How to Start Seeds Indoors: Trays, Light, Soil Temp, and Timing
Start warm- and cool-season crops indoors weeks before your last frost, using soil temperature and light, not a national average date.
Published · Reviewed by Blake Peavy
seed starting indoor frost winter
How to Start Seeds Indoors
Start warm- and cool-season crops indoors weeks before your last frost, using soil temperature and light, not a national average date.
How to Start Seeds Indoors
Count backward from your last frost in My Garden. Open the live seed-start calendar, What to plant now, and the Winter hub. For outdoor windows by zone, see a calendar such as zone 6.
When to Start (Weeks Before Last Frost)
Use these indoor-start offsets from the live seed-start calendar, then harden off and transplant into your zone’s spring window. All lead times are weeks before your last spring frost.
| Weeks before last frost | Crops |
|---|---|
| 10 weeks | Onion (sets OK), Celery, Pansy |
| 8 weeks | Pepper (warm soil), Eggplant, Leek, Parsley (slow germ), Snapdragon |
| 6 weeks | Tomato, Broccoli, Cabbage, Cauliflower, Brussels Sprouts, Marigold |
| 5 weeks | Kale, Collards, Basil (cold-sensitive) |
| 4 weeks | Lettuce (also direct sow), Swiss Chard, Zinnia |
| 3 weeks | Cucumber (short indoor window), Melon, Watermelon, Squash, Pumpkin |
Catalog soil temp, depth, and days-to-harvest for the core crops:
- Tomato: 6 weeks before last frost. Catalog soil 60-85 F; sow 1/4 inch; about 75 days to harvest from transplant timing in the catalog. Harden 14 days; transplant about 7 days after last frost.
- Pepper: 8 weeks before last frost. Catalog soil 65-85 F; sow 1/4 inch; about 70 days. Warm soil for germ. Harden 14 days; transplant about 7 days after last frost.
- Broccoli: 6 weeks before last frost for spring transplants. Catalog soil 45-75 F; sow 1/2 inch; about 65 days. Harden 10 days; transplant at last frost (0 days after).
- Lettuce: 4 weeks before last frost, or sow outdoors when soil allows. Catalog soil 40-75 F; sow 1/4 inch; about 45 days. Harden 7 days; transplant at last frost (0 days after).
Full harden-off and transplant-after values for every crop live on the seed-start calendar. See also How to harden off seedlings.
Example zone 6 outdoor spring windows from the catalog (not frost dates): tomato April 10, May 20; pepper April 15, May 30; broccoli and lettuce March 10, May 10. Shift trays from your own last frost, not these calendar labels alone.
Trays, Mix, and Sowing
- Use clean cell trays or pots with drainage and a sterile seed-starting mix, not heavy garden soil.
- Sow at catalog depth. Label every cell.
- Bottom-water or mist so the mix stays evenly moist, never soggy.
- Cover until sprouted if the room is dry; uncover as soon as seedlings show.
Soil Temp vs Air Temp
Germination tracks soil temperature more than room air. Tomato wants soil at least about 60 F; pepper about 65 F; broccoli about 45 F; lettuce about 40 F (catalog mins). A heat mat helps warm-season crops; cool crops often sprout on a cooler shelf. Once up, most seedlings prefer cooler air than the mat: remove heat or they stretch.
Light and Water
- Give strong light as soon as seeds sprout: grow lights a few inches above the canopy, 14-16 hours, or the brightest south window plus supplemental light.
- Water when the mix surface dulls. Stale wet mix invites damping-off (seedlings collapse at the soil line). Airflow, sterile mix, and no overwatering cut risk.
- Thin to one strong seedling per cell.
Transplant Readiness
Seedlings are ready to harden off when they have 2 to 4 true leaves, hold themselves upright, and fill the cell without being root-bound. Do not rush tender crops out before your last frost unless you can protect them. Next: How to harden off seedlings.
Next Steps
Set last frost in My Garden, open the live seed-start calendar and What to plant now, then stage trays by the week offsets above. After trays, follow harden-off, then plant into your zone calendar. For cold-season alternatives outdoors, see Cold frames and winter sowing on the Winter hub.
Plan your shelf, lights, and heat mat on the Seed Starting Setup Desk. More on the setup: grow lights for seed starting, heat mats: when you need one, and how to fix leggy seedlings.