How the tool works
How we calculate seed-starting dates
Choose when you plan to plant outdoors. The calculator counts backward by the number of weeks the selected crop normally grows indoors.
Calculation
Start seeds indoors = outdoor planting date minus weeks growing indoors
Example
Peppers normally grow indoors for 8-9 weeks. For an outdoor planting date of May 10, 2026, the seed-starting window is March 8-15. About two weeks before planting, gradually introduce the seedlings to outdoor conditions.
Rules currently used by the calculator
These values are stored as structured data and used directly by the calculator. Crop-specific guidance overrides the general table for eggplant, broccoli, cabbage, cauliflower, onions and leeks. Celery keeps the general table value because no matching crop-specific page was available in this review.
| Crop | Weeks indoors | Planning note |
|---|---|---|
| Tomatoes | 5-6 weeks | Start indoors, then transplant after hardening off and when local conditions are suitable. |
| Peppers | 8-9 weeks | Do not transplant from this planning date alone; wait until nighttime lows are above 50°F. |
| Eggplant | 8 weeks | Wait until nighttime lows are above 50°F before transplanting. |
| Broccoli | 5 weeks | Crop-specific spring guidance replaces the general 10-11 week table; fall crops use a different calendar. |
| Cabbage | 4-6 weeks | This range is for spring transplants; the crop-specific guide direct-sows fall cabbage. |
| Cauliflower | 5 weeks | Crop-specific guidance uses about four weeks indoors plus one week of hardening. |
| Onions | 10-12 weeks | Crop-specific guidance overrides the general 14-15 week table. Day-length type still must fit your latitude. |
| Leeks | 10-15 weeks | Transplant after early spring weather settles and daytime temperatures reach at least 45°F. |
| Celery | 12-13 weeks | Celery germination and early growth are slow. |
Why the result is a window
When the source provides a range, the calculator subtracts both ends. This preserves the published uncertainty instead of replacing it with one falsely precise date.
What is not calculated
The tool does not predict weather, germination, soil temperature, transplant success or the correct outdoor date. Users must choose that date from local frost risk, crop conditions, forecasts and variety guidance.
Use the result as a planning checkpoint
Confirm the variety instructions on the seed packet and check current local conditions before sowing or transplanting.
Open the seed calculator