USDA hardiness zones explained: the 2023 map and what it actually tells you

Every plant tag, every seed catalog, every extension publication in the United States eventually references the USDA Plant Hardiness Zone Map.

—- title: "Usda hardiness zones" slug: usda-hardiness-zones hub: care category: How-to guide description: "Every plant tag, every seed catalog, every extension publication in the United States eventually references the USDA Plant Hardiness Zone Map. It is the universal reference point for whether a given." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 11 —-

Every plant tag, every seed catalog, every extension publication in the United States eventually references the USDA Plant Hardiness Zone Map. It is the universal reference point for whether a given perennial will survive winter in a given location. The 2023 version of the map — the most recent — updated zone boundaries across much of the country, shifting some areas one half-zone warmer.

Understanding what the map actually measures — and what it cannot tell you — is more useful than simply knowing your zone number.

What the map measures

Per the USDA Agricultural Research Service: "The Plant Hardiness Zone Map (PHZM) is based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones ranging from zone 1 (coldest) to zone 13 (warmest)."

The key word is "average." Per the USDA: "Zones in this edition of the USDA PHZM are based on 1991-2020 weather data. This does not represent the coldest it has ever been or ever will be in an area, but it simply is the average lowest winter temperatures for a given location for this 30-year span."

That 30-year average matters for practical gardening. A plant rated "hardy to zone 5" will survive most zone 5 winters — but not all. In a year with an unusually severe cold snap, plants at the edge of their rated zone may be killed. Per USDA: "Growing plants at the extreme range of the coldest zone where they are adapted means that they could experience a year with a rare, extreme cold snap that lasts just a day or two, and plants that have thrived happily for several years could be lost."

Zone numbers: what the temperature ranges actually are

Source: USDA Plant Hardiness Zone Map, 2023.

Half-zones: 7a versus 7b

Each zone is divided into two half-zones — 'a' (colder half) and 'b' (warmer half) — representing a 5°F difference. Per USDA: "For example, 7a and 7b are 5-degree F increments representing the colder and warmer halves of zone 7, respectively."

In practical terms: a plant rated "zone 7" is hardy across all of zone 7 — both 7a and 7b. A plant rated "zone 7b" is only reliably hardy in the warmer half of zone 7. Half-zone distinctions matter most at the cold margins of a plant's hardiness range.

My Long Island yard is zone 7a. The standard zone 7 rating on most plant labels covers my location. But plants listed specifically for zone 7b — some Camellia japonica varieties, for example — are more marginal and may die back in the colder winters I sometimes get.

The 2023 update: what changed

The 2023 map replaced the 2012 edition. Per the USDA map creation page: "The new PHZM is generally about one quarter-zone warmer than reported in the 2012 PHZM throughout much of the United States, as a result of a more recent averaging period (1976-2005 vs. 1991-2020)."

The 2023 map is based on the 1991-2020 climate normals, the period used by NOAA for all climate baseline calculations. The 2012 map used 1976-2005 data. Because winters across much of the U.S. have been less extreme in the more recent period, many locations shifted one half-zone warmer.

Per USDA: "Compared to the 2012 and 1990 maps, zone boundaries in this 2023 edition have shifted in many areas." However, the USDA cautions against over-interpreting this as a gardening license to push the limits: "If your hardiness zone has changed in this edition of the USDA Plant Hardiness Zone Map (PHZM), it does not mean you should start removing plants from your garden or change what you are growing. What has thrived in your yard will most likely continue to thrive."

The 2023 map also used data from 13,625 weather stations — "a substantial increase compared to the 2012 map version" — and was generated at a resolution of approximately half a mile per cell, allowing for much more detailed delineation of local conditions including urban heat islands, coastal effects, and elevation differences.

What the map does NOT tell you

This is where gardeners most commonly go wrong. The USDA hardiness zone map is frequently misread as a complete guide to plant suitability. It is not. Per USDA's own guidance on map usage:

1. It does not measure summer heat. A zone 7 gardener in Philadelphia and a zone 7 gardener in coastal Oregon experience radically different summers. Philadelphia summers are hot and humid — routinely above 90°F. Pacific coastal Oregon summers are mild and dry — rarely above 75°F. The same zone, utterly different summer climates. A plant that thrives in Oregon zone 7 may burn out in Philadelphia zone 7 from heat and humidity stress.

2. It does not measure humidity. High humidity affects disease pressure (fungal diseases are much more severe in humid climates), plant transpiration rates, and how plants handle heat. A humid zone 7 and an arid zone 7 are not equivalent for plant selection.

3. It does not measure growing season length. Two locations in zone 6a can have very different frost-free growing season lengths depending on spring timing. Zone 6a in the mountains of northern Georgia might have 150 frost-free days; zone 6a in a valley in the same state might have 180 days.

4. It does not measure snowpack. In zones 4–5, reliable snow cover insulates the soil and protects plant crowns from deep freeze. A zone 4 garden in Minnesota with consistent snow cover all winter may be safer for marginally hardy plants than a zone 5 garden in a maritime climate with freeze-thaw cycles and little snow.

5. It does not measure soil conditions, wind exposure, or rainfall. Per USDA: "Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants."

Microclimates: your zone within a zone

The map is drawn at half-mile resolution. Your individual yard can vary significantly. Per USDA: "Individual gardens also may have very localized microclimates."

Key effects:

• South-facing walls: Absorb and release heat, effectively warming the adjacent planting by 1–2 half-zones. In my zone 7a yard, this is where I push marginally hardy plants.
• Low spots and frost pockets: Cold air drains downhill. A frost pocket can be 2–3°F colder than a site 20 feet away at higher elevation.
• Urban heat islands: Cities are typically 2–4°F warmer than surrounding suburbs due to heat absorbed by pavement and buildings, which is why many large U.S. cities sit in a warmer half-zone.
• Coastal moderation: Proximity to large water bodies (Atlantic, Pacific, Great Lakes) moderates winter extremes. Coastal zone 7 winters are meaningfully milder than inland zone 7.

The AHS Heat Zone Map: the companion tool

The American Horticultural Society developed its Heat Zone Map in 1997 as a complement to the USDA cold hardiness map. Per the U.S. Botanic Garden: "Created by the then president of the American Horticultural Society, H. Marc Cathey, in 1997 the heat zone map was created as a complementary map to the USDA plant hardiness map, which maps the coldest average annual temperature for a given region."

The AHS Heat Zone Map is based on the average number of "heat days" per year — days when the temperature exceeds 86°F (30°C). Per the U.S. Botanic Garden: "At this temperature, many plants begin to experience physiological damage and start to shut down their functioning." The map is divided into 12 zones: Zone 1 has no heat days; Zone 12 has 210 or more heat days per year.

Why it matters: Some plants are cold-hardy but heat-sensitive. Certain alpine plants, cool-season vegetables, and some northern-adapted perennials survive zone 5 winters but fail in zone 5 summers that are too hot. The AHS Heat Zone Map helps identify these situations. Plants with dual ratings (e.g., "USDA zones 3–7, AHS heat zones 1–9") communicate both the cold limit and the heat limit.

The caveat: The AHS Heat Zone Map has not been updated since 1997. It is based on weather data from 1974–1995. As the U.S. Botanic Garden notes, an updated map would be valuable given the warming trends since then.

A plant rated for your USDA hardiness zone may still struggle if it is outside its AHS heat zone tolerance. A Philadelphia gardener (USDA zone 7a, AHS heat zone 7 — roughly 60–90 heat days per year) trying to grow plants rated for AHS heat zones 1–4 will have difficulty in summer regardless of the winter hardiness match.

Where notable cities fall

Sources: USDA Plant Hardiness Zone Map; AHS heat zone approximations from published AHS map data.

Common problems with zone interpretation

Frequently asked

What is my USDA hardiness zone?

Enter your ZIP code at planthardiness.ars.usda.gov for the official determination from the 2023 USDA Plant Hardiness Zone Map. The map resolves to approximately half-mile resolution, which means it can reflect urban heat islands, elevation differences, and coastal moderation effects within a region. If your garden has a notably sheltered or exposed microclimate, your practical zone may differ from the map designation.

Did my zone change in 2023?

Possibly. Per USDA, the 2023 map is "generally about one quarter-zone warmer than reported in the 2012 PHZM throughout much of the United States" due to the shift to a more recent 30-year averaging period. About half of U.S. locations shifted one half-zone warmer. But the USDA's practical advice is that "what has thrived in your yard will most likely continue to thrive" — the shift is a statistical adjustment, not a mandate to change your plant selection.

What does "half-hardy in zone 6" mean?

"Half-hardy" means the plant survives mild zone 6 winters (zone 6b, minimums around -5°F) but may be killed by severe zone 6 winters (zone 6a, minimums to -10°F). The term is more common in British horticulture, but it expresses a real distinction: zone 6a in New England or the upper Midwest can be meaningfully colder than zone 6b in the mid-Atlantic. For borderline plants, provide winter protection and site them with microclimate advantage.

Why do plants sometimes survive well outside their rated zone?

Microclimate, snow cover, and microhabitat. A zone 5 plant may survive in a zone 4 garden if: 1) it is sited against a south-facing wall, 2) the winters have reliable snow cover that insulates the soil, 3) it is planted in a position protected from the desiccating cold winds that are often more damaging than the cold temperature itself. Per USDA, "wind, soil type, soil moisture, humidity… can greatly affect the survival of plants" — zone ratings are averages and do not capture these localized factors.