Methodology & data sources

Hardiness zones

Zones by ZIP code come unaltered from the PRISM Group at Oregon State University ZIP code listings that underlie the 2023 USDA Plant Hardiness Zone Map (1991–2020 mean annual extreme minimum temperature). A city's headline zone is the zone covering the largest share of its land area, using the Census 2020 ZIP Code Tabulation Area–place relationship file; city pages list every ZIP code in the city with its zone and land share. This site does not reproduce the official map, and is not the official USDA Plant Hardiness Zone Map.

Frost dates

Frost dates are 32°F freeze probabilities from NOAA NCEI U.S. Climate Normals, 1991–2020. For each city we pick the closest station whose elevation is within 150 m (about 500 ft) of the city center, widening the search radius and then the elevation limit only when no station qualifies. Each city page shows the station name, ID, distance, elevation difference and years of record.

We show three probability levels. The 30%-risk date is the main "planning" date: in spring, a frost after it happens in about 3 years out of 10. Where 32°F occurs too rarely for NOAA to compute dates, the page says frost is rare instead of inventing a date.

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. Frost-free locations use seasonal windows instead. Zone pages use the median frost dates of the cities in that zone.

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 and land area come from the 2024 Census Gazetteer. Elevations come from the USGS Elevation Point Query Service.

Limitations

Microclimates matter: valleys, slopes, urban heat and nearby water can shift frost by weeks. Confirm with your local Cooperative Extension office.