The USDA Plant Hardiness Zone Map divides the contiguous US, Alaska, Hawaii, and US territories into 13 primary zones based on the average annual extreme minimum temperature over a 30-year period (1991–2020). Each zone spans 10°F and is split into an "a" half (lower 5°F) and a "b" half (upper 5°F), yielding 26 half-zones in total.
Zone numbers describe cold hardiness only — they say nothing about summer heat, humidity, drought, soil type, or rainfall.
All USDA zones — temperature ranges
| Zone | Min temp (°F) | Min temp (°C) | Typical US region |
|---|---|---|---|
| 1a | -60 to -55 | -51.1 to -48.3 | Northernmost Alaska interior |
| 1b | -55 to -50 | -48.3 to -45.6 | Interior Alaska |
| 2a | -50 to -45 | -45.6 to -42.8 | Northern Alaska |
| 2b | -45 to -40 | -42.8 to -40.0 | Northern Minnesota, Alaska |
| 3a | -40 to -35 | -40.0 to -37.2 | Northern Minnesota, Montana |
| 3b | -35 to -30 | -37.2 to -34.4 | Northern Wisconsin, Michigan UP |
| 4a | -30 to -25 | -34.4 to -31.7 | Vermont, Minnesota, Montana |
| 4b | -25 to -20 | -31.7 to -28.9 | Northern Iowa, Wisconsin |
| 5a | -20 to -15 | -28.9 to -26.1 | Chicago IL, Cleveland OH |
| 5b | -15 to -10 | -26.1 to -23.3 | Boston MA, Denver CO |
| 6a | -10 to -5 | -23.3 to -20.6 | Richmond VA, Pittsburgh PA |
| 6b | -5 to 0 | -20.6 to -17.8 | Philadelphia PA, St. Louis MO |
| 7a | 0 to 5 | -17.8 to -15.0 | Washington DC, Oklahoma City OK |
| 7b | 5 to 10 | -15.0 to -12.2 | New York City NY, Seattle WA |
| 8a | 10 to 15 | -12.2 to -9.4 | Memphis TN, Charlotte NC |
| 8b | 15 to 20 | -9.4 to -6.7 | Atlanta GA, Portland OR |
| 9a | 20 to 25 | -6.7 to -3.9 | Sacramento CA, Houston TX |
| 9b | 25 to 30 | -3.9 to -1.1 | Los Angeles CA, Tucson AZ |
| 10a | 30 to 35 | -1.1 to 1.7 | Miami FL, Honolulu HI (sea level) |
| 10b | 35 to 40 | 1.7 to 4.4 | Southern Florida, Puerto Rico |
| 11a | 40 to 45 | 4.4 to 7.2 | Hawaii (coastal low elevation) |
| 11b | 45 to 50 | 7.2 to 10.0 | Southern Hawaii |
| 12a | 50 to 55 | 10.0 to 12.8 | Hawaii (tropical mid-elevation) |
| 12b | 55 to 60 | 12.8 to 15.6 | Hawaii (lowland tropical) |
| 13a | 60 to 65 | 15.6 to 18.3 | US territories (Puerto Rico lowland) |
| 13b | 65 to 70 | 18.3 to 21.1 | US territories (tropical) |
Average last and first frost date windows by zone
Frost dates vary significantly within a zone depending on elevation, proximity to water, and local microclimates. The windows below are broad regional averages derived from NOAA 30-year climate normals (1991–2020). Use them for general planning only — look up your specific location for site-accurate dates.
A "frost" here means 32°F (0°C) at ground level. "Hard freeze" (28°F / -2.2°C) varies further — tender annuals and frost-tolerant crops behave differently at each threshold.
| Zone | Avg last spring frost | Avg first fall frost | Frost-free window (approx.) |
|---|---|---|---|
| 3 | May 1–15 | Sept 1–15 | ~120 days |
| 4 | Apr 15–May 1 | Sept 15–Oct 1 | ~140 days |
| 5 | Apr 1–15 | Oct 1–15 | ~160 days |
| 6 | Mar 15–Apr 1 | Oct 15–Nov 1 | ~180 days |
| 7 | Mar 1–15 | Nov 1–15 | ~200 days |
| 8 | Feb 15–Mar 1 | Nov 15–Dec 1 | ~240 days |
| 9 | Jan 15–Feb 15 | Dec 1–Dec 15 | ~280 days |
| 10 | Frost rare or absent | Frost rare or absent | ~365 days |
How to read a hardiness zone label on a plant tag
Plant tags typically list a hardiness range such as "Zones 5–9". This means:
- The plant can survive winters as cold as zone 5 (−20°F to −10°F)
- It may not tolerate the year-round warmth of zone 10 and above (insufficient winter chill, or heat stress)
Both ends of the range matter. A plant labeled zones 5–9 will not perform well in zone 10 even though zone 10 is "warmer."
Limitations of zone ratings
Zones describe one variable. Average annual extreme minimum temperature is the single criterion. It does not account for summer heat (see AHS Heat Zone Map), humidity, soil drainage, wind exposure, or late frosts after the average last frost date.
Microclimates shift effective zones. A south-facing masonry wall can raise effective zone by one half-step. A low-lying frost pocket can lower it by the same. Your USDA zone is a starting point.
Zone boundaries shifted in the 2023 revision. The 2023 map reflects 1991–2020 normals. Many areas shifted half a zone warmer compared to the 2012 map (1976–2005 baseline). Plants that struggled on the zone boundary a decade ago may now be reliably hardy.
For the interactive ZIP code zone lookup, see the USDA Hardiness Zone Lookup →. For lawn seeding rates by grass type, see the Lawn Seed Calculator →.