USDA Hardiness Zone Map — complete 2023 guide + finder

USDA Hardiness Zone Map — complete 2023 guide + finder

The USDA Plant Hardiness Zone Map is the single most-cited reference in American gardening. Seed packets, plant labels, mail-order nurseries, university extension calendars, and garden books all assume you know your zone. This guide explains exactly what the map is, what the numbers mean, how the 2023 update changed things, and how to actually use your zone to make planting decisions — without falling into the common trap of thinking the zone tells you more than it really does.

Find your zone in 5 seconds: Open Growli's zone finder tool and enter your ZIP code (or UK postcode). You'll get your USDA zone, your average last frost date, and a list of plants suited to your climate — without having to leave the page.

What the USDA Plant Hardiness Zone Map is

The USDA Plant Hardiness Zone Map is a climate classification system maintained by the U.S. Department of Agriculture's Agricultural Research Service (ARS). It assigns every location in the United States, Puerto Rico, and the U.S. territories to one of 13 zones, each defined by a 10°F band of average annual extreme minimum temperature. Each numbered zone is also split into an "a" (the colder half) and "b" (the warmer half), giving 26 named zones in total: 1a through 13b.

The map's first scientific edition was published in 1960 by Henry Skinner of the U.S. National Arboretum. Major revisions followed in 1990, 2012, and the current version released on November 15, 2023. Each revision pulled in more weather stations, a longer climate record, and finer spatial resolution — the 2023 map uses data from 13,412 stations (up from 7,983 in 2012) and renders at roughly 800-metre (half-mile) resolution.

The map's single job is to tell you, on average, how cold your worst winter night gets. That number is the dominant constraint on which perennials, shrubs, and trees survive in the ground year after year.

How USDA zones are calculated

The zone number for any location is derived from a 30-year average of annual extreme minimum temperatures. Here is the precise method:

1. For each year in the climate record, take the single coldest temperature recorded at the weather station.
2. Average those annual coldest temperatures across a 30-year window.
3. Match the resulting average to the 10°F zone band it falls into.

For the 2023 update, the window is 1991 to 2020. The previous (2012) map used 1976 to 2005. The shift forward of 15 years is the main reason most of the country moved half a zone warmer — recent decades have been measurably warmer than the late 20th century baseline.

The underlying weather data is supplied by the National Oceanic and Atmospheric Administration (NOAA) and the PRISM Climate Group at Oregon State University. PRISM interpolates between weather stations using elevation, slope, and proximity to coastlines, which is why the published map shows fine-grained variation around mountains, valleys, and lakes rather than uniform horizontal bands.

It's worth being precise: a zone tells you the average of the worst night each year. In any single year, the coldest night might be warmer or colder than your zone's range. Roughly 1 winter in 10 will be significantly colder than the long-term average.

All 13 USDA zones with temperature ranges

Zones 1 through 8 cover most of the continental United States. Zones 9 through 11 cover the deep south, Hawaii, and the warmest coastal pockets. Zones 12 and 13 only appear in Puerto Rico and the U.S. tropical territories.

How to find your USDA zone (3 reliable methods)

There are three trustworthy ways to look up the zone for any address in the United States:

1. Official USDA tool

Go to planthardiness.ars.usda.gov, enter a 5-digit ZIP code, and the site returns the zone for that postal area. The USDA's lookup is the canonical reference — if a seed packet or plant label disagrees with another source, the USDA value wins.

A caveat: ZIP codes are a postal routing system, not a climate boundary. Large or rural ZIPs can span more than one zone, especially in mountainous areas. The USDA returns the dominant zone for the ZIP, but your specific yard might be slightly different.

2. Growli zone finder

Growli's zone finder tool takes a ZIP code or a UK postcode and returns the USDA zone (US) or RHS hardiness rating range (UK), plus your average last frost date and a tailored list of what to plant this month. It's faster than the USDA site for a planting decision because the answer arrives with the next action, not just the zone number.

3. By city or state

If you only need a rough answer, the table above lists a representative city for each zone. For more detail, browse the full USDA zone library — every zone has a dedicated page with planting calendars and recommended plants.

What hardiness zones do NOT tell you

This is where most beginners go wrong. Your USDA zone is one input to a planting decision, not the only one. The zone tells you nothing about:

• Heat tolerance. A plant rated "hardy to zone 5" might still die in zone 9 summers from heat stress. The American Horticultural Society publishes a separate Heat Zone Map (1 through 12) based on the number of days per year above 86°F (30°C).
• Summer humidity. Many alpine perennials survive zone 6 winters but rot in zone 6 summer humidity in the southeast.
• Soil drainage and pH. Lavender is hardy to zone 5, but it dies in heavy clay regardless of zone.
• Microclimate. A south-facing wall can effectively shift your local zone half a zone warmer; a frost pocket at the bottom of a slope shifts it colder.
• Length of growing season. Two locations can share a zone but have very different last-frost-to-first-frost windows.
• Annual variation. A zone is a 30-year average. In any single winter you might get a "1-in-10" cold event that exceeds the zone's stated range.

For practical year-round planting, you usually need to know: your USDA zone, your average last frost date, your average first frost date, and your summer high temperature range. The zone handles winter cold; the other three handle everything else.

How to use your USDA zone for planting decisions

For each plant you're considering, find the zone range on the label (e.g. "hardy in zones 4-9"). Three rules:

1. Stay inside the range. If a plant is rated for zones 5-8 and you're in zone 4b, expect winter dieback in cold years. You can sometimes push the boundary half a zone using a sheltered microclimate, mulch, or wrapping.
2. Pick perennials at the warm end of your zone for reliability. A plant rated zones 5-9 is more confidently winter-hardy in zone 7 than in zone 5.
3. Don't use zones for annuals. Tomatoes, peppers, basil, and other annuals are killed by the first frost regardless of zone. For annuals, what matters is your last-frost and first-frost dates — not the zone number.

Decision framework

What changed in the 2023 USDA map update

The November 2023 update was the first major revision since 2012. The headline numbers:

• About half of the United States shifted into a warmer half-zone — typically a 5°F change.
• The other half of the country stayed in the same half-zone, though many of those areas still warmed by some amount short of a full step.
• The biggest shifts appeared in the upper Midwest, Northern Plains, and parts of the Northeast, which warmed faster than the national average between the 1976-2005 and 1991-2020 windows.
• Almost no locations shifted to a colder zone.
• The 2023 map uses 13,412 weather stations vs 7,983 for 2012, with rasters at an 800-metre grid resolution that better represents elevation and coastal effects.

What this means in practice: many gardeners can now reliably grow plants that were considered borderline a decade ago. A zone 6a gardener in Pennsylvania who pushed the limits on figs or southern magnolias may now be a 6b gardener for whom those plants are routine. Conversely, a single hard winter can still kill them — the zone is the average, not the floor.

The USDA is explicit that the map is not a direct measurement of climate change. It is a 30-year statistical average, and the shift reflects both long-term warming and natural decadal variability. For climate trend analysis, look to NOAA's climate normals rather than the hardiness map.

Diagnose this with Growli: If a perennial died last winter and you're not sure whether you pushed your zone too hard, open Growli and describe what happened. The app cross-references your zone, the actual minimum temperature recorded in your area, and the plant's hardiness rating to give you a clear "yes you pushed it" or "no, something else went wrong" answer.

How USDA zones relate to UK hardiness ratings

UK readers shouldn't try to use USDA zones directly. The UK uses the RHS hardiness rating system (H1a through H7), which rates each plant rather than mapping geographic regions. The rough conversion:

Most of England and Wales sits in the H4-H5 range, equivalent to USDA zones 7-8. Coastal Cornwall and the Channel Islands behave like USDA zone 9. Scottish highlands behave like USDA zone 6. For more detail, see our UK hardiness zones guide and the UK zone library.

Common USDA zone mistakes

1. Assuming the zone is the lowest temperature you'll ever see. It's the average of the worst night each year. Plan for occasional colder events, especially with marginally hardy plants.
2. Trusting a single ZIP-code lookup for a large rural ZIP. Elevation and aspect can shift the zone half a step. If your property has a known frost pocket or a sheltered south wall, treat the official zone as a starting point.
3. Using zones for vegetable timing. Zones don't tell you when to plant tomatoes. For that, use last-frost dates — see when to plant tomatoes.
4. Ignoring heat tolerance in zones 8-11. A "hardy to zone 5" perennial isn't automatically suited to zone 9 summers. Check the AHS Heat Zone or look for southern-bred cultivars.
5. Comparing to the 2012 map without re-checking. If your zone advice predates November 2023, verify your current zone — it may have changed.

Related articles

• Browse the full USDA zone library — every zone with its own planting calendar
• Find your zone in 5 seconds — ZIP or postcode lookup
• UK hardiness zones explained — for UK gardeners
• UK zone library — RHS-aligned planting calendars
• When to plant tomatoes by zone — using zone + frost date together
• How to start a vegetable garden — beginner playbook
• Get personalized zone advice in the Growli app — plant-by-plant guidance for your specific climate

Reviewed and updated by the Growli editorial team. Source: USDA Agricultural Research Service Plant Hardiness Zone Map (November 2023 edition), PRISM Climate Group at Oregon State University, NOAA climate normals 1991-2020. For questions about anything here, open Growli and ask — or email hello@getgrowli.app.