Where the dates come from

The frost dates are NOAA's 1991–2020 U.S. Climate Normals for freezes at 32 °F [1]. For each weather station, NOAA gives the date in spring after which a freeze still has a 10%, 50% or 90% chance, and the date in fall before which one has that chance. The planner uses the 50% dates as "typical" and the 10% dates as "safer".

A ZIP code is placed at the internal point of its Census ZIP Code Tabulation Area [2], and matched to the nearest stations with freeze records within 60 miles. Of 33,144 ZIP codes, 33,053 have a station; half are within 7.4 miles of theirs and 9 in 10 within 14.1 miles. 7,232 stations are used.

• —Frost-free stations: NOAA records too few freezes to compute a date at some stations (south Florida, parts of coastal California, Hawaii). 704 ZIP codes get one of these as their nearest; the planner says so and asks for your own dates.
• —Year-round frost: at some high mountain stations a freeze is possible in any month. 210 ZIP codes get one of these.
• —Large rural ZIP codes: the ZIP's centre point can be far from where you live. The planner always shows the station, its distance and elevation, and lets you choose another or type your own dates.

The library has 75 crops. Each has its timing in weeks before or after the last spring frost (start indoors, transplant, sow outdoors), a fall window in weeks before the first fall frost where the crop suits a fall crop, days to first harvest and how long the harvest lasts, compiled from US cooperative extension planting guides [3]. Where guides differ, the library takes a middle, conservative range.

A task's dates are a window: the crop's rule placed on your frost dates. A harvest is flagged when its first day falls after the first fall frost would end the crop: tender crops stop at the first frost, half-hardy ones take light frosts, hardy ones keep going about three weeks.

In square-foot mode, each one-foot square holds the square-foot-gardening count for the crop [4]: 1, 4, 9 or 16, or one plant per several squares for squash and melons. In rows mode, each plant takes its in-row spacing times the between-row spacing from the same guides. A planting's count is its area times that density, rounded down.

• —This year's weather. The dates are averages; watch the forecast and cover tender plants when a late frost threatens.
• —Your microclimate: a frost pocket at the bottom of a slope, a warm south wall, a city garden.
• —Heat. In the hottest places, summer limits some crops more than frost does; plant cool-season crops in fall and winter there.
• —Varieties. Days to harvest vary by variety; check the seed packet and adjust.