Plant Hardiness Zone by ZIP Code: 2023 USDA Zone Lookup

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USDA plant hardiness half zones, 2023 map: average coldest winter temperature and ZIP codes in each (PRISM, Oregon State University)

Half zone	Average coldest winter temperature	ZIP codes in PRISM's listing
1a	−60 to −55 °F	5
1b	−55 to −50 °F	11
2a	−50 to −45 °F	27
2b	−45 to −40 °F	17
3a	−40 to −35 °F	23
3b	−35 to −30 °F	230
4a	−30 to −25 °F	814
4b	−25 to −20 °F	1,284
5a	−20 to −15 °F	2,426
5b	−15 to −10 °F	2,880
6a	−10 to −5 °F	5,187
6b	−5 to 0 °F	5,271
7a	0 to 5 °F	4,877
7b	5 to 10 °F	3,855
8a	10 to 15 °F	3,430
8b	15 to 20 °F	3,379
9a	20 to 25 °F	2,199
9b	25 to 30 °F	2,085
10a	30 to 35 °F	1,234
10b	35 to 40 °F	752
11a	40 to 45 °F	205
11b	45 to 50 °F	16
12a	50 to 55 °F	30
12b	55 to 60 °F	137
13a	60 to 65 °F	102
13b	65 to 70 °F	26

In short: Type a ZIP code to get its zone and half zone on the 2023 USDA Plant Hardiness Zone Map, and the band of winter cold that half zone stands for. A zone describes the coldest night of a typical winter; it says nothing about your frost dates or summer heat.

What a hardiness zone tells you

The USDA Plant Hardiness Zone Map is, in the department's own words, the standard by which gardeners and growers can determine which perennial plants are most likely to thrive at a location. It rests on a single measure: the average of each winter's coldest night. The 2023 map was prepared by the PRISM Group at Oregon State University from the years 1991–2020, and it splits the country into zones 1 to 13, each covering 10 °F of that average coldest night. Every zone is cut into two half zones, a and b, of 5 °F each, with a the colder half.

So a zone answers one question: how cold does a typical winter get here? That is what the USDA means when it says the map shows which perennial plants are most likely to thrive: a shrub, a perennial or a fruit tree has to live through that night. The table above lists all 26 half zones in PRISM's listing, from 1a at −60 to −55 °F to 13b at 65 to 70 °F, with how many ZIP codes fall in each.

Where the answer comes from

The lookup reads PRISM's own ZIP code listings for the 2023 map, unchanged: 40,502 ZIP codes in all, of which 39,921 are in the conterminous United States, 272 in Alaska, 133 in Hawaii and 176 in Puerto Rico. PRISM built the map from grids of 800 m cells for the mainland and Alaska and 400 m cells for Hawaii and Puerto Rico, then listed one half zone for each ZIP code. A ZIP code covers a whole area while the grid underneath is much finer, so read the answer as the zone for the ZIP code area rather than a measurement of your own yard.

Oregon State University owns the data behind the official map, and PRISM's terms ask that anything altered from it be labelled as not the official map. This page shows the listing exactly as published and draws no map of its own. For the interactive version, use the USDA's map, which accepts a ZIP code too.

Worked example: same zone, different springs

Take ZIP 80249, whose Census point is the closest ZIP code area to NOAA's weather station at Denver International Airport, about 2.5 miles away. Its half zone is 6a, a band of −10 to −5 °F. Now take ZIP 60106, the ZIP code area closest to the Chicago O'Hare station. It is in 6a as well, inside zone 6, which runs −10 to 0 °F. On the zone map, their winters are alike.

Their springs are another matter. NOAA's normals put the typical last spring frost at the Denver station on May 5 and at O'Hare on April 16, and the typical frost-free season at 156 days against 193. The zone was the same; the planting calendar is not. For comparison, ZIP 55450, beside the Minneapolis-St Paul airport station, is in 5a, at −20 to −15 °F.

Using the zone with the rest of the site

Use the zone for perennials, shrubs and trees that have to live through winter. For annual vegetables, which are planted after the cold and harvested before it returns, the dates that matter are your frosts: the frost dates lookup takes the same ZIP code and finds the nearest NOAA station. From there, the fall planting planner counts back to the last day each fall crop can go in, and the soil temperature chart tells you when the ground is warm enough to sow.

If the zone above differs from the one on an older plant tag or seed catalog, the map itself was redrawn. Our study of hardiness zone changes counts, state by state, how many ZIP codes and how many people moved to a new half zone between the previous USDA map and the current one, and quotes what the USDA says caused the shifts.