One of the factors most important to plant survival is their hardiness in the face of cold temperatures. For each species of plant there is a temperature below which it typically does not survive. If you attempt to grow it, and the temperature gets below this level, it’ll likely die. So it would be handy to know at a glance if temperatures in your area ever get down to where you are running a risk of certain plants dying in a freeze, so you’ll know what to plant and what not to plant.
This is the basic concept behind growing zones. Years ago, botanists and horticulturists for the Arnold Arboretum of Harvard University and the United States Department of Agriculture (USDA) did all the detailed research to compile for every area of the country the average coldest temperature of the year. (Soon similar projects were done in Canada, and eventually many other countries, such as China and Australia).
They then divided the data into eleven “zones.” Any place where the lowest temperature of the year was typically below -50 degrees F (certain places in Alaska, for instance) was designated Zone 1. Any place where the lowest temperature of the year is generally between -50 and -40 was designated Zone 2. And so on in 10 degree increments like that, up to Zone 11, which is all the places (such as parts of Hawaii) where even the coldest temperature of the year is typically over 40.
Subsequently, Zones 2 through 10 were subdivided into 5 degree increments (2a, 2b, 3a, 3b, etc.), increasing the number of zones from 11 to 20.
Don’t think of the zones as contiguous. (A glance at this USDA map will show this.) There might be a spot in Nevada and another in West Virginia that are in the same zone. It just means they have approximately the same lowest temperature each winter.
So, first you ascertain what zone you’re in. Then you check whatever you’re considering planting to make sure it can handle temperatures that go as low as the lowest expected temperatures in that zone.
Generally you don’t even have to do the figuring; you just have to know your zone. Seed packets and gardening websites and such, instead of saying “Avoid planting in areas where the temperature can go below zero” will say something like “Recommended only for Zone 7 and above.”
Growing zones are certainly a helpful concept, but bear in mind they only address one factor. Knowing that you’re in a certain growing zone can tell you that the temperatures will or will not likely go low enough to damage a certain type of plant you’re considering growing, but they tell you nothing about the other factors that determine whether that type of plant will thrive.
Maybe because you’re in Zone 6b, cold is unlikely to kill off a certain type of plant, but what of precipitation, soil type, pests, etc.? There could be two locations, both in 6b, where one is very dry with sandy soil, and one is very wet with clay soil. One 6b area might be ideal for what you want to grow; the other might be terrible.
So think of growing zones just as a convenient source of information concerning one, and only one, of multiple factors relevant to your gardening decisions.
(A handy site for more information on growing zones, in addition to the USDA site above, is “Plant Hardiness Zone Map.”)