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How the calendars are made

Every date on this site is computed from climate data and a short list of crop rules. This page sets out the sources, the method, how well it holds up against weather-station records, and where it can be wrong.

Sources and licences

Frost dates

US and Canada: we show the national weather services’ own figures for the nearest main station: the date by which half of years have had their last spring frost (and the first autumn one), plus the one-year-in-ten dates. A station can sit a few kilometres from the city centre, and cities, valleys and coasts all shift frost by days or weeks.

UK, Ireland, Australia and New Zealand: no open, machine-readable frost normals exist, so we estimate them. From TerraClimate’s twelve monthly average lows we draw a smooth daily curve (a four-harmonic fit that keeps every monthly average), then find where it crosses a threshold: the average low on the night of the 50 % frost date. A frost needs one night well below average, and nights vary more where the climate is continental, so the threshold rises with the annual range of monthly lows:

threshold = 4.70 °C + 0.0155 × (warmest-month low − coldest-month low)

The coefficients were fitted on 88 stations. If the average low never drops to the threshold, the place is treated as having no regular frost. One-in-ten dates for estimated cities add the typical spread seen in the NOAA and ECCC data (16 days in spring, 14 in autumn).

How accurate is it?

Each station was scored with the threshold refitted without that station, so these are out-of-sample errors.

• Average error 8.3 days; half of dates within 7 days, 80 % within 12 days, 90 % within 17 days (132 spring and autumn dates).
• US stations (NOAA): average 8.5 days. Canada (ECCC): 6.2 days. UK, Ireland and Australia: 9.4 days.
• The largest misses are in the US Southeast, where the model puts the last frost two to four weeks early (Atlanta, Austin): cold snaps there are bigger than the climate’s range suggests. That is why US pages use NOAA’s own dates.
• Models that also used latitude or the day–night temperature range fitted North America better but put the UK and Ireland’s last frost three to six weeks early, so we kept the simpler one.

Positive differences mean the model’s date is later than the record. A later spring date and an earlier autumn date are the cautious direction for a gardener.

Hardiness zones

Zones follow the USDA definition: the average of each year’s coldest night, in 10 °F bands split into a and b. For US cities we compute it from the NOAA station’s daily records, 1991–2020. Elsewhere we estimate it as the coldest month’s average low minus a drop that grows with continentality (fitted on the US stations). Tested out of sample on 64 US stations: average error 1.6 °C; 36 of 64 land in the same half-zone and 58 within one half-zone. Estimated zones are marked as estimates.

Crop windows

For each city we build a daily curve of average high, low and mean temperature, then test every day of the year as a possible sowing or planting-out date. A day passes when:

1. The ground is ready: the week’s mean temperature (a stand-in for soil temperature) is at least the crop’s minimum, and highs are not scorching.
2. Frost allows it: tender crops wait until a week or two after the average last frost; hardy crops may go in several weeks before it.
3. It has time to finish: days to harvest are stretched in cool weather in proportion to growing degree-days (above 10 °C for warm-season crops, 4 °C for cool-season ones). Tender crops must give one to two weeks of picking before the first autumn frost or before autumn means fall below 12 °C (9 °C for pumpkins, which finish ripening off the vine); hardy crops must mature before frost has gone on longer than they stand. Where the frost season is shorter than the frost a hardy crop shrugs off, frost does not limit it.
4. It is warm enough to grow, and for tender crops no night in the period averages below 6 °C.
5. Heat doesn’t spoil it: average highs while it matures stay under the crop’s limit (lettuce and spinach bolt, peas and beans drop flowers), and for tomatoes, peppers and beans, nights are not so warm that fruit fails to set.

Indoor sowing is the crop’s raising time before the start of each outdoor window. Harvest is each passing date plus its (stretched) days to maturity, over the picking period. Garlic is handled separately: it is planted as autumn cools (mean below 14 °C), needs about six weeks under 10 °C, and is lifted as early summer warms.

Limits

• These are climate averages: any one year can be two or three weeks early or late. Watch the forecast before setting out tender plants.
• Your garden is not the weather station. Low ground collects frost; city centres, walls and coasts hold off frost.
• Varieties differ a lot. Early tomatoes, bolt-resistant lettuce and short-day onions all shift the windows.
• Rain and irrigation are not modelled: in dry climates everything here assumes you water.
• We don’t cover growing under cover (greenhouses, polytunnels, cold frames), which widens every window.

Climate data fetched 2026-10-26; NOAA normals fetched 2026-10-26. Something wrong for your city? See About.