Your zone-map frost date is based on an area bigger than Connecticut
The frost-date maps every gardener grew up with are derived from a few hundred long-running weather stations, with the gaps filled in by interpolation. That's fine for a national map. It's not fine for your backyard. A cold-air drainage pocket on the north side of a hill can be two weeks colder than the ridge three hundred yards away.
Here's how to get the actual frost dates for your exact latitude and longitude, using open climate data — and what to do with those dates once you have them.
What "frost date" actually means
- Last spring frost — the last day in spring when temperature dropped to 0°C (32°F) or below. After this, warm-season crops can go in the ground (with a buffer — see below).
- First autumn frost — the first day in autumn when temperature dropped to 0°C. Anything still in the ground at harvest needs to come out before this.
- Frost-free growing season — the number of days between the two. For Minneapolis it's about 160; for New Orleans it's about 290.
The cutoff is 0°C by convention, but "frost damage" can happen at 2–3°C under clear-sky, calm-air conditions (radiative frost) because the leaf surface is colder than the air. For frost-tender crops (tomatoes, basil, peppers), treat 2°C as your danger zone.
The two sources of free frost data
1. ECMWF ERA5 via Open-Meteo (global)
ERA5 is the European Centre for Medium-Range Weather Forecasts' reanalysis dataset — hourly weather going back to 1940 at ~31 km grid resolution, with a statistical downscale available at ~9 km. Open-Meteo serves it for free via a simple HTTP endpoint. This is the best global source.
Request: daily=temperature_2m_min for your latitude and longitude, start 10 years ago, end last December.
2. NOAA GHCN (US + global stations)
The Global Historical Climatology Network has actual weather-station observations for 100,000+ stations worldwide, going back decades. Best for locations near a long-running station. Downside: station coverage is uneven — rural areas in the global south have few stations. For those, ERA5 is more reliable.
The math: finding the median frost date
- Group daily temperatures by year.
- Define the hemisphere midpoint — July 1 for northern, January 1 for southern. Days before the midpoint are "spring"; after are "autumn."
- For each year: last spring frost = max(days with min ≤ 0°C in spring); first autumn frost = min(days with min ≤ 0°C in autumn).
- Median across years is your frost date. Mean is noisier because a single outlier year moves it a lot.
This is exactly what AgriLab computes when you enter a location — you don't need to write any of this code. But understanding the method is useful when you want to know why the number you're seeing is what it is.
What to do with your frost dates
- Cool-weather crops (hardy): spinach, kale, peas, lettuce — plant 2–4 weeks before last frost. They tolerate light frost and germinate in cool soil.
- Half-hardy: bush beans, potatoes — plant around the last frost date.
- Tender crops: tomatoes, peppers, basil, cucumber, zucchini — plant 14 days after last frost. Tender plants can survive 0°C briefly but won't grow below 10°C night temps.
- Long-season tender: winter squash, sweet potato, melons — work backwards from first frost: plant date = first frost − 14 days buffer − days to harvest.
The 10-year median isn't a promise
Roughly half of years have a frost after your median spring frost date. That's what "median" means. The 14-day buffer for tender crops drops the risk from ~50% to under 5%. Ninety-fifth percentile "no chance" dates are usually 21 days past the median — useful for high-value crops where a replant is expensive.
Always pair your frost-date calendar with a live 10-day forecast. Even in June, a polar outbreak can hit Minneapolis. The forecast is what saves the plant; the frost date is what plans the season.
How climate change is shifting these dates
Across North America and Europe, last-spring-frost has moved earlier by ~7 days since 1980; first-autumn-frost has moved later by ~5 days. Your growing season is ~12 days longer than your parents' was — but year-to-year variability (±3 weeks) is still much bigger than the trend. This is why a 10-year median captures the current reality: it averages out year-to-year weather, but still reflects the shifted baseline.
A sliding 10-year window is better than an old frost-date map from 1981–2010. Your plot is warmer than it was, and the map is older than you think.
The fastest way to get your numbers
AgriLab pulls 10 years of ERA5 for your exact spot, computes the median frost dates, pairs them with the right per-crop buffer, and shows the next 10 days so you can catch the outlier events. Free, no card, any location.