Seed starting is not complicated, but it is exacting. Weak light and wet soil are the usual failures. You do not need a greenhouse; a shelf, a heat mat for peppers, and an LED shop light that hangs close to the leaves will outgrow a sunny windowsill.
When to start
Count backward from your average last spring frost date. County extension pages list that date. Then match the crop:
| Crop | Start indoors | Notes |
|---|---|---|
| Onion, leek | 8–12 weeks | Cool-tolerant; can go out earlier than tomatoes |
| Pepper, eggplant | 8–10 weeks | Want warmth to germinate and grow |
| Tomato | 5–7 weeks | Too early makes leggy plants in small pots |
| Broccoli, cabbage | 4–6 weeks | Can take some cool outdoor nights |
| Cucumber, squash, melon | 2–3 weeks | Hate root disturbance; often direct-sow instead |
| Lettuce, herbs | 3–5 weeks | Or sow outdoors when soil is workable |
Mix, trays, and moisture
- Use a seed-starting or fine potting mix, not garden soil. Garden soil packs and stays cold.
- Moisten the mix before filling cells so dry peat does not float when you water.
- Sow at the depth on the packet — usually about twice the seed’s thickness.
- Cover trays until sprouts show, then remove the cover so humidity does not invite damping-off.
- Water from below when you can. The surface should not stay soggy.
Light and temperature
- Hang lights a few inches above the tops. Raise the light as plants grow. A distant ceiling fixture makes spindly stems.
- Most vegetables germinate around 70–75°F. Peppers like the warm end; a heat mat helps.
- After germination, cooler nights (60s°F) keep tomatoes stockier than a hot closet.
- A south window alone is usually not enough in late winter. Supplement with light.
Potting up
When roots fill the cell or true leaves are crowded, move to a larger pot with the same kind of mix. Tomatoes can go deeper — bury part of the stem. Peppers prefer to stay at the same depth. See when to transplant seedlings for hardening off and outdoor timing.
Frequently asked questions
Do I need grow lights?
If you start more than a few herbs on a bright sill in late spring, maybe not. For tomatoes and peppers in February–March in most of the US, yes — or accept skinny plants.
Why did my seedlings fall over and die?
Often damping-off from wet, still air, or reused dirty trays. Use fresh mix, airflow, and stop covering trays after sprouts appear.