Data Methodology

Every number on Harman US comes from a named public source. Here is exactly where the data comes from and how we turn it into the pages you see.

Frost and freeze dates

Frost dates come from the NOAA 1991–2020 U.S. Climate Normals, published by the National Centers for Environmental Information. For each weather station, NOAA calculates the probability that a given temperature threshold (36°F, 32°F, 28°F, 24°F, and lower) occurs on or before a date in spring and fall, across the 30-year period.

• The 50% (average) value is the median: in half of years the frost falls on or before that date.
• The 10% and 90% values show the early and late ends of the range, so you can plan on the odds you're comfortable with.
• Growing season length is the number of days between the last spring and first fall 32°F frost, at the 50% level.

Matching a ZIP code to a weather station

NOAA reports normals by station, not by ZIP code. We match each ZIP code's geographic center (from the U.S. Census ZIP Code Tabulation Area gazetteer) to the nearest official NOAA station that has temperature normals, and we show you that station's name and distance on every frost page. The further the station and the more the elevation differs from your yard, the more your local frost timing can vary - so where the nearest station is far away, we mark the page as lower-confidence.

Hardiness zones

USDA Plant Hardiness Zones are from the official 2023 map, developed by the USDA Agricultural Research Service and Oregon State University's PRISM Climate Group. We look up the zone for your ZIP code from their published data. USDA Plant Hardiness Zone data: USDA-ARS and Oregon State University PRISM Climate Group.

Hay prices

Hay prices come from the USDA Agricultural Marketing Service (AMS) Market News weekly Direct Hay reports. USDA reporters collect asking and sale prices directly from growers and dealers in each state. We ingest each new report as it publishes and present the prices exactly as reported - by hay type, quality, bale, and the unit USDA uses (per ton or per bale). We do not adjust or model the prices; we link the original USDA report on each page.

Weather alerts

Active frost and freeze alerts shown on our pages come live from the National Weather Service public alerts feed (api.weather.gov). Alerts cover broad zones and may not apply to your exact location; always check your local forecast.

Sources and limits

All source data is in the U.S. public domain (17 U.S.C. § 105). These are long-term averages and official reports, not a forecast for the current year. Use them for planning and always confirm with your local forecast and, for prices, with the seller. Harman is not affiliated with or endorsed by USDA, NOAA, or the National Weather Service.

Primary sources: NOAA NCEI 1991–2020 U.S. Climate Normals; USDA AMS Market News; USDA-ARS / OSU PRISM 2023 Plant Hardiness Zone Map; U.S. Census Bureau gazetteer; NOAA/NWS api.weather.gov.