Plant Hardiness Zones

Learn more about the USDA hardiness zone map and how to use it.
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The USDA hardiness zone map was revised in 2012 to reflect warming temperatures. There are now 13 gardening zones, and each zone has inched northward. For many gardeners, this means about a half-zone change. If you were used to being in USDA Zone 6b, with an average wintertime low of -5 degrees to 0 F, you're probably now solidly in Zone 7a, which typically registers a low of 0 to 5 degrees.

A half-zone change can make a big difference in your garden. You may be able to grow plants that used to be too tender for your region; on the other hand, a plant that requires a certain amount of cold in the winter may not make it in your yard. Check plant tags before you buy; the zone information is spelled out as a range — for example, USDA Zones 5 to 8.

Q.
How are hardiness zones determined?

A. Hardiness zones are based on the average annual minimum temperature in a given area. Each of the map's colored zones is separated by 10 degrees and divided into subzones A and B, separated by five degrees. To determine your gardening zone, type in your ZIP code at the U.S. Dept. of Agriculture website, or for more precision, use the interactive map there that lets you click down to within a half mile of your home.

The shortcoming of the zone map is that it takes into account only the cold tolerance of plants. The farther south you go, the less dependable it gets because it doesn't take into account the heat and humidity of the South. So while the perennial basket-of-gold, for example is hardy in USDA Zones 8, it can't stand up to the heat and humidity of the Southeast, but it grows well in Zone 8 in the Pacific Northwest.

That's why there's another map — the American Horticultural Society's heat zone map that addresses this very issue, namely the heat tolerance of plants, and rates plants according to their ability to withstand excessive heat. When used together, the two maps will help gardeners in the southern third of the country determine both the cold and heat tolerance of plants.

The shortcoming of the zone map is that it takes into account only the cold tolerance of plants. The farther south you go, the less dependable it gets because it doesn't take into account the heat and humidity of the South. So while the perennial basket-of-gold, for example is hardy in USDA Zones 8, it can't stand up to the heat and humidity of the Southeast, but it grows well in Zone 8 in the Pacific Northwest.

That's why there's another map — the American Horticultural Society's heat zone map that addresses this very issue, namely the heat tolerance of plants, and rates plants according to their ability to withstand excessive heat. When used together, the two maps will help gardeners in the southern third of the country determine both the cold and heat tolerance of plants.

Q. Is it safe to grow plants one hardiness zone north or south of my zone?

A. This depends on where you're actually located within a given zone. For example, if you're at the northern border of USDA Zone 6, you can probably grow a number of Zone 5-hardy plants. Be prepared to mulch those plants heavily before the first hard freeze to protect the rootball. Likewise, if you're at the southern border of USDA Zone 6, you can probably grow a number of plants hardy to Zone 7, which is one hardiness zone south. Of course, if this area were to experience an extremely cold winter, it's possible that a few plants might be lost.

Q. What if a plant tag doesn't have a USDA hardiness zone number?

A. Typically, a plant tag will have a range of numbers — say, Zones 6 through 9 — rather than a single number. In many cases, the farther south you go in terms of the zones, chances are the more shade the plant will need. But if there's no number at all on the tag, chances are the plant is not hardy in your area and is intended to be grown as an annual (for one growing season).

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