Plant Hardiness Zones Q&A

Q. How are hardiness zones determined?

A. Hardiness zones are based on the average annual minimum temperature in a given area, with USDA Zone 1 being the coldest at minus 50 degrees F and USDA Zone 11 the warmest at above 40 degrees F.

Each of the map's colored zones is separated by 10 degrees and broken down into subzones A and B, separated by five degrees. To use the map, simply locate your city and then match up the corresponding color on the map with that on the legend. For example, Tulsa, Okla., is located in Zone 6b and so are sections of southwestern Washington and Long Island, N.Y. One might conclude the same plants can be grown in these three places, but that's not always the case.

The zone map is "flawed" in that it only takes into account 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 hemlocks, for example, are hardy in USDA Zones 4, 5 and 6, and are relatively easy to grow in New York's Zone 6 climate, they're very difficult to grow in the Tulsa, Okla., Zone 6 climate due to the extreme heat and humidity. Of course, that doesn't necessarily keep steadfast gardeners from trying.

Figure B

That's why there's another map—the American Horticultural Society's heat zone map (figure B)—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 those of you 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 root-ball. 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.

Figure C

You can also create warmer microclimates that will allow you to grow plants one hardiness zone south of your own. Placing plants against a south-facing brick or stone wall is one way of doing so, as is the use of stone mulch (figure C). Microclimates may exist due to the buffering effects of water, elevation changes, wind, rainfall and so on, which is why another organization, Sunset Publishing Corporation, has a map that breaks the U.S. into 45 different zones based on these factors.

Q. Is a plant in a pot less hardy than if it were planted in the ground?

A. Great question—the answer is yes. Plants in the ground are buffered by the soil, which rarely drops below freezing. But plants in pots are completely exposed to the elements and soil temperatures can easily drop well below freezing. Bubble wrap placed around pots will buy you a few degrees of protection, but your best bet is to actually bury potted plants in the ground prior to the arrival of freezing temperatures. You can remove them the following spring. This assumes that the plant in the pot is already rated hardy to your zone.

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).

Q. Who actually determines how a hardy a plant is?

A. In most cases the ratings are based on years of observation, but believe it or not, in some cases the rating is little more than an educated guess.