Starting seeds indoors gives tomatoes, peppers and many flowers a head start before the weather is warm enough outside. Two questions come up every spring: how many seeds to sow, and when.
How many seeds?
Not every seed germinates, and some seedlings are lost to damping off, thinning or transplant shock. Work back from the plants you want:
seeds = plants wanted ÷ (germination rate × (1 − losses))
The germination rate is printed on many seed packets, often 70 to 90% for fresh seed; older seed germinates less. For 24 plants at 80% germination and 20% losses: 24 ÷ (0.8 × 0.8) = 37.5, so sow 38 seeds.
With one seed per cell, a 72-cell tray holds them all. Some gardeners sow two seeds per cell and snip the weaker one, which uses more seed but fewer cells.
When to sow
Indoor sowing dates count backward from the average last frost date for your area, which you can find from your local extension service or a frost date lookup by ZIP code. Typical lead times:
| Crop | Sow indoors before last frost | Transplant |
|---|---|---|
| Onions, leeks | 10 to 12 weeks | 2 to 4 weeks before |
| Peppers, eggplant | 8 to 10 weeks | 1 to 2 weeks after |
| Tomatoes | 6 to 8 weeks | 1 to 2 weeks after |
| Broccoli, cabbage | 6 to 8 weeks | 2 to 4 weeks before |
| Squash, cucumbers | 3 to 4 weeks | 1 to 2 weeks after |
The calculator subtracts the weeks before frost to give the sowing date, and adds the weeks after frost for the transplanting date. A negative number means planting out before the last frost, as for cold-hardy crops.
Getting strong seedlings
- Light: a south window is rarely enough. Grow lights 2 to 4 in above the seedlings for 14 to 16 hours a day prevent leggy plants.
- Warmth: a heat mat speeds germination of tomatoes and peppers; remove it once they sprout.
- Moisture: keep the mix damp, not soggy, and water from below to limit damping off.
- Air: a small fan strengthens stems.
- Hardening off: over 7 to 10 days, put seedlings outside a little longer each day before transplanting.
Last frost dates are averages; watch the forecast and protect young plants if a late frost threatens.