Seed Starting Calculator: When to Start Seeds Indoors
Already know your average last frost date? Type it in. The calculator counts back from it to the day to start each crop indoors. Don't know it yet? Look it up by station with the [frost date tool](/frost-dates-and-planting-calendar/).The rules behind the dates
- Cool-season crops (the Idaho guide names broccoli and onions): move outside 2 to 5 weeks before the average last frost.
- Warm-season crops (tomatoes and peppers): move outside about 1 to 3 weeks after it.
- Start date = move-out date minus the weeks the crop needs indoors.
For example, with a last frost of April 15: tomatoes move out April 22 to May 6. At 5 to 9 weeks indoors, the earliest start is 9 weeks before April 22 (February 18) and the latest is 5 weeks before May 6 (April 1).
Weeks indoors for nine crop groups
| Crop | Ideal soil temperature for germination | Days to germination | Weeks to grow indoors |
|---|---|---|---|
| Broccoli, cauliflower | 65–75 °F | 5–10 | 5–7 |
| Cabbage, kale, collards | 65–75 °F | 5–10 | 5–7 |
| Celery | 65–75 °F | 10–18 | 8–10 |
| Lettuces | 65–70 °F | 7–10 | 3–5 |
| Melons | 80–85 °F | 5–10 | 3–4 |
| Onions, leeks, shallots | 65–70 °F | 10–14 | 4–8 |
| Peppers | 75–85 °F | 10–14 | 6–9 |
| Squash | 70–85 °F | 7–14 | 3–4 |
| Tomatoes | 75–80 °F | 7–14 | 5–9 |
Source: University of Idaho Extension, CIS 1209: Start Seeds Indoors with Success (2014), Table 1
The guide gives no move-out timing for cabbage, celery, lettuces, melons or squash, so the calculator shows only their weeks indoors. A start window is wide because the weeks indoors vary and so does the move-out window. If you want a single date, pick a target move-out day inside the window and subtract the weeks for your crop.
Before you start
Pick the right last frost date. The 50% date is an average, so about half of years still have a freeze after it. For tender plants the 10% date is safer. See what frost dates mean.
Sources
- University of Idaho Extension, Start Seeds Indoors with Success (CIS 1209), Table 1 and text on moving transplants outdoors.