annual cycle that includes a winter dormancy period, without which they don’t grow as well in the spring.
There are a lot of factors influencing plant hardiness, including dampness/sogginess of soil (generally bad), duration of cold, the amount of heat the proceeding growing season (usually good - some plants are hardier in interior locations than coastal ones, even if the coastal areas have milder winters) and so on. Much of this has not been studied in detail for individual plants.
It probably doesn’t make a lot of difference whether it’s 30 below or zero as long as the plant is buried under a thick layer of insulating snow (or mulch). For example, I’ve overwintered a flowering sage that generally is regarded as hardy only to zone 7 at best (typical winter lows of 0 to 10) through a 20 below cold snap, because it was insulated by snow.
Also, zone hardiness is based on average winter lows, not the lowest temp you’d get in any one season. A plant rated as hardy to 0 might very well make it through the occasional winter where it gets to 20 below or colder.
Tulips, for example. Tulips in the south (zones 7/8) tend to fizzle after the first year because they don’t get enough cold days in winter to set flower buds. A lot of southern gardeners will dig up their tulip bulbs after the leaves wilt (late spring/early summer) and put them in the refrigerator until October, when they bring them out and replant them for flowering in spring.