Seed Starting Date Calculator
Starting seeds at the right time is critical for a productive garden or market garden. Start too early and seedlings become overcrowded and root-bound; start too late and the transplants won't be ready for spring planting weather. Enter your last frost date and the number of weeks your crop needs indoors to find the ideal seed starting date and target transplant date.
Seed starting date formula
Transplant date = Last frost date + (offset weeks x 7 days)
Seed starting date = Transplant date - (weeks to grow indoors x 7 days)
For crops that go in before the last frost (hardy crops like kale or broccoli), use a negative offset (for example, -2 weeks means transplant 2 weeks before the last frost date). For tender crops (tomatoes, peppers, basil) use a positive offset of 1 to 2 weeks after the last frost date for safety.
Weeks to grow indoors by crop
- Tomatoes, peppers, eggplant: 6 to 10 weeks before transplant.
- Broccoli, cabbage, cauliflower: 4 to 6 weeks before transplant.
- Celery, celeriac: 10 to 12 weeks before transplant.
- Cucumbers, melons, squash: 2 to 4 weeks before transplant.
- Lettuce, kale, chard: 4 to 6 weeks before transplant.
Seed starting date calculator: frequently asked questions
How do I find my last frost date?
The USDA Plant Hardiness Zone Map and NOAA climate data provide average last spring frost dates by ZIP code or climate station. The NOAA Climate Services portal at climate.gov and the USDA Agricultural Research Service both publish historical frost date tables by location.
What does weeks to transplant mean?
Weeks to transplant is the number of weeks a seedling needs to grow indoors before it is large enough to be moved outdoors or into a field. Most vegetable seedlings need 4 to 12 weeks depending on the crop. Tomatoes and peppers typically need 6 to 10 weeks; brassicas need 4 to 6 weeks; cucumbers and squash need 2 to 4 weeks.
Should I adjust seed starting dates for cold or warm years?
The last frost date is a statistical average (typically the date with a 50% probability of frost). In a cold year the actual last frost may be 2 to 4 weeks later than average. Many extension services recommend using the date with a 10% probability of frost (later date) for frost-sensitive crops like tomatoes and peppers.
Can I start seeds too early?
Yes. Seedlings started too early become root-bound, leggy, or overly mature before outdoor planting conditions are safe. This can set back growth after transplanting. Follow the weeks-to-transplant recommendations for your specific crop and variety.
What is hardening off and how long does it take?
Hardening off is the process of gradually exposing indoor-started seedlings to outdoor conditions (sun, wind, temperature swings) before transplanting. This typically takes 7 to 14 days. Build hardening-off time into your schedule so seedlings are ready on your target transplant date.
Official sources
- NOAA Climate Services: Freeze and Frost Data by Location.
- Cornell University Vegetable Production Guides: Commercial Vegetable Production Guides.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.