About Seed Window

Updated 2026-11-08

Seed Window is a seed starting calculator for home gardeners. You enter a US ZIP code to see NOAA frost dates for the nearest weather stations (or type your own last spring frost date), choose crops, and it works back to the dates for sowing indoors, hardening off, planting out and sowing outside. It shows them on one timeline, turns them into a reminder list and exports them as a calendar file (.ics), a CSV spreadsheet or an A4 or US Letter PDF.

Where the timings come from

• Crops started indoors (23 crops): Johnny’s Selected Seeds seed-starting date calculator, weeks indoors and the safe set-out time relative to the frost-free date.
• Crops sown outside (7 crops): Colorado State University Extension, Vegetable Planting Guide (GardenNotes #720).
• Frost dates by ZIP code: NOAA NCEI U.S. Climate Normals 1991–2020 (32°F last-spring and first-fall freeze probabilities at 10%, 30%, 50% and 90%) for the three stations nearest the Census Bureau 2024 ZIP Code Tabulation Area point. Dates are shown as NOAA publishes them. Where NOAA gives no date because a freeze is too rare (its method needs at least a 10% yearly chance), the page says so; stations with no published date for other reasons are skipped. Distances are measured from the ZIP area’s Census point, which for large rural ZIP areas can be far from the post office town.
• Cross-check of weeks indoors: University of Minnesota Extension seed-starting table, shown beside each crop in the seed chart (read 2026-11-08). It does not change the plan.
• Hardening off over 7 to 10 days: Nebraska Extension in Lancaster County.

Each crop in the calculator shows its source and the source’s own wording. The sources were checked on 2026-10-19. Sources do not always agree, and your local extension service knows your climate best.

What it does not do

Seed Window does not use your device location or a weather forecast, and it cannot promise a frost-free date. A published frost date is a long-term probability for a weather station; frost can still come later, and your garden may be colder or warmer than the station. Check the forecast and your soil before planting.

How it works

Everything runs in your browser. Your frost date, crops and ticked-off tasks are saved in this browser’s local storage so they are still there when you come back. Nothing is uploaded and there is no account.

Seed Window is an independent browser tool.

Keep more than one garden plan

Name separate plans for different beds or seasons, switch between them, or archive a plan and restore it later. Download a JSON backup to keep every plan, custom crop and completed task. Import previews a backup and adds copies alongside existing plans. There is no account or automatic cloud sync; clearing browser storage removes local plans.