What changed in the 2023 USDA zone map

What the map is measuring

The USDA Plant Hardiness Zone Map is not a frost calendar and it is not a forecast. USDA’s map site describes it as the average annual extreme minimum winter temperature, drawn as 10°F zones and then split into 5°F half-zones. The 2023 map uses weather-station measurements from 1991 through 2020. That is the same 30-year span NOAA uses for the current U.S. Climate Normals, but the two products answer different questions. The zone is one number: the average of each year’s coldest temperature. A frost date is a day on the calendar, with a probability attached.

The Agricultural Research Service announcement of the 2023 map says the country is divided into 10°F zones and further into 5°F half-zones, and that the map includes data measured at stations from 1991 to 2020. Compared with the 2012 map, the announcement says about half of the country shifted to the next warmer half-zone, and the other half stayed in the same half-zone. That is the change USDA stated. The ZIP table here is the 2023 edition, so a city page reports the 2023 zone. For a side-by-side with 2012, use USDA’s map viewer and read both editions for the same spot.

Half-zones, in the file this site uses

The ZIP table is the PRISM Climate Group file phzm_us_zipcode_2023.csv. Retrieved on October 10, 2026, it has 39,921 rows and four columns: zipcode, zone, trange, and zonetitle. The trange field is the temperature band printed with the zone. Zone 9a in that file is “20 to 25.” Zone 9b is “25 to 30.” Zone 8a is “10 to 15.” Zone 8b is “15 to 20.” Those bands are degrees Fahrenheit, matching USDA’s half-zone width of 5°F. A page on this site prints the band next to the zone so the letter is not left as a code.

The half-zones present in that ZIP file are 3a, 3b, 4a, 4b, 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b, 9a, 9b, 10a, 10b, 11a, and 11b. Colder and hotter half-zones exist on the national map in places this ZIP table does not cover. The table’s ZIP codes run from 00501 to 99403. There are no rows beginning 967 or 968, so Hawaii is absent. There are no rows beginning 995 through 999, so Alaska is absent. Puerto Rico’s 006–009 prefixes are absent too. Anchorage and Honolulu therefore have frost dates and no zone. Anchorage’s nearest qualified station is within a couple of miles. A zone is assigned only from a ZIP within 30 miles, so a lower-48 ZIP is never used for those cities.

How a city gets one zone

A city is not a ZIP code. The Census place record used here is an incorporated place in the 2024 population estimates, joined to the 2024 Gazetteer for a latitude and longitude. The internal point is a Census point inside the place, not a city hall and not a downtown garden. For most large cities it lands close to a ZIP Code Tabulation Area. The site takes the nearest ZCTA centroid that also appears in the PRISM file and prints the distance.

San Francisco is the cautionary case. The city’s legal boundary includes the Farallon Islands, and the Census internal point sits well west of the neighborhoods, far enough that the nearest zoned ZIP was more than 20 miles away. The page then uses a GeoNames populated-place coordinate for the same state and name. With that coordinate, ZIP 94102 (zone 10b in the 2023 file) is about three tenths of a mile from the point, and the qualified station is under 6 miles. The city page says which coordinate source was used. If you garden on the west side of the city or on a ridge, you should still read the zone as the ZIP’s zone, not as a survey of your yard. Microclimates is the reason a single ZIP can be a blunt instrument.

Houston’s city page uses a ZIP a little over a mile from the Census point and comes out 9b. Austin’s comes out 9a. Those are 2023 values for the ZIP that won the distance test. A nursery sign, an older book, or a memory of the 2012 map can disagree. When they disagree, the disagreement is often the half-zone shift USDA described, or it is two different ZIP codes inside one city. This site will not average them.

What a warmer half-zone does not mean

A shift to the next warmer half-zone means the average of the coldest night of each year, over 1991–2020, fell in a band 5°F warmer than the band used for that place on the 2012 map. It does not mean every winter is 5°F warmer. It does not mean the last 32°F date moved by a fixed number of days. It does not mean a plant rated for the new zone will survive a cold night that still happens. USDA’s own framing is an extreme-minimum average. A single Arctic outbreak can undercut that average by a wide margin, which is why the frost-date probabilities exist as a separate product.

Plant tags use the zone as a survival rating for perennial cold, not as a sowing date for tomatoes. A tomato is an annual in most of the country. It dies at a freeze whether the tag says zone 9 or zone 5. The useful question for that tomato is the last spring 32°F probability at a nearby station, plus the planting window in the state extension guide. The zone still matters for figs, rosemary left in the ground, fruit trees, and anything expected to live through January. Use it for that job.

How to read a zone page here

Each half-zone page lists how many ZIP codes in the PRISM file carry that code, and which cities on this site matched it. If several cities share the half-zone, the page also shows the earliest and latest median last-32°F dates among those cities’ stations. That spread is not a USDA statistic. It is a range computed from the stations already attached to those cities, and the page says so. Zone 8 in Texas, for example, covers places with very different spring calendars. The half-zone tells you about the coldest night. The station tells you about the calendar. You want both, and you want them labeled.

If your ZIP is not the one named on a city page, look the ZIP up directly. That page shows the PRISM zone and, when a center point is available, the nearest qualified station and the miles. A city page pairs a zone and a station when both fall inside the distance rules on the methodology page.

Sources

• USDA Agricultural Research Service, “USDA Unveils Updated Plant Hardiness Zone Map” (November 15, 2023): https://www.ars.usda.gov/news-events/news/research-news/2023/usda-unveils-updated-plant-hardiness-zone-map/
• USDA Plant Hardiness Zone Map: https://planthardiness.ars.usda.gov/
• PRISM Climate Group, 2023 PHZM ZIP CSV: https://prism.oregonstate.edu/phzm/data/2023/phzm_us_zipcode_2023.csv
• U.S. Census Bureau, 2024 Gazetteer files and sub-est2024 incorporated-place estimates, used for city points and population.
• GeoNames populated places, used only when the Census internal point was more than 15 miles from the nearest zoned ZIP.