Pocket BotanistA USDA hardiness zone is a number from 1 to 13 that describes how cold a region gets in an average winter. Each zone covers a 10 degree Fahrenheit range of average annual minimum temperature. Half-zones (4a, 4b) divide that range in two. The map is maintained by the United States Department of Agriculture, but the numbers are used informally well beyond the US.
How the zones work
The map asks one question: how cold does it get on the worst night of an average year? It then averages that minimum across roughly 30 years of weather data and assigns each location a zone. Lower numbers are colder. Zone 1 is sub-arctic. Zone 13 is tropical.
According to the USDA Plant Hardiness Zone Map, the 2023 revision is the current authoritative version. Many regions of the United States have shifted half a zone warmer since the previous 2012 edition.
A few useful reference points:
- Zone 3: parts of Minnesota and Montana, average minimums of -40 to -30°F
- Zone 5: much of the northern US, average minimums of -20 to -10°F
- Zone 6: much of the central US, average minimums of -10 to 0°F
- Zone 7: mid-Atlantic and Pacific Northwest, average minimums of 0 to 10°F
- Zone 9: coastal California and parts of Florida, average minimums of 20 to 30°F
- Zone 11: south Florida and Hawaii, average minimums of 40 to 50°F
Why it matters for plant care
Plant tags and seed catalogs list a hardiness range, for example "Zones 5 to 9". If your zone is inside that range, the plant should survive your winters outdoors with reasonable care. If it is outside, you will need to bring the plant inside for winter, plant it as an annual, or expect losses in a hard year.
For houseplants, hardiness zones matter less. Indoor temperatures are similar everywhere people heat their homes. But the zone still tells you something about your outdoor humidity, length of growing season, and how much light your plants get through the year.
How to find your zone
Look up your postal code on the USDA Plant Hardiness Zone Map. The interactive map will return your zone (e.g., 7a) and the corresponding temperature range.
Outside the US, search for your country's equivalent. The most widely used international systems are:
- United Kingdom: the Royal Horticultural Society uses an H1 to H7 rating, where H1a covers heated greenhouses and H7 covers the hardiest plants. See the RHS hardiness ratings.
- Europe: many catalogs list USDA zones alongside their own ratings, since the USDA temperature bands map cleanly onto European climates. Northern Europe is typically zones 6 to 9; Mediterranean climates 9 to 10.
- Canada: Natural Resources Canada publishes a similar Plant Hardiness Zone Map with the same 1 to 9 numbering plus modifications for the country's specific climate.
- Australia and New Zealand: hardiness is less commonly used. Plant tags more often list a heat tolerance and a frost tolerance separately.
The other map: AHS heat zones
Hardiness zones only describe winter cold. The American Horticultural Society publishes a separate Plant Heat-Zone Map with 12 zones based on the average number of days per year above 86°F (30°C). Pairing the USDA zone with the AHS heat zone gives a fuller picture of where a plant can live; many plant tags now list both, such as "USDA 6-9, AHS 9-6".
Microclimates within your zone
The zone you read off a map is the regional average. Your specific garden may run warmer or cooler than that by a full zone, depending on:
- Walls and fences that retain heat (a south-facing brick wall is often a half-zone warmer than open ground)
- Slopes and frost pockets (cold air sinks; the bottom of a slope is colder than the top)
- Wind exposure (windbreaks raise effective hardiness)
- Proximity to water (lakes and oceans moderate temperature swings)
- Urban heat islands (city centers are often a half-zone warmer than rural surroundings)
The practical implication: a zone 6 plant can often survive in a sheltered zone 5 spot, and a zone 7 plant might die in an exposed corner of a zone 7 garden.
Where the system stops being useful
Hardiness zones only describe winter lows. They do not capture:
- How hot your summers get (the AHS heat zone covers this)
- Humidity, which matters for many tropical and arid plants
- Rainfall and soil type
- How long your growing season is, in frost-free days
- The depth and duration of summer drought
Use the zone as a starting point, then read what experienced gardeners in your specific area grow. Local extension services, garden centers, and the RHS's regional advisors are more useful than any map alone.
How Pocket Botanist uses hardiness zones
On Premium, Pocket Botanist derives your hardiness zone from your device location (with permission) and uses it to tailor care notes for your climate. A care tip that mentions overwintering outdoors will say so for zones where it applies, and warn you otherwise. The location stays on your device; only a city-and-country label is ever sent to the model, never raw GPS readings. See our Privacy Policy for details. The full app is free on iPhone and Android and you do not need an account to start.
Sources
- United States Department of Agriculture, "USDA Plant Hardiness Zone Map." planthardiness.ars.usda.gov
- Royal Horticultural Society, "RHS hardiness ratings." rhs.org.uk
- American Horticultural Society, "AHS Plant Heat-Zone Map." ahsgardening.org