Why the 2023 hardiness zone map looks warmer

Last updated October 19, 2026 · Sources: cited below

When USDA released the new Plant Hardiness Zone Map in November 2023, many gardeners discovered they had “moved” half a zone without going anywhere. This guide explains why the 2023 map is warmer, how much it changed — using our own ZIP-by-ZIP comparison of the 2012 and 2023 listings — and what, if anything, you should do about it.

What changed between 2012 and 2023

Three things changed at once:

A newer 30-year period. The 2012 map was based on 1976–2005 data; the 2023 map uses 1991–2020.¹ That drops 15 older years (1976–1990) and adds 15 recent ones (2006–2020).²
Many more stations. USDA says the 2023 map used 13,412 weather stations, compared with 7,983 for 2012.³ PRISM's own write-up counts 13,625 stations from national, regional and state networks — about 68% more than before.²
Better mapping. Improved interpolation, especially in mountains and in Alaska, where the map's resolution became much finer.³

USDA summarizes the result this way: about half of the country shifted to the next warmer half-zone and the other half stayed the same, with warming in the range of 0–5°F.³ The map-creation notes describe the new map as “generally about one quarter-zone warmer” than 2012 across much of the U.S. — and note that some areas came out colder because of the new data and methods.¹

Our ZIP-by-ZIP comparison

The PRISM Group publishes ZIP code zone listings for both maps.^4,5 We matched the two files: 40,480 ZIP codes appear in both with a valid zone. Counting half-zone steps between 2012 and 2023:

Change from 2012 to 2023	ZIP codes	Share
One half-zone warmer	22,543	55.7%
No change	16,927	41.8%
A full zone warmer (two half-zones)	530	1.3%
One half-zone colder	398	1.0%
Three or more half-zones either way	48	0.1%

So USDA's “about half” is borne out at the ZIP level: a little over half of ZIP codes warmed by one half-zone, and most of the rest did not change. Large jumps are rare and mostly in mountainous or thinly populated ZIP codes, where the extra stations made the biggest difference.

The pattern varies by state. Among states with at least 100 ZIP codes in both listings, the highest shares of warmer ZIP codes were in TN (85.9%), AL (79.9%), MO (79.4%), WV (78.9%); the lowest were in IA (24.5%), CA (23.9%), ND (20.0%), PR (19.3%). California is notable for also having 7.0% of its ZIP codes come out colder.

For the 512 cities on this site (using the same land-share rule for both maps), 271 moved one half-zone warmer, 232 stayed the same, and 9 came out colder: Los Angeles, CA (10b→10a), Fremont, CA (10a→9b), Santa Rosa, CA (9b→9a), Provo, UT (7a→6b), Yuma, AZ (10a→9a), Daly City, CA (10b→10a), Westminster, CA (10b→10a), Buena Park, CA (10b→10a), Lakewood, CA (10b→10a). Each city page now states its 2012 and 2023 zone.

Large cities, 2012 versus 2023

Among the 50 most populous cities on this site, 31 have a warmer headline zone on the 2023 map. A selection:

City	2012 zone	2023 zone	Change
New York, NY	7b	7b	no change
Los Angeles, CA	10b	10a	half-zone colder
Chicago, IL	6a	6a	no change
Houston, TX	9a	9b	half-zone warmer
Phoenix, AZ	9b	9b	no change
Philadelphia, PA	7a	7b	half-zone warmer
San Antonio, TX	8b	9a	half-zone warmer
San Diego, CA	10a	10b	half-zone warmer
Dallas, TX	8a	8b	half-zone warmer
Jacksonville, FL	9a	9b	half-zone warmer
Fort Worth, TX	8a	8b	half-zone warmer
San Jose, CA	9b	9b	no change
Austin, TX	8b	9a	half-zone warmer
Charlotte, NC	7b	8a	half-zone warmer

Headline zone = the zone covering the largest share of the city's land, from the PRISM/OSU ZIP listings for each map.

Is this climate change?

Partly, but the zone shift is not a clean measure of it. USDA and the map's developers caution against reading zone changes as evidence of global warming, for two reasons: the yearly extreme minimum is a highly variable statistic, and part of the change comes from more stations and better methods rather than from the climate.^1,3 Climate trends are usually judged from average temperatures over 50–100 years, not the coldest night of the year averaged over 30.¹

NOAA's 1991–2020 normals do show most of the country warmer than the 1981–2010 normals, while parts of the north-central U.S. cooled somewhat, especially in April.⁶ That is consistent with what the zone listings show: the northern Plains changed less than the Southeast and the Ohio Valley.

What it means for your garden

Plants that thrived before will still thrive. A half-zone shift means the average coldest night is up to 5°F milder. Nothing about your existing garden suddenly became unsuitable.
Be cautious about “zone-pushing.” A warmer label does not remove the risk of an unusually cold winter. If you try perennials rated for your new zone but not your old one, start with a few plants in sheltered spots.
Don't change your vegetable calendar because of the zone. Vegetable timing depends on frost dates, which NOAA computes separately from the zone map. Use your city page or read Frost dates explained.
Check your exact ZIP. Some cities now span more zones than before. Our city pages list each ZIP code's zone and share of land.

Why some places came out colder

About 1% of ZIP codes moved to a colder half-zone, and they cluster in particular places. In California, 7.0% of ZIP codes are colder on the 2023 listing, including much of the Los Angeles basin: Los Angeles went from 10b to 10a by land share. In Hawaii the figure is 10.5%. USDA's map notes explain that additional data sources and improved interpolation can produce a cooler zone, not only a warmer one.¹ In places with steep terrain or a sharp coast-to-inland gradient, more stations simply describe the cold spots better.

A colder label does not mean winters there got colder. It means the 2023 map describes those locations more precisely. As with warmer shifts, plants that have done well will keep doing well.

Did my frost dates change too?

Not because of the map. Frost dates on this site come from NOAA's 1991–2020 freeze statistics, a separate dataset from the hardiness map. The two use the same 30-year period, so they are consistent with each other, but a half-zone shift in the map does not move your frost dates. A city can move half a zone warmer and keep essentially the same last-frost date, because the coldest night of winter and the last cold night of spring are driven by different weather.

That is why our city pages show both: the zone (for perennials) and the frost dates (for vegetables and annuals).

How to check your own ZIP code

Enter your ZIP in the lookup to get your 2023 half-zone.
Open your city page and read the question “Did the hardiness zone change on the 2023 map?” — it gives the 2012 and 2023 zones by land share.
If your city spans several zones, check the ZIP table on the city page; neighborhoods on hills or near water may differ.
For the official 2012-to-2023 difference maps, see the USDA map downloads at planthardiness.ars.usda.gov.

Reading the zone pages

Each zone page on this site includes a dot map of every ZIP code listed in that zone, drawn from ZIP code center points, and a count of how many ZIP codes were in the zone in 2012 versus 2023. These maps are our own rendering of the PRISM ZIP listings and are not the official USDA Plant Hardiness Zone Map; for the official map and the official 2012–2023 difference maps, visit planthardiness.ars.usda.gov.

For what the number and letter mean in practice, start with How to read your hardiness zone.

Sources

Figures marked as “our data” or “on this site” were computed by US Grow Zones from the PRISM/OSU ZIP code listings (2012 and 2023) and NOAA 1991–2020 Normals; see the methodology.