Hardiness zones remove some of the mystery of gardening by allowing a gardener to know at a glance whether a plant will survive winter cold in his or her part of the country. So they’re both empowering and budget-conscious, since these zones can help stop you from buying plants that won’t survive from year to year. If you’re not sure what these zones exactly mean, read on:

USDA Plant Hardiness Zone Map

This is the 2012 United States Department of Agriculture Plant Hardiness Zone Map.

Photo Credit: Courtesy of the USDA

When you see “Zones 3-7” on a plant tag, what does it mean?

This is the “hardiness zone,” developed by Harvard’s Arnold Arboretum in Massachusetts and the USDA, who put together guidelines to give gardeners an idea of how well their plants would survive the winter cold. The 1990 hardiness zone map was created based on historical weather patterns.

The US has 13 zones, with lower numbers having colder winters. So Zone 3 is more typical of North Dakota, while Zone 9 is lovely Florida weather. Each hardiness zone differs by 10 degrees F, so the difference in low winter temperatures between zones 6 and 7 is 10 degrees; between zones 6 and 8, 20 degrees; between zones 6 and 9, 30 degrees, and so on.

On the plant tag mentioned above, “zones 3-7” means that the plant will survive in those particular zones, or roughly from North Dakota to North Carolina. It’ll be killed by too much cold (most parts of Canada, for instance), and it won’t receive enough cold to survive if grown in Florida. Almost all plants have been assigned a hardiness zone.

To find your zone, go to the USNA’s website and click on your state. You’ll notice that each zone is divided into “a” and “b,” with “a” having, on average, five degrees colder winters than “b.”

You should also note that being further north doesn’t always mean colder temperatures. For instance, Tallahassee, FL; Portland, OR; and London, England, are all considered to be “Zone 8,” although they exist at 30-, 45- and 51-degrees latitude, respectively. This is because coastal winds and the jet stream modulate the weather in Portland and London. Large metropolitan areas are also slightly warmer, since their “asphalt jungles” retain heat.

When you see “Zones 3-7 / 4-6” on a plant tag, what does it mean?

You now know what the first set of numbers on a plant tag mean – but what does the second set refer to? This covers the plant’s heat zones, or how hot it gets in summer. The heat zone map is based on high temperatures and is divided into 12 zones. The lower the number, the cooler it is in summer. For example, Portland, OR, is in Heat Zone 4, meaning that it has between 15 and 30 days with temperatures above 86 degrees F every year. (Why 86 degrees F? Because plants are damaged at temperatures higher than that.) Much of Texas, on the other hand, is in Heat Zone 9, with 121 to 150 days of above-86-degree F weather.

The heat zone map was developed by the American Horticultural Society. You can also enter your zip code and find your heat zone that way. (Not all plants have been assigned a heat zone, though, so it’s seen less often than the hardiness zone.)

Do other countries have hardiness zones?

Yes. The hardiness zones of Canada and Mexico can be viewed at the USNA’s Website as well. Much of the rest of the world, including Africa, Australia, China, Europe, Russia, Scandinavia and South America, have been categorized according to their hardiness zones.

What don’t plant zones tell us?

While hardiness and heat zones are extremely helpful, they don’t tell the whole story. There are many other weather effects that can determine how well a plant will grow for you, including humidity, rainfall and wind. These maps were compiled based on average temperatures, so they don’t account for unusual weather patterns, such as temperatures hitting 11 degrees F on Halloween night with no previous frosts (which happened to me one year). Nor is the hardiness zone map an assurance that winter temperatures will never be colder than what’s stated on the map. Other factors that can’t be included on a large national map are the effects of soil and soil fertility, and the health of your plants in general.

Zone maps also don’t tell us about microclimates, which are small areas with different weather. A good example of a microclimate is a mountain. The largest microclimate would be the whole mountain, which has different weather based on elevation, but smaller microclimates are the north and south sides of the mountain. Next might come wind-sheltered pockets on the south side or areas beside a mountain stream. Finally, a very small microclimate might be the north side of your house or a south-facing brick wall. Several microclimates can exist in one place, with one adding to or cancelling out the effect of another.

How can you tell if you have a microclimate where you live? Take your own temperature readings around your yard. Or use your plants as a guide: If certain plants that are supposed to be hardy to your zone continue to die in your yard, you may have a cold pocket. Or if you’re growing a Zone 8 plant in Zone 7, you may have a warm pocket.

What are Sunset zones?

Sunset magazine – which is so named because it caters to the West Coast and not because it’s for seniors – created its own weather zone maps years before the USDA. Sunset zones are usually considered to be more precise than the USDA hardiness zones because they take into account many of the microclimates that exist on the “Left Coast.”

There are 24 Sunset zones listed for 13 western states, British Columbia and Alaska. They’re listed and expounded on in the Sunset Western Garden Book – a must-have for West Coast gardeners!

Frost Dates

Why is it important to know your zone’s frost dates? If you are planting annuals, the last frost date tells you when it is safe to get those in the ground. The first frost date lets you know when you will have to bring any tropicals or non-hardy plants indoors for the winter. These dates are guidelines, not absolutes, and can help you in planning your gardening activities. So keep these dates in mind, when purchasing new plants in the spring or closing down your garden at the end of its season.

Zone Last Frost First Frost
1 Mid June Mid July
2 Mid May Mid August
3 Mid May Mid September
4 Mid May Mid September
5 Mid April Mid October
6 Mid April Mid October
7 Mid April Mid October
8 Mid March Mid November
9 Mid February Mid December
10 Mid January Mid December
11 No frost No frost

Take a minute to learn your zones. You only have to learn them once, and you’re set! The next time you hear proficient gardeners saying, “I’m in Zone 7a,” or, “My heat zone won’t allow that plant to grow for me,” you’ll sound like a real pro when you respond, “Interesting. I’m in Zone 7a, but my valley microclimate allows me to grow Zone 8 plants.”