Methods and Sources

Frost dates

Frost dates come from NOAA's U.S. Climate Normals for 1991–2020, version 1.0.1. NOAA describes a normal as “the 30-year average of a particular variable's measurements, calculated for a uniform time period.” The frost dates are statistics from those years at each weather station, not a forecast for any year.

For each station, NOAA gives the last spring frost and the first fall frost at 36°F, 32°F, and 28°F, each at chances from 10% to 90%. The chance points in one direction for each date: a last-frost date with a 10% chance means a 10% chance that the last frost of spring comes on or after it, and a first-frost date with a 10% chance means a 10% chance that the first frost of fall comes on or before it. The chance applies to that one date. It isn't the chance of frost anywhere inside a window, which is higher.

A higher threshold is reached more often, so its last frost comes later and its first frost earlier. All planting windows use the 32°F dates. The extension sources count from the average last and first frost without naming a threshold, and 32°F is the freezing point. The other thresholds are shown in the planner for reference.

The growing season is a separate NOAA statistic for the time between the last spring frost and the first fall frost, given as a length with a chance of being at least that long. It isn't worked out from the two dates.

Where a temperature isn't reached every year at a station, NOAA's dates describe only the years when it is reached. The planner then shows how often the temperature is reached. Where NOAA has no 32°F frost dates for the nearest station, either because frost that cold is too rare or because it's possible in any month, YepPlant shows no planting windows and gives only the hardiness zone.

NOAA marks each station's frost dates with a completeness flag. YepPlant uses stations with any of these flags and doesn't show the flag. NOAA's annual normals documentation defines them as:

• “S Standard - meets WMO standards for data availability for 24 or more years (missing months are filled with estimates based on surrounding stations)”
• “R Representative - meets WMO standards for data availability for 10 or more years (missing months are filled with estimates based on surrounding stations)”
• “P Provisional - meets WMO standards for data availability for 10 or more years (missing months cannot be filled due to lack of surrounding stations)”
• “E Estimated - meets WMO standards for data availability for 2 or more years for all months (nearby stations with standard normals are available to estimate normals statistically)”

The nearest weather station

A ZIP code is matched to the Census Bureau's ZIP Code Tabulation Area (ZCTA) with the same number. ZCTAs approximate the areas of ZIP codes. The Census Bureau gives each one an internal point, usually its center. YepPlant measures the great-circle distance (the haversine formula on a sphere with a radius of 6,371.0 km, with 1 mile = 1.609344 km) from that point to every station with NOAA frost-date normals, in any state, and uses the closest one. If two stations are exactly as close, the one with 32°F frost dates is used, then the first by NOAA station ID.

Distances are shown to the nearest mile. When the nearest station is more than 20 miles away, measured before rounding, the result carries a warning that local frost dates may differ. The 20-mile line is YepPlant's display choice, not a NOAA standard. NOAA notes that normals apply less well as distance from the station grows, without giving a distance.

ZIP codes without a ZCTA, such as PO Box–only ZIP codes, aren't covered. YepPlant covers ZIP codes in the 48 contiguous states and Washington, DC; for other ZIP codes, the planner says that YepPlant doesn't cover them. Washington, DC has no weather station with NOAA frost-date normals, so its ZIP codes use the nearest stations, in Maryland and Virginia, and it has no state page. Elevation differences between your ZIP code and the station aren't measured, so there's no elevation warning.

Risk levels

Each risk level picks the frost dates with one chance, used the same way in spring and fall. The growing season at a level uses the length with that chance of a shorter season.

Planting windows

Each crop rule in the extension sources counts days or weeks from the average last spring frost or first fall frost. YepPlant applies the same count to the 32°F frost dates at each risk level, so the three windows have the same length and differ only in the frost dates they start from. Conservative waits longer in spring and stops earlier in fall; Aggressive plants sooner and later.

• Transplant and outdoor sowing windows start from the last spring frost date plus or minus the rule's days.
• Indoor seed starting counts back the rule's weeks from the transplant window.
• The last sowing date counts back the days to maturity from the first fall frost date. Where the days to maturity are counted from transplanting, it's the last transplant date, and the last indoor sowing date counts back the indoor weeks from it.
• A range in a rule, such as days to maturity that vary by variety, becomes a range in the window: the earlier end is for the slowest varieties, the later end for the fastest.

Windows that run past the start or end of the planting year keep their real dates. The printable calendar draws the planting year and notes any part that falls outside it; the list and the calendar file include the full dates.

Fall planting

For frost-tender crops (basil, bean, cantaloupe, cucumber, eggplant, pepper, pumpkin, summer squash, sweet corn, sweet potato, tomato, and watermelon), the last date also leaves room for Nebraska Extension's Frost Tender Factor of 14 days, because these crops should mature before the first frost. They get no fall planting window, since the sources don't give the harvest period that window needs.

For beet, broccoli, Brussels sprouts, cabbage, carrot, lettuce, peas, radish, spinach, and turnip, the fall window ends on the latest last sowing or transplant date, Purdue Extension's last feasible planting date. It starts 14 days before the earliest one, Nebraska Extension's Fall Factor for slower growth in cool, short fall days. Where a fall window would start on or before the last day of the spring window, it's left out.

Garlic is planted after the first fall frost: from the first fall frost date to 14 days after the first fall frost. The risk levels use the same first-frost dates as for other crops, so Conservative gives the earliest window. For a crop planted after frost, the earlier window isn't the more cautious one; the levels only show how early the first frost tends to come.

Crop rule sources

Crop rules come only from university extension publications, never from blogs, seed catalogs, or other planting calendars. Each rule on a crop's page names its sources, with the date each was retrieved.

• Where sources give ranges that overlap or touch, the window covers their combined range. Where they don't overlap, one source is chosen for a stated reason, so a window doesn't start from an offset that no source gives. Days to maturity vary continuously by variety, so they span the lowest to the highest value in the sources.
• Values aren't averaged. A rule that no source gives, such as a crop's fall planting time, is left out, and no window is shown for it.

Hardiness zones

Hardiness zones are shown unchanged from the ZIP code listing (phzm_us_zipcode_2023.csv) for the 2023 USDA Plant Hardiness Zone Map, prepared by the PRISM Group at Oregon State University and released by the USDA Agricultural Research Service (ARS). Data: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed 10 Oct 2026. Oregon State University retains ownership of the GIS datasets underlying the official map, and the data are provided "as is," without warranty. Each zone is looked up by the ZIP code that matches the ZIP Code Tabulation Area (ZCTA) number; ZIP codes without a ZCTA are not covered. This site shows zones as text only and does not reproduce the map. Nothing on this site is the official USDA Plant Hardiness Zone Map; the official map is at https://planthardiness.ars.usda.gov/. Citation: USDA Plant Hardiness Zone Map, 2023. Agricultural Research Service, U.S. Department of Agriculture. Accessed from https://planthardiness.ars.usda.gov/. Hardiness zones describe average winter lows for perennial plants and are not used to calculate any planting date on this site.

Data sources and citations

Frost dates: Palecki, Michael; Durre, Imke; Applequist, Scott; Arguez, Anthony; Lawrimore, Jay (2021). U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020). ANN-TMIN-PRBLST, ANN-TMIN-PRBFST, ANN-TMIN-PRBGSL, ANN-TMIN-PRBOCC. NOAA National Centers for Environmental Information. Dataset. https://doi.org/10.25921/4ek7-fk11. Accessed October 10, 2026. Version 1.0.1, published May 4, 2021 and revised May 15, 2023. NOAA data are in the public domain in the United States and come without warranty.

ZIP Code locations: U.S. Census Bureau, 2026 Gazetteer Files, ZIP Code Tabulation Areas (internal points). State assignment: U.S. Census Bureau, 2020 ZCTA to County Relationship File. A ZIP Code Tabulation Area (ZCTA) approximates the area of a ZIP Code; some ZIP Codes, such as PO Box–only ZIP Codes, have no ZCTA.

What YepPlant doesn't include

• Weather forecasts, frost warnings, soil temperature, or growing degree days.
• Days to maturity for specific varieties, yields, detailed crop rotation plans, or planting times based on summer heat.
• Planting times for perennials, bulbs, or lawns based on hardiness zones.
• Alaska, Hawaii, Puerto Rico, and other US territories.
• Plant identification, houseplants, pests, or plant diseases.

Updates

YepPlant checks the NOAA, Census Bureau, and PRISM download pages and every extension source once a month. NOAA says it produces official normals “every 10 years.” When NOAA, the Census Bureau, or PRISM publishes a new version, the data are rebuilt from the new files and checked before they replace the current version. When an extension source changes, the change is reviewed before any crop rule is revised.

A station that fails a check in newer NOAA normals keeps showing its earlier normals, with a note that the newer normals are still being checked. Other stations and ZIP codes that fail a check are held back rather than shown with doubtful dates.

Related pages

• Planting calendar and frost dates by ZIP code