Seed starting calculator

Enter your average last spring frost date, tick the crops you grow, and get a dated schedule for starting seeds indoors, setting transplants out and sowing directly in the garden.

How to use it

Find your average last spring frost date (see finding your frost dates).
Enter it and select your crops.
Sow indoors inside the shown window; set transplants out inside the transplant window after hardening them off; direct-sow crops follow the soil or date rule shown.

The formula

Set-out window = last frost + (earliest … latest set-out weeks) Sow-indoors window = earliest set-out date − (longest … shortest seed-to-transplant weeks) Example (UNH): tomato, 6–8 weeks indoors, set out 1–2 weeks after last frost. Last frost May 31 → set out Jun 7–14 → sow indoors Apr 12–26.

Weeks are taken directly from the University of New Hampshire Extension fact sheet. Negative weeks mean before the last frost (cold-hardy crops such as cabbage, onion and spinach). Crops with a soil-temperature rule (beet 45°F, carrot and corn 60°F, squash family 70°F) are shown as a note because the date depends on your soil, not the calendar.

Gear for this job

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Frequently asked questions

When should I start tomato seeds indoors?

About 6–8 weeks before you plan to set them out. UNH Extension says tomatoes are safe to set out 1–2 weeks after your last frost, so count back 6–8 weeks from that date.

Should I use my hardiness zone instead of a frost date?

Use a frost date. USDA hardiness zones are based on average winter minimum temperatures, not on spring frost timing, so two places in the same zone can have quite different last-frost dates.

Is the last frost date a guarantee?

No. It is an average from historical records, so frost can still happen after it in some years. Keep row covers handy and watch the forecast before setting out tender crops.

Why do broccoli and cabbage go out before the last frost?

They are cool-season crops that tolerate light frost. UNH lists broccoli as safe to set out 2–4 weeks before last frost and cabbage 4 weeks before.

Sources

Last reviewed 2026-10-22. Found an error? Tell us.