USDA Hardiness Zones, Explained

What the zone number actually measures

Hardiness zones are built from a single statistic: the average annual extreme minimum temperature. Every winter has one coldest night; the USDA averages that coldest-night temperature over 30 years (1991-2020 for the current map) for each location. It is an average, not a record low, so a once-a-decade cold snap can still dip below your zone's range.

Zones step down in 10 degree Fahrenheit bands, and each is split into half-zones: zone 7a averages 0 to 5 degrees F, zone 7b averages 5 to 10 degrees F. Plant labels and catalogs use these numbers as a survival floor: a shrub rated to zone 5 should normally come through a zone 5 winter, and will find zone 7 winters easy. Growing something rated only to zone 7 in zone 5 means it will likely be killed in a normal winter, not just an unusual one.

What zones do NOT tell you

The zone map is a winter-cold map and nothing more. It does not measure summer heat, which is why a plant can be 'hardy' in both Phoenix and Seattle yet thrive in only one of them; the American Horticultural Society publishes a separate heat-zone map for that. It also ignores rainfall, humidity, soil type, snow cover (which insulates plants), wind exposure, and how long cold spells last.

It also cannot see your yard. A bed against a south-facing brick wall, a frost pocket at the bottom of a slope, or a balcony above a heated garage can each behave like a half zone (or more) warmer or colder than the map says. Cities in general trend warmer than surrounding countryside because pavement and buildings hold heat. Use the zone as a starting filter, then let your own observations of what survives refine it.

• Zone = winter cold only; heat tolerance is a separate question
• No information about rain, humidity, soil, wind, or snow cover
• Microclimates in one yard can differ by a half zone or more
• An average, not a record: rare extreme winters can undershoot your zone

How to find your zone (and why the 2023 update matters)

Go to planthardiness.ars.usda.gov and enter your ZIP code or click the interactive map. The current map was released in November 2023, the first update since 2012, and was built from 13,412 weather stations, up from 7,983 for the previous version, so it is noticeably more precise at the local level.

Compared with the 2012 edition, about half of the country shifted into the next warmer half zone. The USDA itself cautions that these zone shifts are not automatically evidence of climate change, because the statistic (extreme minimums) is volatile and the newer map also uses more stations and better mapping methods. Practically, it means many gardeners' 'official' zone changed without them noticing, so re-check before you order borderline-hardy plants.

✔ If a plant you love is rated exactly to your zone, plant it in your warmest microclimate (near a wall, sheltered from wind) rather than in an open, low-lying bed.

The UK equivalent: RHS hardiness ratings

UK plant labels and the RHS website use RHS hardiness ratings instead of USDA zones. There are nine ratings from H1a (tropical, needs a heated greenhouse, above 15C) through H4 (hardy in an average UK winter) to H7 (very hardy, below -20C). Note the direction is reversed: higher H numbers mean hardier plants, whereas higher USDA zone numbers mean milder winters.

The two systems also measure differently: RHS ratings are pegged to absolute minimum winter temperatures in Celsius, not the 30-year average annual extreme minimum the USDA uses, so conversions between them are approximate. If you garden in the UK, use the H rating on the label; if you are reading American advice, translate loosely rather than literally.

Sources

Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.

• USDA ARS — Plant Hardiness Zone Map: Map Creation — 2023 map based on 30 years (1991-2020) of average annual extreme minimum temperatures; zones 1-13 in 10F bands with 5F half-zones; cities can map warmer because pavement and buildings retain heat; zone shifts are not by themselves reliable evidence of climate trends.
• USDA ARS — USDA Unveils Updated Plant Hardiness Zone Map (Nov 2023) — About half of the country shifted to the next warmer half zone versus the 2012 map; 13,412 weather stations used versus 7,983 in 2012; zones are 30-year averages of lowest annual winter temperatures; USDA notes updates are not necessarily reflective of global climate change.
• Purdue Extension — USDA Releases Updated Plant Hardiness Zone Map — 13 zones in 10F ranges subdivided into 5F half-zones (a/b); 2023 map uses 1991-2020 data versus 1976-2005 for the 2012 map; zones indicate what plants are likely to survive winter in an area; 'average' means colder winters will still occur.
• RHS — RHS Hardiness Rating — Nine RHS hardiness categories H1a-H7, from above 15C (heated glasshouse) to below -20C (very hardy); ratings are defined by absolute minimum winter temperatures in Celsius, not the long-term average annual extreme minimum used for USDA zones; system introduced in 2012.

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