Frost dates come from NOAA NCEI's U.S. Climate Normals, 1991-2020: for each of Hawaii's ZIP codes, the median last and first 32°F dates at the nearest weather station (an airport station within 15 miles where there is one), with ZIP code points from the U.S. Census Bureau's 2023 ZCTA Gazetteer. The same station data drives our Frost Date Calculator. Planting-offset rules are drawn from US Cooperative Extension sources including UGA, UMN, NC State, Utah State, Iowa State, Penn State, and UConn Extension, cited crop-by-crop in the table above.
For more on how we build and verify reference data across this site, see our methodology page.