For gardeners who want to optimize their results, the Zone System is indispensable.

The Zone System was created by the USDA (U.S. Dept of Agriculture) and divides the country into eleven zones. The divisions are based on the minimum average winter temperature with zone 1 the coldest, zone 11 the warmest. Maps of the zone system are readily available online and they’re very helpful for pinpointing which zone you’re in.

The USDA Zone System isn’t the only one around, however. Sunset, the famed gardening publication company, developed its own system, and it is even more extensive. The Sunset Western Climate zone system has 24 divisions for the western portion of North America, with additional zones (A1-A3) for Alaska and Hawaii (H1-H3).

 [The Zones are:
 Cold and Snowy: 1-3
 Rainy Northwest Zones:4-6
 Northern and Interior-alley California Zones: 7-9, 14-17
 Southern California: 18-24
 Southwest Desert Zones: 10-13]

Sunset uses more than just minimum winter temperatures. Its algorithm also includes summer temperatures, annual rainfall, elevation, humidity and other climate factors such as marine and mountain geography. But that’s not all. The Sunset system factors in the length of the growing season - by examining the average number of days between the beginning of Spring and the first frost of Fall.

Even with all this extensive information, zones can be divided into micro-climates, for those who want to narrow down their area as much as possible. Some areas like the panhandle of Northern Idaho are protected from the harshest Winter weather by its unique configuration of mountains. Just on the other side, Montana has much more severe weather.

Mountain slopes can channel warm or cold air into an area in the same way they keep other influences out. In the wine growing region of Southern California, for example, the narrow gap in which most of the vineyards sit, traps hot summer air. At the same time the valleys get a cool breeze in the evening from the nearby Pacific Ocean. The combination of these (and other) factors creates a zone that’s perfect for growing wine grapes.

Using micro-climate information, it’s possible to grow some plants that otherwise you might suspect wouldn’t thrive (if you considered only the zone number). There are limits, however. Lemon trees will still do better in Southern California than they do in Eastern Washington.

Some experimentation will always be needed to determine which plants will grow best in your particular environment. But using the zone system as a starting guide will help you avoid costly mistakes. You can grow a plant that’s rated, say, as hardy up to Zone 5. But if you live in a Zone 2 you are almost certainly wasting your time and money in that instance.

Since the hardiness of a plant is largely genetic, there is little you can do to adjust the plant, apart from hybridization. But selecting which plants thrive best in which zones is a straightforward exercise. There’s ample information online to guide you.

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