By: Mary Ellen Ellis
If you are new to gardening, you may be confused by some ofthe terminology associated with plants. For instance, a USDAzone explanation may be necessary. This is a useful system for determiningwhat plants will survive and grow in certain areas of North America. When youunderstand how these hardiness zones work, you will be able to better plan yourgarden.
What Do Hardiness Zones Mean?
The USDA plant hardiness map is created and updated everyfew years by the U.S. Department of Agriculture. It divides North America intoeleven zones by minimum average annual temperatures. The lower the number is,the lower the temperatures in that zone.
Each zone represents ten degrees of temperature difference.Each zone is also divided into “a” and “b” segments. These represent fivedegrees of temperature difference. For example, zone4 represents minimum temperatures between -30 to -20 F. (-34 to -29 C.). Thea and b subdivisions represent -30 to -25 F. (-34 to -32 C.) and -25 to -20 F. (-32to -29 C.).
Hardiness refers to how well a plant will survive coldtemperatures. Where the USDA zones fall short, however, is that they don’taccount for other factors. These include freeze dates, freeze-thaw cycles, theeffects of snow cover, precipitation, and elevation.
How to Use Hardiness Zone Information
Understanding hardiness zones means you can pick plants foryour garden that will be most likely to survive your local winters. The zonesare not important for annualssince these are plants you would only expect to survive the summer months, orone season. For perennials, trees, and shrubs though, be sure to check the USDAzones before you put them in your garden.
The limitations of the USDA zones are most felt in thewestern U.S. If you live in this area, you may want to use the Sunset climatezones. This system uses more than just the minimum temperatures to determinewhich plants grow best where. They also use the length of the growing season,summer temperatures, wind, humidity, and rainfall.
No zoning system is perfect and even within your own gardenyou may have important microclimatesthat impact how plants grow. Use the USDA or Sunset zones as a guide and alwayscheck them to give you the best chance of success in your garden.
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Read more about USDA Planting Zones
Plant Hardiness Zones
Learn more about the USDA Plant Hardiness Zone Map and how to use it.
The USDA Plant 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 US 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.
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).
Difference Between Planting Zones
Separated by 10-degree differences, hardiness zones run from the lowest to highest possible cold conditions. Vegetation with the lowest numbers can withstand the chilliest weather, while higher numbers mean warmer climates are a better match. If a plant belongs to hardiness zones three to seven, this means it's hardy enough to survive some cold, but not enough to endure extreme and prolonged icy periods. On the other hand, a three to seven zone plant needs a certain amount of cold and will likely perish in the desert heat.
If you've ever bought plants online or at a nursery, you'll likely notice they’re tagged by hardiness zones. Zone numbers are further subdivided into “a” and “b.” Plants marked as “a” have average winter tolerance temperatures five degrees cooler than their zone “b” counterparts.
You might assume that the farther north an area is, the colder its average temperature will be, but that’s not always true. Seattle, for example, is at a latitude of 47 and a hardiness zone of eight, while Baltimore is farther south and boasts a zone of six or seven.
Besides the cold-tolerance level of plants, there are other local factors to account for when learning about your hardiness zone. Unique climate conditions in your area also affect which types of plants will do well. These factors include:
- Winds: Weather factors like winds from jet streams and coastal air movement can change growing conditions in specific areas. Winds dry out vegetation. Additionally, high winds can damage plants. If you reside in a windy microclimate, be sure to select stronger, low-lying shrubs or evergreens.
- Urban Heat: Metropolitan areas are generally warmer than rural areas, even if they belong to the same region. That’s because of the urban heat island effect — urban environments absorb and trap heat in their concrete jungles, maintaining higher year-round temperatures.
- Rainfall: The greater the rainfall, the moister the soil. That’s why it’s important to consider how much precipitation your area sees. Residents of the Pacific Northwest need to choose wet-tolerant plants or improve their soil drainage, whereas gardeners in dryer areas won’t be concerned about rainfall.
- Humidity: Even though California and Florida have parts of their respective states with the same hardiness zones, their relative humidity levels are vastly different. High humidity slows down a plant’s transpiration rate, so Floridian gardeners need to consider the humidity tolerance of their plant selection.
AHS Heat Zones
To partly resolve some of the above issues, a Heat Zone Map has been developed by the American Horticultural Society, based on the National Weather Service (NWS) daily high temperatures recorded between 1974 and 1995. While the effect of heat is not as immediate and radical as severe cold, it may slowly damage your plants and kill them. The most obvious heat symptoms include withering flower buds, drooping leaves, leaves discoloration (leaves may turn white or brown), or non-growing roots.
The AHS Plant Heat Zone Map includes 12 zones, each indicating the average yearly number of days with temperatures over 86°F (30°C). This threshold represents the point when the plants start suffering from the heat. These Heat zones range from less than one heat day (Zone 1) to more than 210 heat days (Zone 12). Similarly to the hardiness zones, most garden plants provide heat tolerance information. Therefore, you will find 4 numbers on each plant: Maximum Hardiness Zone, Minimum Hardiness Zone, Maximum Heat Zone, Minimum Heat Zone. For example, a tulip may be 3-8, 8-1. If you live in USDA Zone 7 and AHS Zone 7, you will know that you can leave tulips outdoors in your garden year-round. An English wallflower may be 5-8, 6-1. It is relatively cold hardy, but can't tolerate extreme summer heat.
Again, this Heat Zone map should be used as a guideline and gardeners may find that many plants will survive outside their respective heat zone. The reason is that other factors could have an impact on the life of your plant such as a lack of water (resulting from the heat), light (cloud cover, dappled shade), day length (the longer the summer day, the more impact on plant survival), air circulation (fast-moving air on hot days may quickly dehydrate the plants), surrounding elements (hard structures of stone, concrete) emit heat, raise the air temperature and soil pH.