Frost dates in Ottawa: last spring frost and first autumn frost
When is it safe to plant out in Ottawa, and when does the first frost come? Dates from 45 years of nightly minimum temperatures (1981–2025), with how early or late they can fall.
Frost dates in Ottawa and how sure they are
| Early year (1 in 10) | Typical (median) | Late year (1 in 10) | |
|---|---|---|---|
| Last spring frost | 26 April | 6 May | 18 May |
| First autumn frost | 29 September | 10 October | 27 October |
In a typical year the last frost in Ottawa falls around 6 May and the first one around 10 October, leaving about 161 frost-free days (between 145 and 174 in 8 years out of 10). To be safe, plant tender crops after 18 May: by then the last frost has passed in 9 years out of 10. Bring them in before 29 September: in 1 year in 10 the first frost comes earlier.
Frost month by month in Ottawa
| Month | Average low | Coldest night (avg) | Record low | Frost nights | Years with frost |
|---|---|---|---|---|---|
| January | -17.2 °C | -32.2 °C | -46.3 °C | 30.4 | 100% |
| February | -15.8 °C | -29.5 °C | -40.5 °C | 27.5 | 100% |
| March | -8.8 °C | -22.9 °C | -33 °C | 28.5 | 100% |
| April | -0.3 °C | -7.3 °C | -14.5 °C | 16.3 | 100% |
| May | 6.7 °C | -0.9 °C | -4.1 °C | 1.7 | 71% |
| June | 12.3 °C | 5.8 °C | 0.9 °C | 0 | 0% |
| July | 15.2 °C | 9.9 °C | 6 °C | 0 | 0% |
| August | 14.2 °C | 8.5 °C | 3.2 °C | 0 | 0% |
| September | 9.8 °C | 2.8 °C | -1.9 °C | 0.2 | 13% |
| October | 3.6 °C | -3.6 °C | -6.5 °C | 6.7 | 96% |
| November | -2.8 °C | -10.9 °C | -20.3 °C | 22.2 | 100% |
| December | -11.3 °C | -25.5 °C | -44.4 °C | 29.8 | 100% |
Frost here means a night minimum at or below 0 °C at 2 m. Frost nights is the average count per month; years with frost is how many years had at least one.
Frost alerts for your garden or field
LayEarth Business Agro sends an alert when the forecast low at your saved site drops below your threshold (0 °C by default), with frost risk, hours below zero and white or black frost for each forecast day. LayEarth Pro, from 9 USD a month, adds alerts for the places you follow.
Temperature map around Ottawa
The live temperature layer of the map (NOAA GFS forecast): watch the cold nights coming hour by hour.
More for Ottawa
Will it rain today in Ottawa? · Open the temperature map on Ottawa
Questions
- When is the last frost in Ottawa?
- On average around 6 May. In an early year the last frost comes by 26 April, in a late one (1 year in 10) after 18 May.
- When is the first frost in Ottawa?
- On average around 10 October. In 1 year in 10 it comes before 29 September, in a mild year not until 27 October.
- How long is the growing season in Ottawa?
- About 161 frost-free days between the last spring frost and the first autumn frost, from 145 in a short year to 174 in a long one.
- When can I plant tomatoes in Ottawa?
- Frost-tender plants such as tomatoes, peppers, beans and courgettes go out after the last frost: after 18 May the risk is down to 1 year in 10. Hardy crops can go earlier.
- How cold does it get in Ottawa?
- The coldest night of winter averages -33.8 °C; the lowest since 1981 was -44.4 °C in 1994.
Frost dates nearby
Montreal · Toronto · Buffalo · Quebec City · Boston · New York
How these dates are computed
Daily minimum temperature at 2 m from NASA POWER (MERRA-2 reanalysis, public domain), 1981–2025, corrected from the ~50 km grid to the real altitude of Ottawa (6.5 °C per km). For each year: the last night at or below 0 °C before midsummer and the first one after it; the dates are the median and the 10% / 90% of those years. A garden in a frost pocket or a city centre can differ by days. Live temperatures: the temperature layer of the map.