Methodology

The data

Frost dates come from the NOAA NCEI U.S. Climate Normals 1991–2020, published by NOAA’s National Centers for Environmental Information. For each weather station, NOAA estimates the dates of the last spring and first fall freeze at 32°F and 28°F, at several probability levels, from thirty years of daily minimum temperatures.

Typical date and safer date

We show two dates for each season. The typical date is the median (p50): in half of the years that get a freeze, the last spring frost comes later. The safer spring date is the one with only a 1-in-10 chance of frost after it in those years (column p10); in fall we use the date with a 1-in-10 chance of frost before it (column p10). NOAA computes these dates only over the years that actually freeze, so where a freeze does not happen every year we also show the share of years that get one. Planting windows are counted from the safer dates.

Matching a place to a station

• Each city or ZIP code is matched to the nearest station that has both spring and fall 32°F dates.
• The station must be within 40 km. We also skip stations whose elevation differs by more than 250 m from the station nearest to the place (used as a stand-in, since we do not have the exact elevation of every town). If none qualifies, we leave that place out rather than guess.
• Every page names the station and its distance, so you can judge how well it represents your garden.

Planting windows

For each crop we use week offsets from university extension services and the USDA: weeks before the last frost to start seeds indoors, weeks after it to transplant, weeks relative to it to sow outdoors, and weeks before the first fall frost to sow a fall crop. Each crop page lists its sources. See all crops.

Places where frost is rare

When a station does not record a 32°F freeze in most years, or a 32°F freeze happens in fewer than half of years, frost dates stop being a useful planting guide. We do not invent one: those pages say frost is rare and give a calendar by season instead (cool-season crops in fall and winter, warm-season crops outside the peak of summer heat).

Hardiness zones

Zones come from the 2023 USDA Plant Hardiness Zone Map, produced by the PRISM Group at Oregon State University. A zone is set by the average of each winter’s lowest temperature over 1991–2020, in 5°F half-zones from 1a to 13b. See all zones.

• For a ZIP code we show the zone from PRISM’s official table for that ZIP. If the table has no zone for it, we show none.
• For a city we use the zone of the ZIP nearest the city center, within 15 km, and name that ZIP on the page. Zones can vary within a city, with elevation and terrain.
• A zone describes winter lows, not frost dates. Planting windows on this site are always counted from NOAA frost dates, never from the zone.

Hardiness zones: 2023 USDA Plant Hardiness Zone Map, PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed 9 Nov 2026.

Limits

• Climate normals describe the past thirty years, not this year’s forecast. Check the forecast before setting out tender plants.
• Low spots, valleys and open ground can freeze when the station does not; south walls and cities stay warmer.
• 32°F is a light freeze. Hardy crops survive it; tender crops may not survive even a near-freezing night.