Your last frost date is a range, not a day

The date you were told

A single “last frost date” feels useful. It's a useful starting point, but it hides how much spring varies. In a cold spring the last frost can land weeks later than in a warm one. If you plant tender crops to a published average, you are betting that this year looks like that average — and the archive shows how often that bet fails.

Extension calendars and almanacs still quote one day because gardeners ask for one day. Plan & Plant starts from daily weather for your area, then shows the spread across the springs in this sample. That spread is the planning tool; the Typical mark is only a midpoint, not a promise.

What the springs actually did

Each dot below is one spring’s last frost: the last day that season when the daily minimum air temperature at two metres was at or below freezing. Orange mark = typical last frost. The axis runs from early March through June so you can compare places side by side.

• A last frost
• Average last frost
• Usual range

Place	Early (1 in 10 years)	Typical (half of years)	Late (1 in 10 years)	Earliest / latest on record
Denver	Apr 21	May 8	May 19	Apr 16 2012 / May 24 2019
Seattle	Mar 3	Mar 25	Apr 14	Feb 26 2003 / Apr 20 2008
Chicago	Mar 27	Apr 12	Apr 21	Mar 10 2012 / May 9 2020

Data: ERA5-Land via Open-Meteo, air and soil temperatures, 35 seasons (1991–2025). Algorithm 2026-10-season-12. Frost = daily min 2 m air ≤ 0 °C32 °F0 °C (32 °F).

In Denver the last frost swung by 28 days across the seasons in this archive. That is not a rare outlier year — it is the width of the set. The latest-ever last frost in the range is the rare worst case, not the day to plant by. Planting “on the average day” ignores half the springs that came earlier or later.

Same latitude, different spring

Latitude alone does not set your frost range. Elevation, continentality, and coastal air all shift the pattern. The three places below are examples only: a high short-season city, a mild coastal city, and an inland continental city. Open each place page for its own climate chart and planting dates. Your garden is not any of these three — look yours up in the search at the end.

What to do

• Plant tender crops (for example tomatoes) after your place’s Safe last-frost date, or keep cover you trust ready.
• Sow hardy cool-season crops from the early end of the last-frost range when you accept a light frost.
• Treat the latest-ever last frost in the range as the rare worst case, not your default plant-out day.
• Recheck your place page before planting out — a weather model estimate for your area is not a thermometer in your garden.