Turn last-spring and first-fall frost dates into frost-free days and simple plant/transplant windows for cool- and warm-season crops.
Use this when planning garden or field vegetable calendars from your local average frost dates (Extension or NOAA normals).
Frost-free season = days between last spring frost and first fall frost. Cool-season crops can start before the last frost; warm-season transplants usually wait until after it.
Frost-free days count from the day after last spring frost through the first fall frost date (inclusive planning variant shown in steps).
frost_free_days = first_fall − last_spring
cool_start = last_spring − cool_lead_days
warm_plant = last_spring + warm_wait_days
latest_plant = first_fall − days_to_maturity
Last frost 15 Apr 2026, first frost 20 Oct 2026 → 188 frost-free days. Cool crops from ~18 Mar; warm transplants ~22 Apr; a 75-day crop’s latest plant ≈ 6 Aug 2026.
Assumptions: calendar arithmetic on the dates you enter; averages are not guarantees.
Limits: microclimates, elevation, and unusual cold snaps override 30-year normals.
Responsibility: verify dates with local Extension; protect transplants on forecast freeze nights.
Cooperative Extension, NOAA/NCEI normals, or trusted local garden calendars for your ZIP/county.
Light frost (~32°F) vs hard freeze (~28°F) calendars differ — use the threshold that matches your crop sensitivity.
Count backward from first fall frost by days-to-maturity plus a buffer for slowing autumn growth.