Data & methodology
Every date on when.garden is traceable to a source and carries a confidence score. Here is exactly how a planting date is produced — by whichever method fits the place — and where it can be wrong.
Live dataset: every figure is produced by the automated pipeline, town by town, from raw NOAA GHCN-Daily station records and NASA POWER over the 1991–2020 normal. “Station-based” means a real station is close enough to use directly; “modeled” means we fell back to the grid. We show the method, not just the number.
How a planting date is decided
A useful planting date depends on the climate a place actually has, so when.garden picks the method that fits — and tags every figure with which one produced it:
- Frost-anchored (temperate). From a 30-year daily record we compute the last-spring and first-fall frost at the 50% and 10% probability levels, then offset each crop by its frost tolerance and days-to-maturity. This is the method behind our temperate-climate figures.
- Wet/dry-season (tropical). Where nothing freezes, sowing is timed to the rainfall calendar (monsoon onset) from gridded precipitation normals.
- Observed calendars. Where an authoritative planting calendar exists (e.g. FAO GIEWS, national agriculture bulletins), we ingest it directly and label it observed.
- ECOCROP modelling. Everywhere else, we run FAO ECOCROP's per-crop temperature and rainfall requirements against a town's monthly climate normals (a limiting-factor suitability model) to derive the window — labelled modeled, with lower confidence.
All four methods are live: the frost method — detailed next — covers temperate towns, while the wet/dry-season, observed-calendar and ECOCROP methods cover the tropics and everywhere else.
Deriving a frost date
For the frost method, the underlying data comes from one of two places:
We use its daily minimum-temperature record — ideally a 1991-2020 normal — to compute the dates when the probability of frost crosses 50%. We show the source and its distance.
We fall back to gridded reanalysis (NASA POWER) when the nearest station is over 50 km away or differs by 300 m in elevation. Useful, but wider error bars — always labelled "modeled", never dressed up as measured.
The confidence score
A 0–100 score combining three factors (example: a dense-station location scores ~88).
Frost dates are probabilities, not guarantees. We headline the safe date — the P90 last-spring / P10 first-fall date, about a 10% frost risk, the standard threshold for setting out tender crops — and show the P50 median as “typical”. Cold-hardy crops can be planted nearer the median. Cold-air pockets, slopes, cities and coastlines still shift your real risk by a week or two. Treat these as a well-informed starting point — then watch your own sky.
Sources
All datasets are open or openly licensed · computation code published open-source · derivation method station-derived, reference window 1991-2020, min 20 years.