Why Start Seeds Indoors?
For crops with long growing seasons (tomatoes, peppers, eggplant, celery), starting indoors lets you get a jump on the season that direct sowing can't provide. A tomato transplant set out after last frost is 6–8 weeks ahead of a direct-sown seed — the difference between a full harvest and a partial one in shorter-season climates.
Timing: Count Back from Last Frost
The standard approach: find your average last frost date (available from the Old Farmer's Almanac by zip code), then count backward based on each crop's recommended indoor weeks.
- 10–12 weeks before last frost: peppers, celery, celeriac, onions, leeks
- 8–10 weeks: tomatoes, eggplant
- 6–8 weeks: broccoli, cabbage, cauliflower, Brussels sprouts, kale
- 4–6 weeks: lettuce, Swiss chard, fennel, basil
- 2–4 weeks: squash, cucumbers, melons (note: these resent transplanting — start late and transplant carefully)
For zone-specific planting timing, also see our fall planting guide by zone.
Grow Lights: What Actually Works
A south-facing window is almost never enough — seedlings stretch and lean toward inadequate light. Full-spectrum LED grow lights are the current standard: energy-efficient, low-heat, and effective. Look for a minimum of 30–50 micromoles of PAR (photosynthetically active radiation) at seedling level. Run lights 14–16 hours per day. Keep the light 2–4 inches above seedling tops and raise as plants grow.
T5 fluorescent shop lights are a cheaper alternative and work well for most seedlings, though they're less efficient than LED. Avoid incandescent bulbs — they produce heat but very little usable light for plants.
Starting Mix vs Potting Mix
Seed starting mix is finely textured, low in nutrients, and well-draining — ideal for germination. Potting mix is coarser and nutrient-richer, better for growing seedlings on once they have their first true leaves. Start in seed starting mix, then pot up (move to a larger container with potting mix) at the first true leaf stage. Never start seeds in garden soil — it compacts in trays and may carry pathogens.
Hardening Off: The Critical Step Most Growers Rush
Seedlings grown indoors under ideal conditions are not ready for outdoor wind, temperature swings, and direct sun. Hardening off — a 7–14 day gradual acclimation — prevents transplant shock. Start with 1–2 hours outdoors in a sheltered, part-shade spot. Add an hour per day, gradually exposing to more sun and wind. By the end of two weeks, seedlings can tolerate full outdoor conditions.
FAQ
When should I start tomato seeds indoors?
6–8 weeks before your last frost date. Starting tomatoes too early (10+ weeks) produces large plants that become pot-bound before it's safe to transplant outdoors — they're harder to establish and often flower prematurely indoors.
Can I start seeds on a sunny windowsill?
You can start seeds on a windowsill, but seedlings will likely stretch toward the light (etiolation) and become leggy. Rotating the tray daily helps but doesn't fully compensate for inadequate light. Supplemental grow lights produce noticeably sturdier plants.
What is damping off and how do I prevent it?
Damping off is a fungal condition where seedlings emerge normally then suddenly collapse at the soil line. Prevention: use sterile seed starting mix, avoid overwatering, ensure good air circulation (a small fan helps), and water from below (fill trays and let the mix absorb water rather than watering from above).
How many seeds should I plant per cell?
2–3 per cell, then thin to one per cell after germination by snipping (not pulling) the weakest seedlings at soil level. Pulling disturbs the remaining seedling's roots.
Do I need a heat mat for seed starting?
For peppers and eggplant: yes. Soil temperature below 70°F significantly slows germination. For tomatoes: helpful but not required at normal room temperature (65–70°F). For most other crops: not necessary if your indoor temperature is 65°F or above.