Where the dates come from

Frost dates

A frost here means a night when the temperature at the weather station drops to 0 °C (32 °F) or lower. For each station and each year from 1991 to 2020 we find the last frost of spring and the first frost of fall, leaving out years with too many missing days. A station needs at least 15 usable years.

Each town page shows two spring dates:

• Safe to plant out: only 1 year in 10 had a frost after this date. Tender crops like tomatoes are timed from it.
• Average last frost: about half of years had a frost after it. Hardy crops like peas are timed from it, because a late frost won't kill them.

We checked our method against official figures. At 256 Canadian stations, our averages match Environment and Climate Change Canada's 1991–2020 normals to within a week at about 97% of stations. At 281 US stations, our safe dates match NOAA's 1991–2020 freeze probabilities to within a week at about 85–90%.

Matching towns to stations

Each town uses the weather station that best matches it, weighing distance against height: 100 m of elevation difference counts the same as 10 km of distance, because frost dates change quickly with height in hilly country. Every town page names the station it uses, its distance and both elevations.

When the station is more than 40 km away or more than 150 m higher or lower than the town, the page says so, because the dates may then be off by a week or more. Towns with no reliable station within 100 km don't have a page yet.

Warmth as well as frost

Frost-free isn't the same as warm. In mild coastal places the last frost can come in February, long before it is warm enough for beans or tomatoes. So planting dates also wait until the area's average daily temperature reaches a level each kind of crop needs: about 6 °C for hardy crops, 8 °C for semi-hardy ones, 11–12 °C for tender crops and 14 °C for heat-lovers like melons and okra. These levels are rules of thumb checked against local practice.

Crop timing

How many weeks before the last frost to start seeds, when to transplant, and how long each crop takes come from seed company and university extension guides, including Johnny's Selected Seeds and the extension services of Colorado State, Purdue, North Dakota State, Montana State, Utah State, Iowa State and Alabama, and Manitoba Agriculture. Real gardens vary: use the dates as a starting point and adjust with what you see.

Sources and credits

• Daily weather records: NOAA Global Historical Climatology Network – Daily (GHCN-Daily). Canadian records in it come from Environment and Climate Change Canada.
• Climate normals used for checking: Environment and Climate Change Canada, Canadian Climate Normals 1991–2020 (Open Government Licence – Canada); NOAA U.S. Climate Normals 1991–2020.
• Place names, populations and elevations: GeoNames, licensed under CC BY 4.0.

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