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Seed Starting Indoors for Beginners: Lights, Timing and a $60 Setup

What to start, when, and the cheapest light setup that actually grows stocky seedlings.

Updated 2026-09-11

Which seeds to start indoors (and which not to)

Start indoors: tomatoes, peppers, eggplant, broccoli, cabbage, kale, lettuce, basil, onions. Direct sow instead: beans, peas, carrots, beets, radishes, corn, squash and cucumbers (they hate transplanting or grow fast enough outside). Squash and cucumbers can go in 3 in pots three weeks before transplanting if your season is short, never longer.

When

Count back from your last spring frost: peppers and onions 8–10 weeks, tomatoes 6 weeks, brassicas 6 weeks, lettuce and basil 4 weeks. The planting calendar does the counting for you.

With a last frost of May 15 (zone 5), tomatoes are sown around April 1 and peppers around March 10. Zone 7 (mid-April) sows tomatoes March 1; zone 9 (mid-February) at New Year. Sowing tomatoes in February in zone 5 is the classic mistake: by May they are 2 ft of leggy stem in a yogurt cup.

The $60 setup

A windowsill produces leggy, weak seedlings. What works: one shelf, two 2-ft LED strip lights 2–3 in above the seedlings for 14–16 hours a day on a $10 timer, and a heat mat under peppers and tomatoes until they germinate. Standard 1020 trays with cell inserts and a humidity dome. That is the whole list.

The math: two LED strips at about $12 each (a 4-pack for $45 lights two shelves), a $10 outlet timer, a $20 heat mat, and $10–15 of trays, inserts and a dome. One shelf lights two 1020 trays, 100 cells: more seedlings than a first-year bed can take.

Temperatures that decide everything

Germination wants warmth: peppers and eggplant 80–85°F, tomatoes 75–80°F, brassicas and lettuce 65–70°F (lettuce stalls above 80°F; keep it off the mat). After sprouting, seedlings want it cooler: 65–70°F by day and 55–60°F at night grows stocky plants. A warm room with the mat still on grows tall, soft ones.

Step by step

1. Fill cells with moist seed-starting mix (not garden soil). Sow 2 seeds per cell, 1/4 in deep. Label every row; tomato and pepper seedlings look identical for two weeks.
2. Dome on, heat mat on, until the first sprouts show. Then dome off, mat off, lights on.
3. Keep lights 2–3 in above the tops as they grow. Water from the bottom.
4. Thin to one seedling per cell with scissors. Feed at half strength once true leaves appear.
5. Pot up tomatoes into 3–4 in pots when they have two sets of true leaves. Bury the stem up to the first leaves; tomatoes root along it.
6. Harden off: 7 days of increasing outdoor time, shade first, then sun, then overnight.

Watering and feeding

Set the tray in an inch of water for 20 minutes; pour off what is left. Water when the surface is pale and the tray feels light, usually every 2–3 days under lights. Seed-starting mix has no food in it, so from the first true leaves feed weekly with liquid fertilizer at half strength. Yellow lower leaves at week three mean hungry, not sick.

Hardening off, day by day

• Days 1–2: two hours outside in full shade, out of wind.
• Days 3–4: four hours, morning sun only.
• Days 5–6: all day in sun, back inside at night.
• Day 7: outside overnight if the low stays above 50°F (40°F for brassicas and lettuce).
• Day 8: transplant, ideally on a cloudy afternoon, and water in.

Skip a windy or cold day rather than pushing through; a seedling burnt on day 3 takes two weeks to recover.

Common failures

Leggy stems (light too far away), damping off (too wet, no airflow: run a small fan), sunburn after transplanting (skipped hardening off). Also: seeds that never come up because the mix was cold (no mat) or dried out (no dome), and pepper seedlings stalled at two leaves because the room is 60°F; peppers want warmth their whole indoor life.

After transplanting

Water daily for the first week, then follow the first raised bed guide. The same trays start a second round of lettuce, kale and broccoli in July for the fall garden, when outdoor soil is too hot for them.

FAQ

Can I start seeds in a sunny window without grow lights?

Only for lettuce and brassicas that go out in four weeks, and even those stretch. Tomatoes and peppers need 14–16 hours of light 2–3 in away; a south window in March gives half that.

How far should grow lights be from seedlings?

2–3 in for LED strips, raised as the plants grow. Further than 4 in and seedlings stretch within days. Hang the light on chains so it adjusts.

When should I start tomato seeds indoors?

Six weeks before your last spring frost, no earlier. A 6-week-old tomato is 6–8 in tall and transplants without a check; a 10-week-old one is root-bound and leggy.

Do I need a heat mat to start seeds?

For peppers and eggplant, yes; they germinate slowly and unevenly below 75°F. For tomatoes it halves the germination time. Brassicas, lettuce and onions do not need one. Turn it off as soon as the seedlings are up.

Why are my seedlings falling over?

Damping off, a fungus that rots the stem at the soil line in wet, still air. Water from below, run a small fan and remove the dome the day sprouts appear. Affected seedlings do not recover; resow.

What I'd buy first

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