How the frost dates are built

The zone and its temperature range are taken, unchanged, from the ZIP-code files the PRISM Group at Oregon State University publishes on its 2023 Plant Hardiness Zone Map page, which the official USDA map (USDA-ARS) is built from: phzm_us_zipcode_2023.csv (39,921 ZIP codes), phzm_ak_zipcode_2023.csv (272 ZIP codes), phzm_hi_zipcode_2023.csv (133 ZIP codes) and phzm_pr_zipcode_2023.csv (176 ZIP codes), 40,502 ZIP codes in all; the files are dated November 17, 2023 and were downloaded October 27, 2026. The columns used are zone and trange, the range of the average coldest night of the year in °F; the °C figures are converted by this site. 18,634 of the 18,659 ZIP pages have a zone; the others are not in the files.

This site draws no maps from the PRISM data and does not compute zones itself. Oregon State University owns the datasets; on 26 October 2026 the PRISM Group confirmed by email that showing the ZIP-code zones, with this attribution, on this ad-supported site is permitted. The hardiness zone by ZIP tool and the zone pages read the same files.

Each ZIP code takes the nearest station, measured from the ZIP code's centre, that publishes the 32°F freeze normals. Half of the pages are within 5.6 miles of their station. 24 pages are farther than 25 miles and say so; a station more than 100 miles away is not used at all.

NOAA cautions that normals describe the station: the farther a place is from it, and the bigger the difference in height, the more its weather can differ. Frost in particular settles in low ground on clear, still nights, so a garden in a hollow can freeze when the station does not.

A spring date at 10% is the date after which a night that cold came in only one year in ten; at 50%, half of years; at 90%, nine years in ten. A fall date at 10% is the date by which the first night that cold had come in one year in ten. Most frost date websites print one of these levels — often 30% or 50% — which is why they disagree with each other by a week or more for the same town. This site prints all nine.

NOAA computes the dates from the years that reach each temperature. Where that is every year, the dates are the whole story. Where it is not, the pages give the share of years first and treat the dates accordingly:

• a 32°F night in at least 90% of years — the dates are shown as they are (16,809 pages);
• in 50–90% of years — the dates are shown with the share of years, and risks quoted on the page are scaled by it (651 pages);
• in fewer than half of years — no 32°F date is given; the page reports how often a freeze happens and in which month (830 pages);
• never in 1991–2020 — the page says so (356 pages).

Where a freeze comes most years but only after New Year, the dates are called the first and last freeze of winter, not a fall and spring frost.

Some ZIP pages draw the same figures as a curve: the chance of another 32°F night after each spring date and of the first one by each fall date, with straight lines between NOAA's nine dates (the same interpolation the risk tool uses) and the 28°F curve dashed. Where a freeze comes in fewer than nine years in ten, the curve is scaled by that share. The days between the two 10% crossings, and how long each curve takes to pass from 90% to 10%, are read off the same lines.

Neighbouring ZIP codes that share a station would repeat the same figures, so one page per station is offered to search engines: the most populous ZIP code whose nearest station it is, when the station is within 25 miles. Pages that readers were already landing on stay in search as well (13 pages, visits from 2026-09-26 to 2026-10-26). That makes 5,028 of the 18,659 ZIP pages. The others stay online for readers and links, marked noindex, and each names the page that presents its station in full.

A state has its own page when at least 5 NOAA stations stand behind its indexed ZIP pages (51 states). The guides compare those stations with each other and with NOAA's previous normals, for 1981–2010, matched by station ID; a station counts only where it is in both sets. The risk by date and compare tools read the same figures for any ZIP code with a station within 100 miles, including ZIP codes without a page; the zone tool answers for every ZIP code in the zone files.

• 27 October 2026 — hardiness zones now come straight from the PRISM/Oregon State University ZIP-code files instead of phzmapi.org, a third-party copy of the same map. Of the 31,617 ZIP codes the site knows, 31,421 had the same zone and range in both, 0 differed, 16 had no zone before and have one now, and 180 (25 with a page) had a zone from phzmapi.org but are not in the official files, so they show none.
• 26 October 2026 — warm-climate pages showed a "first fall frost" in January and a 365-day season, because they printed the 50% date even where frost comes in fewer than half of winters. They now give the share of winters with a freeze instead.
• 26 October 2026 — Puerto Rico ZIP codes were matched to a station in Florida, about 1,000 miles away, because Puerto Rico stations were left out when the data was loaded. They now use the nearest Puerto Rico station.