How we work it out

Fixed rules, one source per value, no guessed charts. Here is how we calculate and where the numbers come from.

Last frost

For every station of the German weather service (DWD) with at least 24 usable years between 1991 and 2020, we find the last spring day each year on which the air at 2 m fell below 0 °C. The median of these days is the “average last frost”. The 90% date is the day after which it froze again in only 1 in 10 years. Mountain-top stations above 1,000 m are left out.

Your postcode

Each German postcode gets the nearest station (straight-line distance to the centre of the places with that postcode). The coordinates come from GeoNames. We don’t correct for height differences between your garden and the station.

Planting calendar

Our sources give the number of weeks before or after the last frost for each job. Hardy plants go by the average. Frost-tender plants go outside on the 90% date at the earliest; starting them indoors moves back by the same amount.

Companion planting

Every pair has an evidence level. A: several trials. B: one study. C: an expert rule with a reason, such as a shared disease. D: gardeners’ lore, only if at least two sources name it. X: sources disagree. Green and red are only used for A to C.

Crop rotation

Breaks per plant family come from the Schleswig-Holstein Chamber of Agriculture and the South Tyrolean Farmers’ Association. A “4-year break” means 4 full years without that family in the bed.

What we don’t do

No moon calendar, no pesticides or home remedies, no yield promises. We don’t copy other people’s charts; we take single facts, in our own words, with a source.

Data and licences

Data: Deutscher Wetterdienst, daily minimum temperatures 1991–2020, our own calculation. Licence CC BY 4.0. Postcode coordinates: GeoNames (CC BY 4.0).

Sources

Frost dates are statistics from past years. They don’t tell you when it will freeze this year.