Where our gardening data comes from
ZIP-based recommendations combine three different kinds of data. Hardiness zones describe winter extremes. NOAA climate normals estimate freeze timing. Crop rules turn those dates into broad planting windows.
Hardiness zones
We use the 2023 USDA Plant Hardiness Zone ZIP Code Listings published by the PRISM Climate Group at Oregon State University. The zones are based on mean annual extreme minimum temperatures from 1991 to 2020.
A hardiness zone is useful for perennial survival. It is not a vegetable planting calendar and does not describe summer heat.
Freeze dates
We use the NOAA NCEI 1991 to 2020 U.S. Climate Normals. For each ZIP, we select the nearest station with valid probability dates for temperatures at or below 32°F.
The site defaults to the 50 percent probability date and shows the station distance. This is a planning estimate, not a forecast. Frost can occur above 32°F, and a garden may be warmer or colder than the station.
Place names and coordinates
ZIP place names and approximate coordinates come from GeoNames, licensed under Creative Commons Attribution 4.0. Coordinates are used during the offline data build to locate the nearest NOAA station. They do not represent a user's street address.
Planting windows
Crop rules are based on guidance from university extension programs, including Iowa State University, Utah State University, the University of Maryland, and the University of Minnesota. The calculator applies week offsets to local freeze dates.
Seed packets and local extension guidance take priority when they are more specific. Cultivar, soil temperature, elevation, weather, and microclimate can all shift the right date.
How to use the result
Use the generated date as a starting plan. Check the current forecast before planting, and adjust from your own garden records over time.