Last Frost Date

How it works

Everything on this site is worked out from public data published by NOAA, the USDA and the Census Bureau, using planting rules published by university extension services. This page explains each step, so you can judge the numbers for yourself.

Last checked October 25, 2026. Data last loaded October 29, 2026.

Frost dates: NOAA's climate normals

The frost dates come from the U.S. Climate Normals for 1991 to 2020, published by NOAA's National Centers for Environmental Information. For each weather station with a long enough temperature record, NOAA gives the dates of the last spring frost and the first fall frost at several probabilities, and the length of the frost-free season.

NOAA's method, which it describes in its methodology document, fills gaps in each station's daily low temperatures from its nearest neighbors, builds 10,000 simulated years by reshuffling the real ones, and counts how often the frost falls on each date. We use three of the probabilities it publishes: the date that only 1 year in 10 beats (the early year), the date half of years beat (the typical year), and the date only 1 year in 10 goes past (the late year).

"Frost" in our dates means a night when the station's thermometer reads 32°F or colder. We also show NOAA's typical dates for 28°F (a hard freeze) and 36°F (cold enough for frost on plants on a clear, still night).

The normals include 7,274 stations in the 50 states, the District of Columbia and Puerto Rico with frost statistics we can use. At 6,948, NOAA gives frost dates. At 146, frost comes in fewer than 1 year in 10, so there are no dates; we say frost is rare. At 180, all high in the mountains or far north, frost is possible in every month, so NOAA gives no dates but does give the length of the longest frost-free stretch.

In warm places where frost doesn't come every year, NOAA's dates describe only the years that have one. Our pages say so, and say how often frost comes. Where it comes in fewer than half of years, we don't build a planting calendar on it.

We copy NOAA's numbers as published. We don't adjust them for elevation, distance or recent weather.

Matching your ZIP code to a station

A ZIP code is a set of mail routes, not a place on a map. The Census Bureau publishes ZIP Code Tabulation Areas, areas built from census blocks that approximate them, with a central point and a land area for each. We use those points, from the Census Bureau's 2026 gazetteer files.

For each ZIP code we look up the elevation of that point, from NOAA's ETOPO1 elevation grid, and find the station that best matches it. "Best" means the smallest effective distance: the distance in a straight line, plus a charge for any difference in elevation, at 25 kilometers (about 16 miles) for every 100 meters (about 330 feet). Elevation changes frost dates, so a station a little farther away at your height usually describes your garden better than a closer one far above or below you. Stations whose figures NOAA marks as estimated, because their own records are short, get a further 15-kilometer charge. One check keeps the elevation charge from going too far: a station more than twice as far as the nearest one can win only if your ZIP code's point is at least 150 meters (about 500 feet) higher or lower than that nearest station, so a town on a valley floor isn't handed the dates of a station up in the hills.

If another station within about 15 miles has typical dates two weeks or more away from the one we picked, we say so, name both stations and give both sets of dates, because that is a sign that local terrain matters where you live.

We then tell you the station's name and NOAA ID, how far it is, and how much higher or lower it sits than your ZIP code's point. If it's more than about 25 miles away, or more than about 500 feet higher or lower, we say what that means for your dates.

Some rural ZIP codes cover hundreds of square miles, and the Census point can land on a mountainside where nobody lives. (Death Valley's ZIP code, 92328, has its point in the Panamint Range.) For ZIP code areas larger than 500 square kilometers (about 190 square miles), we use the town inside the ZIP code that takes up the most of it, using the Census Bureau's ZIP-to-place relationship file, and we name that town in your result. For smaller areas, down to about 8 square miles, we do the same when the Census point is at least 100 meters (about 330 feet) higher than the town with the most residents in the ZIP code, because that point is then up the hillside from where people live.

Not every ZIP code has an area. The Census Bureau notes that not every valid ZIP code is represented by a ZIP Code Tabulation Area; post-office-box and single-address ZIP codes often aren't. For those we can give the hardiness zone, which the USDA lists for them, but not frost dates, and the lookup asks for the ZIP code of the garden's street address instead.

Hardiness zones

Your zone comes from the ZIP code lists for the 2023 USDA Plant Hardiness Zone Map, published by the PRISM Group at Oregon State University, which prepared the map for the USDA. We show your 2012 zone from the matching 2012 list for comparison. We don't calculate zones ourselves, and we don't draw maps.

The planting calendar

The calendar counts from your typical last spring frost and typical first fall frost, using published rules:

Where the season is too short for a separate fall crop, the calendar leaves it out and says why. The calendar also tells you how many days to shift the spring dates to plan around the late-year date instead of the typical one.

Place pages

269 places have a page of their own: the largest places in each state by the Census Bureau's 2025 population estimates, at least three per state where the data allows, plus towns in Hawaii and Puerto Rico chosen by hand because the estimates don't rank them. A station serves one page at most. Places use the same matching as ZIP codes, except that when the area's main National Weather Service station (usually at the airport) is within about 7 miles of effective distance of the best match, the page uses it, because those stations have the longest, most complete records. A place whose best station is more than 60 kilometers (about 37 miles) away, or where frost is too rare for dates, has no page.

What the numbers can't tell you

Your privacy

The lookup runs in your browser. It downloads one data file covering every ZIP code that begins with the same three digits as yours, and finds your ZIP code in it on your computer. Your ZIP code is never put in a web address or sent to us or anyone else. If you tick "Remember my ZIP code," it's stored in your browser only. The privacy policy has the details.

The data, and when we checked it

Every source behind the guides is listed at the end of each guide. If you find a number that doesn't match its source, please tell us.