Plant Hardiness Zones and Chill Hours
There are two opposite, but essential ways of telling if a particular plant variety will grow well in your part of the country, cold and cold. While at first that may sound like nonsense, it really isn't.
Plant Hardiness Zones are an approximation of the maximum amount of cold weather a plant can tolerate over Winter, while Chill Hours (sometimes called Chill Units) are an approximation of how many hours of weather between 32 degrees and 45 degrees (F) a plant requires to properly go dormant so it can wake up and blossom and/or set fruit. For plants to thrive in your geographical area, it may require matching optimum Plant Hardiness Zone compatibility with the optimum Chill Hours.
Let's start with Plant Hardiness Zones. The USDA released a new Plant Hardiness Zone chart in February of 2012 that tries to account for how well a particular plant will do when grown in a particular area by averaging out the minimum temperatures across the country into thirteen bands with a ten degree spread in temperatures. Each zone is further broken up into two zones (the a & b parts). The a part of a zone will be the cooler of the two.
You can determine your Plant Hardiness Zone using Plant Me Green's online lookup at Plant Me Green Plant Hardiness Zone Lookup. All you have to do is enter your zip code, and it will tell you what zone you're in.
Say you live in Nashville, TN and your zip code is 37212. The lookup will tell you that you are in zone 7a, which means that on average your winter lows will range from zero to five degrees Fahrenheit. So if a gardening guide says a particular plant will do best in zones 5b - 8a, you usually don't have anything to worry about if you're in zone 7.
Keep in mind that no two years are the same weatherwise, and you may get some years that are considerably colder or warmer than average. Especially if you live near the edge of a Plant Hardiness Zone, you'll want to be alert to the need of taking some precautions if some of your plants are near the edge of their range. You may want to consider covering plants during cold snaps or bringing them indoors.
Some plants like fruit trees and flowers like tulips require a minimum number of chill hours to thrive. This process of setting a plant's dormancy framework is called vernalization (which comes from the Latin vernus, meaning Spring). It is a process where the plant kind of sets its internal alarm clock to wake up in the Spring instead of Summer or Fall, so that it can take advantage of the warm weather to blossom, set fruit, and finish the seed cycle before the coming of another Winter's nap.
You may be surprised to find that the middle of the country around Nashville, Kentucky, Virginia, etc. get some of the biggest Chill Hour counts in the country because farther North, the temperature may fall below freezing and stay there for months at a time. Only time spent between 32 and 45 degrees (F) count.
There are a number of ways to calculate Chill Hours, and all of them are an approximation, so you want to make sure any variety of fruit you're planning to fits within the range your area gets annually. If you pick the wrong variety of fruit tree for your area, you may be disappointed because if the tree doesn't get the needed Chill Hours, it may never flower or set fruit. Your county Extension Agent can advise you about Chill Hour averages for your area.
Fortunately, botanists are constantly working on creating new varieties of plants that have different Chill Hour requirements, so in recent years varieties have been developed that let folks safely grow apples as far south as Florida and Texas. In fact, Israeli research has resulted in the Apple Ein Shemer variety that needs only approximately 300 chill hours, making it a very good choice for Southern gardeners who can finally get good apples to grow as far South as Zone 9.
As always, you should take advantage of the services of your county's Extension Agent, who will be able to give you information about plant hardiness in your area, and recommendations on fruit trees and other plants that do well in your area because you get just the right combination of Plant Hardiness Zone and Chill Hours.