If you’ve ever dug a hole and stuck in a tomato plant, or flipped through a seed catalog, you’ve probably heard about USDA Plant Hardiness Zones. If you’re not in North America, you might have climate zones instead of hardiness zones, but the idea is the same.
Plant hardiness zones are simply a way of classifying plants and regions of the country so you can tell at a glance if you’ve made a good plant choice for your garden. Think of Garanimals for Mother Nature. If you live in zone six and your plant is tolerant of zone six, you have a match. If your plant is only tolerant to zone eight, you can try it, but it’ll be a bit like wearing stripes and plaid.
Hardiness zones were first defined in the 1920s, and were based on the average low temperature for a region. Each ten degree variance in temperature denoted a new zone, with zone one being the coldest, and zone ten the warmest. The USDA printed the first hardiness zone maps in 1960, and in 1990 they updated the map with more current data and added Mexico. They also added a zone eleven, and further subdivided the map into five degree increments by adding a suffix of either a or b to each zone.
No Map is Perfect
Because the current version of the hardiness zone map is based on temperature data collected from 1974 through 1986, the USDA is in the process of overhauling the map again, which means that some areas will find themselves in a whole new zone.
Another problem with the map is regional. If you live in the Midwest or in the eastern third of the United States, the zones are pretty clearly defined, looking something like crooked stripes drawn by a four year-old. In the western part of the country, though, mountainous regions have an effect on weather patterns that is not seen in the flatlands of the Midwest. Those rather orderly stripes become chaotic and difficult to decipher. So difficult, in fact, that Sunset Magazine has developed their own system of defining growing regions based on length of growing season, timing and amount of rainfall, winter lows, summer highs, and humidity. Sunset’s system is very successful in the West, where it is used even more frequently than the USDA system, but it fails in the eastern half of the country.
Don’t think that all you have to worry about is the average cold temperature of a zone, though. The average warm temperature can be a problem as well, especially with cold hardy plants like tulips. This led the American Horticultural Society to develop the AHS Plant Heat Zone Map. This map, like the plant hardiness zone map, divides the United States into regions, but instead of basing those regions on average cold temperature, they are based on the average number of days in which the temperature peaked above 86 degrees Fahrenheit.
Zones Are Really Just A Guide
Does all this mean that we can’t rely on hardiness zone maps at all? Of course not. But it may mean that they are best thought of as guides instead of laws.
Bananas, for example, grow best in zones eight through eleven. If you live in Atlanta and you plant a banana tree, you’ll probably still get bananas, even though you’re in zone seven. You might have to take a little extra care, especially if your weather turns unusually cold, but otherwise you shouldn’t have any problems with your banana tree.
If you try to grow bananas in Duluth, though, you will be disappointed. Zone 3b is just too far removed from the banana’s native climate.
The key to successful gardening is to know your zone, keep it in mind when planning your garden and shopping for seeds, but remember too that nothing is absolute. With reasonable precautions, you can enjoy many varieties of plants which are not native to your region. If you are in doubt, ask for advice from your local garden center or cooperative extension office.
