How the dates are worked out
- Your last frost. For your ZIP code we take the last spring freeze (32 °F) from NOAA’s 1991–2020 climate normals at the nearest weather station, the same date as on our frost dates page. “Average” is the date after which half of years have no more frost; “Safer” is the date after which only 1 year in 10 has one.
- Count back. Each plant’s start date is that many weeks before the frost, from North Dakota State University Extension’s seed-starting chart.
- Count forward to move them out. Cool-season seedlings like broccoli and lettuce go out before the last frost; tender ones like tomatoes and peppers go out 2 weeks after it.
The dates are a guide, not a rule: NDSU notes the best times vary with your location and how you garden. A warm spot under lights can grow seedlings faster; a cold spring can hold them back.
Seed starting chart: weeks before the last frost
| Plant | Start indoors (weeks before last frost) | Seed temperature | Sprouts in | Move outside (from the last frost) |
|---|---|---|---|---|
| Vegetables | ||||
| Broccoli | 10 | 70–75 °F | 5–10 days | 3 weeks before |
| Cabbage | 10 | 70–75 °F | 5–8 days | 3 weeks before |
| Cauliflower | 10 | 70–75 °F | 5–10 days | 3 weeks before |
| Kale | 10 | 70–75 °F | 5–8 days | 3 weeks before |
| Leeks | 10 | 75–85 °F | 5–10 days | 2 weeks before |
| Onions | 10 | 75–85 °F | 5–8 days | 2 weeks before |
| Eggplant | 8 | 80–90 °F | 10–14 days | 2 weeks after |
| Lettuce | 8 | 65–75 °F | 3–5 days | 4 weeks before |
| Peppers, sweet and chile | 8 | 80–85 °F | 10–15 days | 2 weeks after |
| Tomatoes | 6 | 80–85 °F | 7–10 days | 2 weeks after |
| Herbs | ||||
| Parsley | 10 | 70 °F | 10–15 days | 3 weeks before |
| Oregano and marjoram | 6 | 65–70 °F | 8–10 days | 2 weeks after |
| Basil | 3 | 70–80 °F | 5–7 days | 2 weeks after |
| Flowers | ||||
| Carnations | 10 | 65–70 °F | 10–20 days | 2 weeks before |
| Foxglove | 10 | 65–70 °F | 10–15 days | 2 weeks before |
| Lobelia | 10 | 75–80 °F | 14–21 days | 2 weeks after |
| Pansies and violas | 10 | 65–75 °F | 10–15 days | 2 weeks before |
| Petunias | 10 | 75–78 °F | 7–10 days | 2 weeks after |
| Columbine | 6–8 | 70–75 °F | 20–25 days | 2 weeks before |
| Schizanthus | 8 | 60–70 °F | 10–15 days | 2 weeks after |
| Snapdragons | 8 | 65–70 °F | 7–10 days | 2 weeks before |
| Marigolds | 6 | 75–80 °F | 5–7 days | 3 weeks after |
| Nicotiana | 6 | 75 °F | 14–21 days | 2 weeks after |
| Statice | 6 | 70 °F | 10–15 days | 2 weeks after |
| Stock | 6 | 65–70 °F | 7–10 days | 2 weeks after |
| Canterbury bells | 2–4 | 70–80 °F | 10–20 days | 4–6 weeks after |
| Delphinium | 2–4 | 65–75 °F | 10–18 days | 4–6 weeks after |
| Phlox | 4 | 60–65 °F | 10–15 days | 2 weeks after |
From NDSU Extension’s Time and Temperature Chart (H-1139). Seed temperature is the soil warmth for the quickest sprouting.
Tips for strong seedlings
From University of Minnesota Extension’s guide to starting seeds indoors:
- Light: 12 to 16 hours a day, with lights no more than 4 inches above the seedlings (2 inches is ideal), on a timer.
- Warmth: keep the air above 60 °F; the mix can run 5 °F cooler than the room, so a heat mat helps seeds that like it warm.
- Mix: a soilless seed-starting mix, moist before you sow, in containers with drainage holes.
- Feeding: nothing until seedlings have several sets of true leaves, then a quarter-strength liquid fertilizer once a week.
- Hardening off: about two weeks before planting out, start with a few hours outside in shade and add a little more time and sun each day.
What not to start indoors
Some crops are sown straight into the garden instead, such as beans, peas, carrots and radishes, which Purdue and UGA Extension list for direct sowing. Our planting calendar gives their sowing windows for your ZIP code. For the dates themselves, see frost dates by ZIP code and your growing zone.
Questions people ask
When should I start seeds indoors?
Count back from your last spring frost: about 10 weeks for onions, leeks and the cabbage family, 8 weeks for peppers, eggplant and lettuce, and 6 weeks for tomatoes, according to NDSU Extension’s chart. The calculator does the counting for your ZIP code.
How do I find my last frost date?
Type your ZIP code above or on our frost dates page. Both use NOAA’s 1991–2020 climate normals.
Should I use the average or the safer frost date?
The average date still has a 1-in-2 chance of a later frost. If your seedlings are tender, or you’d rather not cover them on cold nights, plan from the safer date.
What if my area has no frost?
Then there’s no last frost to count back from: plant for the cool season instead, or type a date to plan from.
Sources
- North Dakota State University Extension, Time and Temperature Chart (H-1139): weeks before the last frost, seed temperatures, days to sprout and when to transplant.
- University of Minnesota Extension, Starting seeds indoors (reviewed 2026): light, warmth, feeding and hardening off.
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020: last spring freeze dates by station.
Checked October 15, 2026. All other numbers are arithmetic.