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How frost dates work

ZIP Frost Dates turns NOAA’s official climate normals into frost dates you can plan a garden with, and pairs them with the USDA hardiness zone for your ZIP code.

Where frost dates come from

NOAA’s National Centers for Environmental Information computed freeze probabilities for 7,283 U.S. weather stations from daily low temperatures in 1991–2020. For each station and temperature (36, 32, 28 and 24°F shown here) NOAA lists the dates by which the last spring freeze has a 10% to 90% chance of still being ahead, the same for the first fall freeze, and the growing-season length. We show those values unchanged.

  • Last spring frost, 10%: only 1 year in 10 has a freeze after this date. This is the cautious choice for tender plants.
  • 50%: the median. Half of years have a later freeze.
  • 90%: 9 years in 10 still have a freeze after this date.
  • Fall dates work the other way: the 10% date is the date by which 1 year in 10 has already had its first freeze.

Some stations are flagged by NOAA as “too cold to compute” (a freeze is possible in any month). Others rarely or never reach 32°F; for those we show the yearly chance of a freeze instead of dates. Stations without enough data are left out.

How a ZIP code gets its dates

We take the ZIP code’s center from the Census Bureau’s ZIP Code Tabulation Areas and pick the closest station by straight-line distance. 211 ZIP codes are more than 40 km from a station; the calculator warns you when that happens. Elevation matters a lot in the West, so compare the station’s elevation to yours. 7,027 ZIP codes (mostly PO boxes and single buildings) have no Census location, so we show only their hardiness zone.

Hardiness zones

Zones come from the 2023 USDA Plant Hardiness Zone Map ZIP code tables produced by USDA-ARS and the PRISM Group at Oregon State University. Each zone is based on the 1991–2020 average of the coldest winter night of each year. We show the zone and temperature range exactly as published.

Not the official USDA Plant Hardiness Zone Map. Zones shown here are the values published in the USDA-ARS / Oregon State University PRISM Group 2023 ZIP code tables, shown unaltered. For the official interactive map, visit planthardiness.ars.usda.gov.

Sources

Who we are

ZIP Frost Dates is a free reference site run by Olsen Industries. We are not affiliated with NOAA, USDA or Oregon State University. Questions or corrections: [email protected].