Data sources & method

Hardiness zones

USDA Plant Hardiness Zone Map 2023, produced by the USDA Agricultural Research Service and the PRISM Climate Group at Oregon State University, using the ZIP code table published by PRISM. For a city we use the zone of the ZIP code whose center is closest to the city center, and we list the zone of every ZIP code of the city.

Frost dates

NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020 (annual/seasonal, by station). We use the 32°F freeze probability dates: the "average" date is the 50% probability date; we also show the 10% and 90% dates. For each city we pick the nearest NOAA station with frost data within 40 km (25 miles) and with an elevation difference of no more than 300 m (about 1,000 ft) from the city. If no station qualifies, the page says Info missing instead of guessing. The station, its distance and elevation are shown on every page.

Places and ZIP codes

ZIP code centers: U.S. Census Bureau Gazetteer Files 2024 (ZCTA). City names, ZIP-to-city and population: GeoNames (CC BY 4.0).

Planting calendar

Planting windows are counted from the average last spring frost with the typical timing recommended by U.S. university extension services (for example: start tomatoes indoors 6–8 weeks before the last frost and transplant 1–2 weeks after). Local conditions vary: check your state extension service and the forecast.

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