Frost dates and planting dates on plantingzonesbyzipcode.com come from NOAA 1991–2020 weather-station records, matched to each ZIP code’s nearest weather station, and from planting rules published by university extension services. This page explains each step, so every date on the site can be checked.
Where Do the Frost Dates Come From?
The frost dates come from the NOAA 1991–2020 U.S. Climate Normals, published by NOAA’s National Centers for Environmental Information. The normals summarize 30 years of daily temperature readings at more than 7,000 U.S. weather stations, and they are the same records used by the National Weather Service and university extension services.
What Does “In 7 of 10 Years” Mean?
“In 7 of 10 years” means the frost date is a cautious date, not an average. The last spring frost date is the date after which frost occurs in only 3 of 10 years. The first fall frost date is the date before which frost occurs in only 3 of 10 years. Both dates use 32°F, the temperature at which water freezes. NOAA calls these the 30% probability dates; the Old Farmer’s Almanac uses the same convention.
What Is a Hard Freeze?
A hard freeze is a night at or below 28°F, cold enough to kill most tender plants. Planting tables on this site show the first hard freeze (28°F) next to the first frost date (32°F), using the same “3 of 10 years” convention.
How Is the Weather Station for a ZIP Code Chosen?
The weather station for a ZIP code is the nearest NOAA station with frost records, measured from the center of the ZIP code’s area. The search covers every state, so a ZIP code near a state line uses the closest station across the line. Every result names the station and its distance.
3 notes appear when a result needs care:
- Short record: the station has fewer than 10 years of records, so its frost date is less reliable.
- Stations disagree: the 2 nearest stations are about the same distance away and their first frost dates differ by more than 3 weeks, often across a hill, valley, or coastline. The result names the second station and its date.
- PO-box ZIP code: a PO-box or single-address ZIP code has no delivery area, so the station is chosen from the post office location.
Where Do the ZIP Code Locations Come From?
ZIP code areas come from the Esri USA ZIP Code Boundaries (July 2024). PO-box and single-address ZIP codes, which have no area, use the post office location from GeoNames.org, licensed under Creative Commons Attribution 4.0.
Where Do the Planting Zones Come From?
Planting zones come from the 2023 USDA Plant Hardiness Zone Map, the current official map. A planting zone measures how cold winter gets, so it decides which plants survive the winter; the frost date decides when to plant.
How Are Planting Dates Calculated?
Planting dates are calculated from the weather station’s frost dates with the planting rules of university extension services. Each crop page names its sources. For garlic, the rule comes from the University of Minnesota Extension, Penn State Extension, Cornell Cooperative Extension, the University of Georgia Extension, UC Master Gardeners, Texas A&M AgriLife Extension, and UF/IFAS Extension.
Where winters are too mild for a frost-based rule, the planting rule uses the number of nights a year below 40°F, also from the NOAA 1991–2020 records. Garlic, for example, needs 6–8 weeks below 40°F to form a full bulb, according to Texas A&M AgriLife Extension.
What Are the Limits of These Dates?
The dates describe a typical year at the nearest weather station. 3 limits apply:
- A garden in a valley, on a hilltop, or near a lake or coast can frost earlier or later than the station.
- Any single year can be colder or warmer than the 1991–2020 records.
- High-elevation stations with frost in every month have no frost date; their results give a rule instead of a date.
How Are the Dates Checked and Updated?
Before each update, an automated check compares every station’s dates with the NOAA records, confirms that every ZIP code has a location and a station, and lists any ZIP code whose dates changed. Each crop’s planting rule is tested against the dates its university sources give for their own areas. The current data version is 2026.10.21-1.
To see the frost dates for a ZIP code, use First and Last Frost Dates by ZipCode; to see garlic planting dates, use When to Plant Garlic.