Ice Saints: what do 30 years of weather data say?

German gardeners wait for the Ice Saints in mid-May before tomatoes go out. We looked at the last frost at 299 weather stations from 1991 to 2020.

In short

84%
On average, the last frost is over in April.
250 of 299 stations
55%
In 9 out of 10 years, frost is over before 11 May.
165 of 299 stations
25%
Here the safe date only comes after 15 May.
75 of 299 stations

So for most places the Ice Saints are a cautious date, and for 25% of stations too early.

Based on the years 1991–2020, not a weather forecast.

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

By postcode

We also counted all 10,815 German postcodes, each with its nearest station. For 92%, the average last frost is over in April; for 66%, frost is over before 11 May in 9 out of 10 years. For 16%, the safe date only comes after 15 May.

Average across all postcodes: last frost on 19 April, safe from 6 May.

What about your garden?

The higher, the later

Share of stations where frost is over before 11 May in 9 out of 10 years:

Below 100 m77% · 68 of 88safe from 5 May (median of stations)
100 to 300 m60% · 46 of 77safe from 5 May (median of stations)
300 to 500 m47% · 35 of 75safe from 12 May (median of stations)
500 to 700 m41% · 15 of 37safe from 15 May (median of stations)
700 to 1,000 m5% · 1 of 22safe from 17 May (median of stations)

Altitude isn’t everything: München-Stadt (515 m) is safe from 22 April, Genthin in Saxony-Anhalt (35 m) only from 29 May.

Earliest and latest places

Safe the earliest

Helgoland (4 m)28 March
Norderney (12 m)29 March
Cuxhaven (5 m)5 April

Safe the latest

Oberstdorf (806 m)8 June
Carlsfeld (895 m)3 June
Elster, Bad-Sohl (558 m)31 May

Based on the years 1991–2020, not a weather forecast.

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

What does this mean for your beds?

Hardy plants like lettuce, peas and carrots go by the average date.
Tomatoes, cucumbers and beans go out once there is no frost in 9 out of 10 years.
Go by the date for your area, not by the calendar.

Last frost date by postcode
What to sow and plant in May

How we worked it out

For every station and every year, we find the last spring day on which the air at 2 m dropped below 0 °C (DWD daily records). From that come the average (median) and the date after which it did not freeze in 9 out of 10 years.

We count 299 stations up to 1,000 m with at least 24 usable years. “Before 11 May” means 10 May at the latest. “In April” includes earlier months.

Each of the 10,815 postcodes gets its nearest station, including postcodes for companies and PO boxes. The Ice Saints (11 to 15 May) are dated as in the DWD weather glossary.

How we work it out

Questions and answers

Do the Ice Saints still hold?

As a rough rule, yes, but not precisely. At 165 of 299 German weather stations, frost is over before 11 May in 9 out of 10 years; at 75, the safe date only comes after 15 May. Check the date for your area.

When is the last frost in Germany?

Averaged across all postcodes on 19 April; in 9 out of 10 years it is over by 6 May (DWD data 1991–2020). Depending on the place, the safe date ranges from 28 March (Helgoland) to 8 June (Oberstdorf).

When can tomatoes go outside?

Once there is no frost in 9 out of 10 years. Averaged across Germany that is 6 May, earlier in many places, later at higher altitudes.

When are the Ice Saints?

From 11 to 15 May, as dated in the glossary of the German weather service (DWD). It is a folk rule, not a measurement.

Sources

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