Gardenia Organic

Frost dates & seed-starting calendar Calculator

Frost dates & seed-starting calendar

Season dates and crop timing → sowing windows, succession dates and calendar downloads.

Open full calculator · Calculation schema v2 · Sources checked 2026-10-28

Season dates & provenance
Only applies to a selected historical station. Spring and fall probabilities describe different tails.

Station examples describe historical normals, not forecasts or your garden. Manual dates remain editable. A first autumn freeze date is not a guarantee that a crop can grow until then.

Crop timing template

Templates populate only the cited indoor lead range. All other timing values are editable planning assumptions; confirm cultivar, weather and soil requirements. Source checked 2026-10-28; Gardenia human horticultural review is pending.

Planting windows

Succession & fall harvest

At most 100 succession events are generated. A truncation warning reports any remaining events. Dates are all-day values and do not shift with time zones.

Manual dates: user supplied, probability unknown. Choose a reference station to inspect historical provenance.

JavaScript enables calculations and share links. The formulas and worked example below remain readable without it. Shared inputs stay in the URL fragment.

Enter your measurements, then select Calculate.

Your estimate

Indoor sow window
—
Hardening-off window
—
Transplant window
—
Direct-sow window
—
Last planting date for fall target
—
Succession events
—
Assumptions and warnings appear after calculation.

Worksheet generation time appears after calculation.

Calendar events and export selection

Select events to include in CSV and all-day ICS downloads.

Formula and worked example

Indoor sow = transplant date − lead days. For 2027-05-15 and 42–56 days, sow 2027-03-20 through 2027-04-03. Hardening 7–10 days earlier gives 2027-05-05 through 2027-05-08. Fall cutoff = harvest target − maturity − explicit buffer: 2027-10-15 − 60 − 14 = 2027-08-02.

University of Maryland indoor lead templates describe time to grow transplants, not a guaranteed outdoor planting date. Check your cultivar and local forecast.

Hardiness zones are a separate reference

The USDA 2023 map uses 1991–2020 average annual extreme minimum winter temperature: 10°F zones and 5°F half-zones. It does not supply freeze dates. Canadian plant hardiness zones use a different system with multiple climate factors; a Canadian zone is not interchangeable with a USDA zone.

Read the frost protection guide.

Bundled historical examples: NOAA Indianapolis station CSV (1991–2020, 29 years), and ECCC Regina station (probability series 1884–2016, 116 years). Spring: a freeze on or after the date. Fall: a freeze on or before the date. Dataset snapshot: 2026-10-28 ET. These are illustrative station selections, not a location search.

All gardening tools

Latest Posts