What Is Hardiness Zone and How Is It Set?

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A hardiness zone is a band on a map that groups places by how cold their typical winter gets. What is hardiness zone and how is it set? In the United States the standard reference is the USDA Plant Hardiness Zone Map. It sorts locations into numbered zones using the average annual extreme minimum temperature, which is the coldest reading a place is expected to hit in a typical year, averaged over many years. A zone number tells a gardener roughly how cold the winter is, so they can compare that with the cold tolerance a perennial plant is listed for.

Quick answer

• A hardiness zone is based on the average annual extreme minimum winter temperature for a location.
• The USDA map uses 10-degree Fahrenheit zones from zone 1 (coldest) to zone 13 (warmest), and each zone is split into a and b halves that are 5 degrees apart.
• Elevation, latitude and proximity to the coast are the main factors that set the minimum temperature pattern.
• The zone does not capture snow cover, summer heat, humidity, soil moisture or spring frosts, so it is a starting point rather than a full growing guide.

What a hardiness zone measures

The zone is built from one number: the average annual extreme minimum temperature. Each year has a coldest night, and the map averages those coldest nights across a long record for each place. The result is a picture of how cold winter tends to get, not a forecast of any single winter. The Oregon State University landscape plants resource says the zones are based on that average annual extreme minimum temperature and gives the reading in both Fahrenheit and Celsius.

That focus on the coldest point is why the zone is useful for perennial plants. For how plants themselves build tolerance to cold, see how cold hardiness works in plants. A perennial that survives in its zone has to get through the coldest nights of an ordinary winter. The USDA guidance describes the maps as general guides for growing perennial plants, which is the main use the maps are built for.

The measure also has limits. The OSU page states plainly that the zones do not take into account other aspects that affect plant survival and performance, and it lists snow cover, summer heat, humidity, soil moisture and spring frosts. Two gardens in the same zone can therefore perform quite differently if one is shaded, wet, windy or exposed to late frost.

How the zones are set

The USDA map is a mapped summary of temperature records. According to the USDA page, the Plant Hardiness Zone Map is based on the average annual extreme minimum winter temperature. The zones are drawn across the country using that average, so a location’s zone reflects its typical coldest winter reading rather than the coldest reading it has ever recorded.

The USDA says the current edition uses weather data from 1991 to 2020. That 30-year span sets the baseline for the averages. A place can therefore look warmer or colder on the map than it did in an older edition, because the averaging window is different. The USDA also notes that the zones are based on the average lowest temperatures, not the lowest ever, and it warns that a rare cold snap lasting a day or two can still damage plants that are grown at the edge of their zone.

The Oregon State page says the main factors that set the average minimum temperature are elevation, latitude and proximity to the coast. Higher ground tends to be colder, and places far north tend to be colder than places far south. These are the factors the OSU page names behind the pattern of zone lines on the map.

How the a and b half zones work

Each zone spans 10 degrees Fahrenheit, which is wide enough that a plant label listing a single zone can hide a lot of variation. The USDA splits every zone into two halves, labeled a and b. The page gives the example that 7a and 7b are 5-degree increments representing the colder and warmer halves of zone 7.

In practice, the letter tells you which end of the zone you sit in. A garden in zone 7a is in the colder half, so a plant rated for 7b may be at risk in a normal winter. A garden in zone 7b has a little more margin. The USDA describes the half zones as convenient labels for comparing winter temperatures across the country, and that is the right way to read them.

The zone numbers themselves are the figure most nursery catalogs print. The USDA notes that zone numbers are typically listed with the descriptions of perennial plants in catalogs and other sales information from commercial nurseries and plant suppliers. When a catalog says a plant is hardy to zone 6, it means the plant is expected to survive in zone 6 and, usually, in warmer zones.

How to use a zone on a plant label

To find your zone, the USDA page says to enter your ZIP code in the search box of the online map and then click the map at your location. The map then shows the classification for that spot. The USDA also explains that digital maps in various resolutions for each state are available from the Maps Downloads tab, and that the agency has not printed paper copies of this edition for public distribution. Printed maps you see may therefore be from older editions or from other publishers.

Once you know your zone, read a plant label as a comparison. If the label lists zone 5 and you are in zone 6, the plant is rated for a colder area than yours and should handle a typical winter. If the label lists zone 8 and you are in zone 6, the plant may not survive your winter without protection. The USDA page recommends checking with established nurseries, Master Gardeners or extension services that know local conditions, because they can speak to the effects of the local site.

The zone is also only one factor in whether a plant will live. Conditions within a single yard can differ a great deal, which is the subject of what a microclimate in the garden is. The USDA lists wind, soil type, soil moisture, humidity, pollution, snow and winter sunshine as factors that can greatly affect survival. It also warns that light exposure matters: a plant that prefers partial shade and sits near the limit of its hardiness can be hurt by winter sun that warms and cools its tissue quickly. Soil moisture matters too, because a plant that goes into dormancy dry in late autumn may be injured even if it is hardy for the zone.

A practical approach is to treat the zone as the first filter. Check the zone on the label against your own zone, then look at the site factors the USDA lists, such as wind, soil moisture and winter sun, before deciding where the plant goes. The sources reviewed here name these factors but do not give a step-by-step method for managing each one.

How we chose

This guide is built from two sources. Both were chosen because they are official and non-retail: one is a university landscape plants resource and the other is the USDA page that explains the current map. Neither source sells plants, and neither is a product page. Both pages were opened on 2026-10-27 and the claims above are tied to text from those pages. Where a point is general knowledge rather than something a source says, this guide does not state it as fact.

What we could not verify

The two pages give a clear definition and a clear method for reading the map, but they leave some questions open. Each claim below is marked by the source it came from, and anything the sources did not say has been left out rather than filled in from memory.

What the sources do not say

• How many weather stations feed the averages, or how the zone lines are drawn between them. Neither page describes the station network or the interpolation method.
• A table that converts each zone into an exact temperature in degrees. The pages describe the zones as 10-degree and 5-degree steps but do not give the full temperature list for each zone.
• How much a zone shift changes plant survival in a given species. The pages warn that other factors matter but do not give survival rates.
• Whether the 1991 to 2020 averages will be updated on the same schedule as earlier editions. The pages describe only the current edition.

Frequently asked questions

What does the number in a hardiness zone mean?

Each number stands for a 10-degree Fahrenheit band of average annual extreme minimum winter temperature. Zone 1 is the coldest band and zone 13 is the warmest, according to the USDA page.

What do the a and b letters mean on the map?

They split each zone into two 5-degree halves. The a half is the colder side and the b half is the warmer side, as the USDA example for 7a and 7b shows.

Does a hardiness zone tell me when to plant?

No. The zone describes average winter cold and does not set planting dates. The OSU page says the zones do not account for spring frosts, which affect planting timing, so local frost dates need a separate source.

Is my zone the coldest temperature my garden will ever see?

No. The USDA page says the zones are based on the average lowest temperatures, not the lowest ever, so a rare cold snap can still fall below the zone’s typical minimum.

Where can I find my zone?

Use the USDA map and enter your ZIP code in the search box, then click your location on the map. The USDA also offers state maps in several resolutions through its Maps Downloads tab.

Sources

• Oregon State University, Landscape Plants, “USDA Hardiness Zone Maps of the United States,” https://landscapeplants.oregonstate.edu/usda-hardiness-zone-maps-united-states (opened 2026-10-27)
• USDA Plant Hardiness Zone Map, “How to Use the Maps,” https://planthardiness.ars.usda.gov/pages/how-to-use-the-maps (opened 2026-10-27)