How we calculate the dates

Every date on Greenly comes from ten years of weather at your own place, not from a generic chart: the median last spring frost and first autumn frost, the day the soil warms up, and each crop's own needs from cited sources.

Weather data

Daily minimum air temperature at 2 m and soil temperature at 0–7 cm from the Open-Meteo Historical Weather API (reanalysis, the Copernicus ERA5 family), for the grid cell of the place: 0.25°, about 25 km.
The last ten full calendar years; a year counts only with at least 300 days of data.

Frost dates

A frost is a day whose minimum falls below 0 °C. Each year we take the last frost of the spring half (January–June in the north, July–December in the south) and the first of the autumn half.
The date shown is the median over the years that had such a frost. A place with no frost in any of the ten years has no frost season, and its crops follow warmth and rain instead.

Soil warm-up

For each temperature from 0 to 20 °C, the first day the seven-day mean of the top 7 cm of soil reaches it; again the median over the years.

Sowing and planting rules

Tender crops go into open ground a week after the last frost, and not before the soil is as warm as the crop needs (its card gives the minimum).
Frost-hardy crops go in three weeks before the last frost; an unheated greenhouse is planted three weeks before open ground.
Seedlings are sown so that they are ready on the planting day: the crop's days to transplanting plus its days to sprouting before it.
Garlic and spring bulbs are planted a month before the first autumn frost.
Harvest is counted from sprouting by the crop's days to harvest; in a greenhouse 10% sooner.
Each window starts on the Greenly calendar date and ends on the last day that still leaves the crop’s days to harvest (from its card) before the average first autumn frost, plus six weeks of picking for crops harvested over weeks (tomatoes, peppers, aubergines, cucumbers, courgettes, pumpkins, watermelons, beans) — our rule; earlier in the window is safer.

Crop figures

Days to sprouting, transplanting and harvest, the minimum soil temperature, frost tolerance and care come from the crop's card, where every figure is checked against cited sources (the RHS, university extension services); the sources are listed on each plant guide.

What the dates do not know

They are medians: in a cold year the last frost can come a week or two later. Your own garden — a slope, a wall, a city centre — can be warmer or colder than the 25 km cell. The app lets you set your own frost dates and recalculates the calendar.
The moon is never used: the lunar calendar pages show folk tradition, labelled as such.

Weather warnings in the app

Frost: the forecast minimum in the next 48 hours below +2 °C while tender plants grow outdoors. Heat: above +32 °C while plants grow in a greenhouse. Rain of 5 mm or more counts as watering for plants outdoors.

Hardiness zone

The USDA Plant Hardiness Zone is the average of each year’s coldest night (the annual extreme minimum temperature) in 10 °F zones split into 5 °F halves «a» and «b»; the official map (USDA ARS and the PRISM Climate Group, 2023) uses 1991–2020 data on an 800 m grid.
We compute the same measure from the city’s ten years of daily minimum temperatures (ERA5 via Open-Meteo, the 25 km grid cell of the frost dates): the lowest value of each calendar year with at least 300 days of data, averaged over at least eight such years.
Reanalysis smooths the coldest nights and ten years are not thirty, so the zone is labelled «USDA-equivalent, our estimate»; the official map can differ by half a zone or more. Canada’s official zones (Natural Resources Canada) use a different index; the RHS ratings (H1a–H7) are temperature bands too, and we give the matching one for UK cities.

2016-01-01 – 2025-12-31