How we work out planting dates
Every planting date on this site is calculated from two things: the frost records of a weather station near you, and the timing that university Extension services recommend for each plant.
Last updated October 24, 2026
Frost dates
Frost dates come from NOAA's U.S. Climate Normals for 1991 to 2020. For thousands of weather stations, NOAA publishes the date by which the last spring freeze has passed in 90%, 50% and 10% of years, and the same for the first fall freeze. We show them as three dates instead of one average:
- Risky: in spring, frost still comes after this date in 9 years out of 10; in fall, frost comes before this date 1 year in 10.
- Typical: the median, the date most calendars call "the" frost date. Half of years have a later spring frost.
- Safe: in spring, a later frost happens only 1 year in 10.
"Frost" here means a low of 32°F at the station's thermometer, about five to six feet above the ground. Plants at ground level, in hollows and in rural open ground often see frost on nights when the station stays above freezing. We also show 28°F "hard freeze" dates, which matter for hardy crops.
Which station
Each ZIP code is matched to the most suitable NOAA station with freeze records: close by, at a similar elevation and with enough years of data. Every page names the station, its distance and its elevation difference. City pages are only published for the 819 cities with a suitable station within 30 miles and 500 feet of elevation. Neighboring cities often share a station, and then they share its frost dates and planting calendar; each city page lists its own ZIP codes and zones, and names the larger city it shares a calendar with. Only the larger city's page is listed for search engines. Where frost happens in fewer than half of years, as in much of Florida, southern Arizona and coastal California, we say so instead of printing a median frost date that would describe no real year.
Hardiness zones
Zones are the 2023 USDA Plant Hardiness Zone Map, from the ZIP code listings published by the PRISM Climate Group at Oregon State University. A zone describes the average coldest night of the year, which decides whether a perennial survives the winter. It says little about when spring arrives: across the 25 half zones, the typical last frost within one zone often spans three to four weeks. That is why zone pages show ranges, and why your ZIP code gives better dates than your zone. Our zone pages and tables are not the official USDA map.
Planting dates
For each of the 96 outdoor plants that are planted by frost dates (of 125 plant guides; houseplants are not), our researchers recorded the timing that Cooperative Extension services at land-grant universities recommend, such as "start indoors 6 to 8 weeks before the last frost" or "sow 10 weeks before the first fall frost", along with soil temperatures, days to maturity and the sources, which are listed on every plant page. Where guides give only phrases such as "as soon as the soil can be worked", we convert them to a conservative number of weeks. The calendar then applies that timing to the typical frost dates for your station.
Two checks keep the dates realistic:
- Warm soil. Frost dates alone put warm-season crops far too early where springs are frost-free but cool, such as Seattle, whose last frost comes in early March while tomato soil is not warm enough until June. NOAA's normals do not include soil temperature, so we use the normal daily average air temperature as a stand-in: a crop such as tomatoes, which needs 60°F soil, is not planted out before the normal daily average reaches 60°F at your station, and indoor sowing moves back to match.
- Fall crops of warm-season plants. Advice to plant tomatoes, peppers or potatoes again in summer for a fall crop comes from Southern Extension services. We only show it where the frost-free season is at least 210 days and spring warms early.
Where the dates can mislead
- Normals describe 1991 to 2020. Any single spring can be weeks earlier or later than these averages.
- Your garden may be warmer or colder than the station: a south wall, a city center or a hilltop can be warmer; a valley floor or frost pocket colder.
- Varieties differ. Days to maturity on seed packets vary by weeks between early and late varieties.
- Where frost is rare, heat and rain, not frost, set the seasons; plant guides list the planting months that Florida, Texas, Arizona, California and Hawaii Extension services recommend.
When the weather and the calendar disagree, trust the weather: check the forecast before setting out tender plants, and a soil thermometer before sowing warm-season crops.
Writing and review
The growing guides are written from the same Extension sources, which are cited on each page. Companion planting advice is labeled as tradition and gardener experience where no study supports it. If you find a mistake, please tell us; we correct pages and note the update date.