Frost dates and growing weather

Why a frost date is not a date

A last-frost date is really a percentile, drawn from 30 years of records at a nearby weather station. In about half of years a frost still comes after the median date. The tool above shows the median beside the 1-year-in-10 date, so you can choose how much risk to take.

Spring frost and fall freeze, side by side

Gardeners ask about both ends of the season. The last spring frost decides when it’s safe to set out a tomato. The first fall freeze decides whether what you sow in August has time to finish. Each is a range of dates, and in each the 1-in-10 date is the one to watch. In spring that’s a frost that comes late; in fall, a freeze that comes early. Enter a US ZIP code and you get both, from the same station with the same percentiles. Whichever is closer to today shows first.

Where these numbers come from

NOAA NCEI 1991–2020 U.S. Climate Normals from 4,350 weather stations. 33,047 of the 33,144 U.S. ZIP codes have a station within 80 km. Each ZIP code reads the station the Milpa planner uses for it, the nearest one, and every date is that station’s own recorded percentile, unshifted. Near a few cities the planner uses a hand-built site instead, where a station’s dates were corrected for terrain with PRISM temperature maps, and the answer says so. Nothing else in a US answer is modeled or averaged. It is the same committed table behind every location page on this site, read a second way. A city abroad, typed by name, has no NOAA normals station to read. Where a station in NOAA’s GHCN-Daily records stands within 25 km of it, with 10 or more winters on record, whether frost is rare or comes in some or most winters is that station’s own count, and the answer names the station. Where one of them has 20 or more years of daily lows recorded in full, the city’s frost dates are that station’s own too, and the answer names it. Elsewhere a city’s frost dates come from a weather model’s daily lows. Either way they are graded and credited as the city’s own frost page credits them.

Frost counts and dates from GHCN-Daily, version 3.35-upd-2026101817, NOAA National Centers for Environmental Information, cited as NCEI asks: Menne, Matthew J., Imke Durre, Bryant Korzeniewski, Shelley McNeill, Kristy Thomas, Xungang Yin, Steven Anthony, Ron Ray, Russell S. Vose, Byron E.Gleason, and Tamara G. Houston (2012): Global Historical Climatology Network - Daily (GHCN-Daily), Version 3. Daily minimum temperature (TMIN), stations outside the United States, 1991-2020. NOAA National Climatic Data Center. doi:10.7289/V5D21VHZ. Accessed 2026-10-20.

What this tool will not do

It won’t call a weather service, guess a number for a ZIP with no station near it, or tell you your hardiness zone. The zone answers a different question, which perennials survive your winter, and it can’t tell you when to sow. This tool gives frost odds from records, or says it can’t.