Frost Date Math

Last Frost Date and First Frost Date by State and Station

By Tam Vu · Updated

On this page
  1. How to read the tables
  2. How the seed-starting calendar works
  3. About the data
  4. Sources
Choose your state and the weather station closest to you. You get the dates when freezes usually end in spring and begin in fall, at three levels of risk, and a seed-starting calendar built from the average last freeze date.

Frost dates by station

Pick a state, then a weather station.

Station dates are for the station's location. A yard on a hill, near water or in a city can differ by days.

How to read the tables

  • Spring table. Each cell is a date. On the 50% date, a freeze at that temperature is as likely to still come as not. On the 10% date, the chance of a freeze after it is only 10%. On the 90% date, it is still very likely that a freeze is coming.
  • Fall table. The 10% date is early: only a 10% chance a freeze has already happened by then. The 90% date is late, and by then a freeze has very likely happened.
  • Temperatures. NOAA reports three thresholds: frost (36 °F), freeze (32 °F) and hard freeze (28 °F). The seed-starting calendar uses the 32 °F date. The guide to what frost dates mean explains the three levels and which one to trust for tender plants.

How the seed-starting calendar works

The calendar is arithmetic on one date, the last spring freeze at 32 °F:

  1. Move-out dates. The University of Idaho guide says cool-season crops such as broccoli and onions can go outside 2 to 5 weeks before the average last frost, and warm-season crops such as tomatoes and peppers about 1 to 3 weeks after it.
  2. Start dates. Take the move-out window and count back the weeks each crop needs indoors. The earliest start is the earliest move-out date minus the longest indoor time. The latest start is the latest move-out date minus the shortest.
  3. Other crops. The guide gives indoor weeks for cabbage, celery, lettuce, melons and squash but no outdoor timing, so those rows show only the weeks. Follow the seed packet for the move-out date.

Worked example: a station with a 50% last freeze of April 18. Tomatoes move out April 25 to May 9 (1 to 3 weeks later). They need 5 to 9 weeks indoors, so the earliest start is 9 weeks before April 25 (February 21) and the latest is 5 weeks before May 9 (April 4).

Seedlings grown indoors need to be hardened off before they go out, which the Idaho guide says usually takes at least a week. Count that into your plan.

About the data

Dates are from NOAA's National Centers for Environmental Information, U.S. Climate Normals 1991–2020, for 6,948 stations in the 50 states. Some stations in warm areas have a last freeze date before January 1 or a first freeze date after it. Those show "before Jan 1" or "after Jan 1" and are not errors. They mean that in most years the last cold night of the winter comes before the new year, or the first one after it.

Sources