Methodology & data sources
This page documents every step from the raw public datasets to the numbers shown on the site, so that anyone can check or reproduce them.
1. Hardiness zones
Zones by ZIP code come unaltered from the PRISM Group at Oregon State University ZIP code listings for the 2023 USDA Plant Hardiness Zone Map (1991–2020 mean annual extreme minimum temperature): 40,502 ZIP codes for the contiguous U.S., Alaska, Hawaii and Puerto Rico. A city's headline zone is the zone covering the largest share of its land area, using the Census 2020 ZCTA–place relationship file; each city page lists every ZIP code in the city with its zone and land share.
The 2012-to-2023 comparisons use the same rule with PRISM's 2012 ZIP listings (1976–2005 data). Of 40,480 ZIP codes present in both, 55.7% moved one half-zone warmer, 41.8% stayed the same and 1.0% moved one half-zone colder.
Zone dot maps plot the Census 2024 Gazetteer center point of every ZIP Code Tabulation Area, projected with an Albers equal-area projection (Alaska, Hawaii and Puerto Rico as insets) and binned into small squares. They are our rendering and not the official USDA Plant Hardiness Zone Map. ZIP codes used only for PO boxes have no ZCTA point and are not drawn.
2. Frost dates and probabilities
Frost dates are 32°F freeze probabilities from NOAA NCEI U.S. Climate Normals, 1991–2020 (annual/seasonal files, variables ANN-TMIN-PRBLST-T32FPxx and ANN-TMIN-PRBFST-T32FPxx). Hard-freeze dates use the 28°F series, and growing season length uses ANN-TMIN-PRBGSL-T32FPxx. Spring probabilities are the chance of a freeze after the date; fall probabilities are the chance that the first freeze has occurred by the date. The charts on city pages plot all nine probability levels (10% to 90%) for both thresholds.
The 30% date is the headline “planning” date. Where 32°F occurs too rarely for NOAA to compute dates, we say frost is rare and report the share of years in which the station reached 32°F, instead of inventing a date.
3. Matching a city to a weather station
- Candidate stations: NOAA stations with 1991–2020 freeze statistics and at least 10 years of record (shorter records are used only if nothing else exists within 200 km).
- Search radius and elevation limit widen step by step: within 25 km and 150 m (about 500 ft) of the city center's elevation; then 50 km; then 100 km; then the elevation limit relaxes to 300 m; then 150 km and 500 m; finally the nearest station.
- Within a step, the station with the lowest score wins, where score = distance in km + elevation difference in m ÷ 20.
City elevations come from the USGS Elevation Point Query Service (with Open-Meteo as a fallback). Each city page shows the station's name, ID, distance, elevation difference and years of record. We checked the method against published 30%-probability dates for 20 large cities before launch.
4. Planting calendars
Each crop has timing rules counted in weeks from the last spring frost or the first fall frost (for example, tomatoes: start indoors 6–8 weeks before the last frost; plant out 1–3 weeks after). These are general horticultural rules of thumb, not recommendations from any agency. City calendars apply them to the city's 30% dates. Zone calendars use the median 30% dates of the cities in that zone; zones without a city on this site use the median of NOAA stations whose nearest ZIP code (within 15 km) is in that zone, or the nearest zone with data. Fall windows are shown only when they start at least 30 days after the last spring frost. Frost-free locations use seasonal windows.
5. Questions and answers
The question-and-answer sections on city, zone and crop pages are generated from the same data: station probabilities, growing-season percentiles, state rankings, 2012-versus-2023 zone listings and comparisons with the nearest city that uses a different station. Questions that do not apply to a place (for example, zone mixes for a city entirely in one zone) are left out.
6. Cities
The 512 cities are the 500 most populous incorporated places in the U.S. (Census Vintage 2024 estimates) plus at least two places per state; coordinates come from the 2024 Census Gazetteer, with a few corrections where the Census internal point falls offshore or in mountains.
7. Limitations
Microclimates matter: valleys, slopes, urban heat and nearby water can shift frost dates by weeks (see Microclimates). Probabilities describe 1991–2020, not any particular year. Confirm local timing with your Cooperative Extension office.
Software and updates
The site is generated as static pages; the ZIP lookup runs in your browser from static files. Last full data rebuild: October 19, 2026.