PlantAlong
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How we date things

Every date on this site comes from a public dataset or a cited extension publication, through rules written down here. Where we had to choose, we say so.

Last reviewed
Sources
53
This page is for grown-ups who love details. Every source is listed at the bottom.

Hardiness zones

Zones come from the 2023 USDA Plant Hardiness Zone Map ZIP code files published by the PRISM Group at Oregon State University (1991-2020 average of each winter's coldest night). We use PRISM's files directly; they match USDA's own ZIP lookup for all 40,502 ZIP codes. Each ZIP has one zone, the one covering most of its area, so a ZIP can span more than one zone. Territories other than Puerto Rico have no published zone.

A zone says which perennials survive winter. It does not set a frost date, so we never turn a zone into a frost date.

Frost dates

Frost dates are NOAA's 1991-2020 U.S. Climate Normals: for each weather station, the dates of the last spring and first fall 32°F temperature at 10%, 50% and 90% probability. The median (50%) is the "typical" date. In spring the 10% date is the later, safer one: only one year in ten has a frost after it.

Table 1. Station records used (7,305 stations with freeze statistics).
Station typeStationsWhat we show
Dated6,949Median and 10% dates for spring and fall
Frost too rare to date176The share of years with any 32°F night; no frost-date planting rules
Frost possible any month180No planting windows; 28°F hard-freeze dates for reference

NOAA counts its frost season from August 1, so a station's "last spring frost" can fall in December and its "first fall frost" in January; the calendar handles both. Where 32°F happens in fewer than all years, NOAA's dates describe only the years with a frost, and we say so.

Matching a ZIP to a station

Each ZIP is placed at its 2026 Census ZIP Code Tabulation Area point, or, for PO-box and other ZIPs without an area, at a GeoNames point (city-level). We take the nearest station with freeze statistics and record the distance and the elevation difference (USGS elevation data). We never skip to a farther station to get a nicer answer.

No authority sets how far a station can be from a garden before its record stops applying, so the thresholds below are our own, informed by WMO guidance that local uses need stations within about 10 km, by temperatures dropping about 3.5°F per 1,000 ft of height (University of Idaho Bulletin 494), and by cold-air drainage and coastal effects.

Table 2. Confidence rules. Any warning makes a match low confidence; any caution makes it medium.
CheckCautionWarning
Distance to stationover 25 kmover 60 km
Elevation differenceover 100 mover 300 m
Location—this ZIP can only be placed at city level; location sources disagree for this ZIP
Station recordthe station record is partly estimated—
Zone checkstation 2+ half-zones from the ZIP—

Today 61.8% of ZIPs are high confidence, 16.3% medium and 21.9% low.

Planting rules

We hold 91 planting rules for 12 crops, each copied as written from a university extension publication with the passage that states it. When sources disagree we keep every version and show them side by side. We never average or blend them.

For your calendar, each crop and method uses, in order:

  1. a month calendar written for your region, where one exists;
  2. a rule written for your state;
  3. the crop's primary source, applied to your own frost dates and labelled as coming from another state;
  4. otherwise the first source with that method in this order: Colorado State, Utah State, Nebraska (G1343), Purdue (HO-66), Wisconsin, Minnesota, … with Texas last because its guidance is written for a hot climate.

Fall rules from hot-climate sources (Texas A&M (E-502), Texas A&M (E-517)) are not applied outside their state below zone 9a; they stay visible as other guidance. Rules written for a named area without a stated boundary ("central Oregon") are never treated as covering the whole state.

How a window is computed

Week offsets. "Transplant 1-2 weeks after the last frost" becomes a window from 7 to 14 days after your median last frost. Spring rules count from the last frost; fall rules from the first.

Count-backs. Where a source gives a formula (Nebraska G1343), the latest fall planting date is the first frost minus days to maturity, a 14-day fall factor and, for frost-tender crops, another 14 days.

Harvest. Days to maturity are added to the planting window, using a figure counted from the same starting point (seed or transplant), preferably from the same source. Fall plantings that grow through winter, like garlic, get no harvest date, because winter dormancy breaks that arithmetic.

Short seasons. If a frost-tender crop planted in its window would not reach its earliest maturity before the median first frost, the calendar warns you.

Conditions. Some guidance is a condition, not a date ("plant basil when the soil is at least 60°F"). We show it as written and do not invent a date for it.

Hot and frost-free climates

Frost-date rules give poor advice where frost is rare or summer heat sets the season. Where an extension service publishes a month calendar we use it instead: UF/IFAS for North, Central and South Florida, and the University of Arizona's AZ1005 for Maricopa County (44 windows in all).

  • North Florida (UF/IFAS: north of State Road 40): 34 counties
  • Central Florida (UF/IFAS: between State Roads 40 and 70): 12 counties
  • South Florida (UF/IFAS: below State Road 70): 11 counties
  • Maricopa County, AZ (low desert, AZ1005): 1 county

UF/IFAS draws its regions along State Roads 40 and 70, not county lines, so we assign only counties that lie wholly on one side. Ten Florida counties crossed by those roads have no regional calendar yet. Where no source covers a frost-free climate (coastal Southern California, Hawaii) the calendar says so instead of guessing.

Companion evidence

Each of the 38 pairings is graded. Tested in a study: a field or controlled trial of this pair and effect, with its conditions recorded. Extension guidance: recommended by an extension publication without a cited trial. Traditional: found on companion charts that trace to gardening tradition. A pairing where credible sources contradict each other is marked disputed.

Soil volumes

Bed volume is length × width × depth. Conversions use exact factors from NIST Handbook 44: 1 cubic yard = 27 cubic feet; 1 cubic foot = 28.316846592 litres. Mix proportions are the parts by volume an extension source states. Bag sizes vary by product, and compressed bales are labelled at their compressed volume (NIST Handbook 130), so check the label.

Limits

  • A weather station is not your garden. Frost pockets, slopes, walls and city heat can shift your dates by days or weeks.
  • Climate normals describe 1991-2020. Individual years vary, and recent years may differ from the average.
  • Several extension pages could not be read by our tools and are left out rather than guessed at.
  • There is no sourced guidance yet for high-mountain sites, Hawaii, Alaska or coastal Southern California calendars.

Update history

  1. : First data build: PRISM 2023 zones, NOAA 1991-2020 normals, Census 2026 ZIP areas with GeoNames fallback, NED/SRTM elevations; 12 crops entered from extension sources; companion pairings graded; Florida and Maricopa County month calendars added.

All sources

Datasets

Publications