What a ZIP lookup returns
A ZIP lookup answers two questions and keeps them apart. The zone is the 2023 USDA Plant Hardiness Zone for that ZIP, taken from the PRISM Climate Group table of the Plant Hardiness Zone Map. It is the average annual extreme minimum temperature, grouped into 5°F half-zones. That number describes how cold winter usually gets. It is not a date to sow seed, and it is not a forecast for this year.
The frost dates, when a station can be matched, are 1991–2020 probabilities from NOAA’s U.S. Climate Normals. A median last-32°F date is the 50% point in that station’s record. A warm spring or a cold one can fall outside the 10% and 90% dates. The probability guide walks through that distinction with a worked station.
How to use the form
Type five digits and press Look up. The lookup reads the 2023 ZIP table. If the ZIP is not in that table, the form says so. Hawaii, Alaska, and Puerto Rico are outside the table, so those codes do not return a zone.
A ZIP in the table shows the zone and the temperature range from the source. It shows a station when one is matched. The match uses the Census 2024 center point for the ZIP and the nearest NOAA station that meets the qualification rules. The result names the station, the station id, and the miles. Those miles are straight-line distance. A long distance means the frost dates belong to that instrument. Some ZIPs have a zone and no station, because no center point is available. A neighboring ZIP’s station is not substituted.
How to read the dates
Read the 32°F table in National Centers for Environmental Information order. On the last freeze, 10% is the later date and 90% is the earlier date. On the first freeze, 10% is the earlier date and 90% is the later date. The 50% date is the median. The colored bar runs from the median last 32°F date to the median first 32°F date. A growing-season length, when shown, is NOAA’s published length.
The other tools can use the same station. A planting window appears when this ZIP’s state has a cited extension guide. The fall calculator and the seed countdown use the numbers you enter. The station rules are on the methodology page.