How the planting dates are calculated

Each planting date is a local frost date plus a planting rule. Frost dates come from NOAA climate normals; the rules (for example "1–3 weeks after the last spring frost") follow U.S. university extension guidance.

Frost dates

NOAA's National Centers for Environmental Information publish 1991–2020 climate normals for weather stations. For each first-order station we use the date of the last spring and first fall 32°F freeze at 50% probability (the "average" frost date) and the 10% spring date (after which a later freeze happens 1 year in 10). Stations without freezes in most years (south Florida, Hawaii) are left out.

Hardiness zones

Each station is assigned the USDA 2023 Plant Hardiness Zone of its nearest ZIP code. A zone's frost dates are the median of its 1045 stations. Zones with fewer than three stations are not shown.

Planting rules

The 60 plant rules are set in weeks before or after the frost date, plus a minimum soil temperature, following common extension-service recommendations (University of Minnesota, University of Illinois, Texas A&M AgriLife, Clemson and others). Fall windows count back from the first fall frost. Harvest estimates add the typical days to maturity to the middle of the planting window.

Limits

Average dates mean a frost after the "last frost" date happens in about half of years. Low spots, hilltops and cities can be a week or two different from the nearest station. Always check the 10-day forecast before planting tender plants, and cover them if frost is predicted.