Planting Date Calculator
Find indoor start and transplant dates from USDA zone and last frost date.
Covers tomatoes, peppers, cucumbers, and 30+ crops with days-to-maturity guidance.
Planting date calculation determines the optimal time to start seeds indoors or transplant seedlings outdoors based on your last frost date and each crop’s cold tolerance.
The Formula:
Outdoor planting date = Last Frost Date + (Cold hardiness offset in days)
Indoor start date = Outdoor planting date − (Weeks to transplant size × 7)
Cold Hardiness Categories:
| Category | Plant Examples | Offset from Last Frost |
|---|---|---|
| Hardy (survive frost) | Kale, broccoli, lettuce, peas, radish, onion | −3 to −5 weeks (plant early) |
| Semi-hardy | Chard, beets, carrots, potato, celery | −2 to −3 weeks |
| Tender (frost kills) | Tomatoes, peppers, basil, beans, corn | +1 week (after last frost) |
| Very tender | Cucumber, squash, melons, eggplant, okra | +2 to +3 weeks |
Indoor Start Lead Times:
| Crop | Weeks Indoors Before It Goes Out |
|---|---|
| Onion, leek, celery | 10 weeks |
| Peppers, eggplant | 8 weeks |
| Tomatoes | 7 weeks |
| Broccoli, cabbage, kale, basil | 5 weeks |
| Lettuce, chard, melons | 4 weeks |
| Cucumber, squash, pumpkin | 3 weeks |
| Beans, corn, peas, carrots, beets, radish | none, sow direct |
That last row matters more than it looks. Root crops and legumes resent having their roots disturbed, and a direct-sown bean overtakes a transplanted one within a fortnight. Starting them indoors is work that makes the harvest later, not earlier.
Worked Example:
Zone 6, last frost April 15, growing tomatoes:
- Tomatoes are tender, so they go out 1 week after the last frost: April 22
- They want 7 weeks indoors first: April 22 − 49 days = March 4
- Days to maturity is 72, so the first pick lands around July 3
- Zone 6 has 183 frost-free days and the crop needs about 79 from the last frost, which leaves plenty of room
Practical Tips:
- Find your last frost date from a USDA Hardiness Zone map or your local agricultural extension office. Entering it in the field above beats the zone average, which can be two weeks out either way.
- Late frosts happen. Keep row cover fabric ready until nights consistently stay above 4 °C (40 °F).
- Soil temperature matters as much as air temperature. Most seeds germinate best at 18–24 °C (65–75 °F) soil temp, and a tomato set into cold soil sits there sulking while a later one passes it.
- Harden off transplants over 7–10 days before moving them outdoors permanently.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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