The Self-Reliant Garden
field note · garden · the calendar that matters

Your frost dates, from the federal record instead of a rule of thumb

"Plant after Mother's Day" is a rule of thumb for one part of one state. The actual number, the date your location normally sees its last freeze, is published by NOAA for more than nine thousand weather stations, and it is a probability rather than a promise.

What "frost date" actually means

NOAA defines a freeze as temperatures dipping to 32 °F or below. The dates you see published are not the earliest or latest freeze ever recorded; they come from the 1991 to 2020 US Climate Normals, which are thirty year averages, and they are expressed as probabilities of a threshold being crossed.

That distinction is the whole practical point. A "last frost date of 5 May" does not mean 6 May is safe. It means that on average, by that date, the freezing weather is behind you, and in some years it is not.

NOAA NCEI, last spring freeze, published 29 March 2016 and updated 22 March 2024. US Climate Normals 1991 to 2020, dataset published 4 May 2021, current version 1.0.1 revised 15 May 2023.

How to look up yours

  1. Open the NOAA Climate Normals Quick Access tool at ncei.noaa.gov/access/us-climate-normals/.
  2. Choose the Annual/Seasonal dataset for 1991 to 2020.
  3. Find the station nearest you, by map or by name. There are over 9,800 of them for temperature and precipitation.
  4. Look in the agricultural normals, which carry the probabilities of the first and last time temperature thresholds are crossed, and the normal length of the growing season.

Pick a station that shares your terrain, not just your distance. A valley station and a hilltop station ten miles apart can differ by two weeks, and cold air pools downhill.

NOAA NCEI, US Climate Normals product page, read 17 September 2026. We could not confirm that an interactive frost map as friendly as the old one exists for the 1991 to 2020 period; the tool above is the reliable route, and it is a table rather than a map.

Using the two dates

The last spring freeze tells you when to move tender plants outdoors, and the first autumn freeze tells you how long you have. Between them is your growing season, which is the number that decides whether a long season plant is worth starting at all.

  • Hardy perennials like thyme, sage and oregano do not need the date; they are already outside.
  • Tender annuals like basil and calendula started indoors want the last freeze behind them.
  • Rosemary in a pot needs the first autumn freeze marked on your calendar, because it goes back inside before it.

Which of your herbs is in which group depends on your zone: herbs by USDA zone. And if you are starting seeds indoors, the light numbers are here.

Why we do not print a table of dates by city

Because it would be a copy, and a copy goes stale without telling anyone. The normals get revised, stations move, and the useful version is the one you pull from the source for the station that matches your ground. The three minutes it takes are the same three minutes we would have spent, and you get the current number.

Sources, with the date of each

  • NOAA NCEI, US Climate Normals 1991 to 2020, dataset published 4 May 2021, version 1.0.1 revised 15 May 2023.
  • NOAA NCEI, last spring freeze, published 29 March 2016, updated 22 March 2024.
  • NOAA NCEI, Climate Normals Quick Access, read 17 September 2026.