Our cold and soil maps, our département pages and our plant lists rest on public data and on a simple method that you can check. Here is how we make them, and their limits.
1. The weather records
We use the daily minimum temperatures measured in a screen, 2 m above the ground, over the last thirty winters (1996-1997 to 2025-2026): 1,575 Météo-France stations in France, plus those of the Royal Meteorological Institute in Belgium, the Deutscher Wetterdienst in Germany and MeteoLux at Luxembourg-Findel. A winter runs from 1 July to 30 June. For each station and each winter we keep a single value: the coldest night. A winter only counts if December, January and February are almost complete (80 days out of 90), and a station is only kept with at least 20 winters.
2. Five ways to read the cold: the caution profiles
With a station's 30 coldest nights, ranked from the coldest to the mildest, we work out five values. A real example at Muret (Haute-Garonne):
| Profile | Calculation | Muret |
|---|---|---|
| Conservative | the coldest of the 30 nights | −14.7 °C (February 2012) |
| Cautious (starting setting) | the 3rd coldest: one winter in ten | −9.9 °C |
| Normal | the 6th coldest: one winter in five | −8.9 °C |
| Gambler | the average of the 30 nights (USDA hardiness zone method) | −7.0 °C |
| Daredevil | the average of the last 5 winters | −6.6 °C |
It is not the average that kills a plant but the hardest frost it meets. The Gambler and Daredevil profiles are therefore bets: choosing a citrus tree on the 30-winter average means accepting damage about one winter in three; on the average of the last 5 winters, betting that winters stay as mild as in recent years. Our plant lists start from the Cautious profile.
3. What about the official methods?
Weather services work out “return values” by fitting an extreme-value law (Gumbel) to the same annual minimums. We ran this calculation on our 1,519 stations: our Cautious cold is on average 0.3 °C colder than the 10-year return value; the gap stays below 1 °C for 82 % of stations and below 2 °C for 97 % (at Muret: −9.9 °C against −9.7 to −10.0 °C). The largest gaps, always colder on our side, are in mountain hollows (Doubs, Jura), where hard frosts come back more often than the law predicts. For the Conservative profile the gap with the 30-year value is wider: it is more severe than the law where an exceptional winter occurred (at Muret, −14.7 °C in February 2012 against −11.5 to −12 °C), milder where there has been none in 30 years.
The American USDA hardiness zones, used by many catalogues, rest on the average of the minimums: −7.0 °C at Muret, zone 8b. That is our Gambler profile, which explains why some foreign hardiness ratings look optimistic.
4. From the station to your town
For each town (34,386 in France, 565 in Belgium, 100 in Luxembourg, 10,939 in Germany) we take the 5 nearest stations within 40 km (at least 3), the nearest counting most (weight inversely proportional to the square of the distance). If the town is higher than the station, we take off 0.5 °C per 100 m, never more than 3 °C; if it is lower, we do not warm it up, because valley floors are often the coldest. The result is rounded to the degree, towards the cold in case of a tie. Finally the order of the profiles is enforced in every town: Conservative ≤ Cautious ≤ Normal ≤ Gambler ≤ Daredevil.
The frost mask counts, for each town, the average number of days per winter when the minimum falls below the chosen threshold, from the daily records of nearby stations and altitude.
5. The suggested plants
The hardiness on our product pages is the threshold beyond which an adult, healthy, hardened citrus tree may die during a short frost. Damage comes well before: scorched leaves, then lost shoots and crop, often several degrees above that threshold (see Understanding citrus hardiness). That is why we keep a margin. A plant is suggested “without protection” if it withstands at least 2 °C more than the cold of the chosen profile; “with a fleece on frosty nights” if the winter fleece, which gains about 3 °C, is enough to close the gap. The others stay in a pot. Only plants available today are shown.
6. Soils and rootstocks
Each town's soil comes from the map of dominant soils of France (INRAE, Gis Sol), read at the centre of the town. We derive its constraints (chalky, acid, wet in winter, clayey, silty, salty), then apply the rules of our 5-question test to rate each rootstock: recommended, possible with precautions or not recommended. Outside France, with no soil map, the advice is based on the cold only.
7. The limits
- These are estimates: your garden may be milder (town, south-facing wall, seaside) or colder (hollow, valley floor, windy plateau) than the nearby station.
- With 30 winters, a value can move by about 1 °C if a winter is added or removed.
- The soil map gives the most frequent soil; your plot may be different. A soil analysis settles it.
- The climate is changing: the last five winters were on average 1.7 °C milder than the thirty-winter average, but a winter like 2012 remains possible.
Our tools: the cold map, the soil and rootstock map (in French), the frost alert (for our customers), and the Belgium and Luxembourg pages (in French).
Sources: Météo-France, daily basic climatological data (Licence Ouverte 2.0); Royal Meteorological Institute of Belgium (CC BY 4.0); Deutscher Wetterdienst (Datenlizenz Deutschland 2.0); MeteoLux, Administration de la navigation aérienne (data.public.lu, CC0 1.0); map of dominant soils INRAE / Gis Sol (Licence Ouverte); towns, altitudes and boundaries: INSEE, IGN, geo.api.gouv.fr (Licence Ouverte), Destatis, Administration du cadastre et de la topographie, Wikidata, GeoNames (CC BY 4.0), Copernicus DEM GLO-90, © EuroGeographics (GISCO). Calculations by the Greffeur d'agrumes nursery.