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Take a look at the USDA Plant Hardiness Zone Map, either in the color section of this book or online by searching for it. It’s available on many different garden sites. Notice that Zone 1, located at the top or northernmost part of the map, is coldest; Zone 11 is at the bottom, or southernmost part, and is warmest. In terms of hardiness zones established in the USA, Zones 1 and 11 represent the extremes. The bulk of the United States, though, fits into Zones 5, 6, 7, and 8. Originally, the zones were conceived to be 10°F apart.

As you study the map or chart, say you find that you live in USDA Zone 7. So you determine that this means

 You should be able to grow any tree, shrub, or perennial labeled “hardy to USDA Zone 7.”

 You probably can’t grow plants that are less cold-hardy, such as Zone 8 or 9 ones — your colder winters may harm or kill them.

 You can grow plants labeled for farther north, even more cold-tolerant ones said to be “hardy to Zone 6 or 5.”

However, every rule has an exception. Most gardeners can stray one, maybe two USDA zones from their own when making plant choices and the gamble will pay off. See “Zoning Out: Breaking the Zone and Growing Season Rules,” later in the chapter.

You often see a plant’s projected USDA Hardiness Zones expressed as a range. For instance, most clematis hybrids are said to be “hardy in Zones 3 to 8.” This statement means anyone gardening in Zones 3, 4, 5, 6, 7, or 8 ought to be able to grow one; the plant should survive your winters.

Basic zones are based on the average annual minimum temperature — in other words, as cold as winter gets. Thus, in the USDA Plant Hardiness Zone Map, Zone 6’s lowest winter temperature (on average) is –10°F (–12°C).

Why base zone maps on cold temperatures? After all, other things doom plants, such as high heat, lack of water, too much water, too much or too little sun, and the wrong soil. But over the centuries of gardening, people found that cold is a better predictor of what will survive than any of those other (admittedly important) factors.