Starting seeds indoors means sowing warm-season crops inside 4–8 weeks before your last frost date, growing them under strong light in seed-starting mix, and hardening them off for a week before transplanting outside. Done right, it gives you stronger plants, earlier harvests and far more varieties than any garden centre sells — for the cost of a packet of seed.
Why Start Seeds Indoors at All?
Some crops need a longer, warmer season than your climate gives them outdoors — tomatoes, peppers and eggplant are the classic examples. Others (cabbage, broccoli, lettuce) transplant so well that indoor sowing is simply the fastest, cheapest route. Indoor starting also protects tiny seedlings from slugs, birds and sudden cold snaps during their most fragile week.
Not everything belongs indoors, though. Beans, peas, corn, squash, carrots and other root crops resent transplanting or grow so fast they do better sown directly outside. The table below sorts the common vegetables.
When to Start: Count Back From Your Last Frost
Timing is the whole game. The method: find your area’s average last spring frost date, then count backwards by the weeks shown on the seed packet. Sow too early and you get leggy, root-bound plants waiting for weather; too late and you lose the head start you were after.
| Vegetable | Weeks before last frost | Notes |
|---|---|---|
| Peppers | 8–10 | Slowest of the common crops — start these first |
| Eggplant | 8–10 | Needs real warmth to germinate |
| Onions (from seed) | 8–10 | Very slow; surface sow, keep moist |
| Celery | 10–12 | Slowest of all; needs light to germinate |
| Tomatoes | 6–8 | Fast once warm; do not rush them |
| Cabbage / broccoli / cauliflower | 4–6 | Sturdy transplants, tolerate cool rooms |
| Lettuce | 4–5 | Also easy direct-sown |
| Cucumber / melon / squash | 2–4 | Fast and resent root disturbance — use big pots, sow late |
| Herbs (basil) | 6 | Basil after tomatoes; hates cold at every stage |
The Setup That Actually Works
You do not need a greenhouse or expensive kit. Four things matter:
Light — the non-negotiable. A windowsill is not enough for most seedlings; it produces the tall, pale, flopping plants every beginner knows. Use an LED or fluorescent light hung 2–4 inches above the seedlings, raised as they grow, on for 14–16 hours a day (a cheap plug timer makes this automatic).
Containers. Anything 2–3 inches deep with drainage holes: cell trays, yoghurt pots, toilet-roll tubes for beans. Wash reused containers with soapy water so last year’s fungi do not move in.
Seed-starting mix. Fine, sterile, and moisture-retentive — not garden soil and not heavy potting soil with big bark chunks. Pre-moisten it before filling containers; dry mix repels water at first.
Warmth for germination, cool for growing. Most seeds germinate fastest warm (see the seed germination temperature chart), then grow stockiest in a cooler room, 60–65°F, under strong light. A heat mat is for the germination week only — leaving it on grows soft plants.
Sowing Indoors, Step by Step
- Fill containers with pre-moistened mix and firm it gently.
- Sow at twice the seed’s width deep; surface-sow tiny or light-needing seed and press it in.
- Label everything with variety and date. You will not remember — nobody does.
- Cover with a clear lid or film to hold humidity; place somewhere warm.
- Check daily. At the first sign of green, remove the cover and put the tray under lights immediately — this single moment decides whether seedlings are sturdy or leggy.
- Water from below where possible, and run a small fan on low nearby for a few hours a day. Moving air grows thick stems and prevents damping off.
- Feed weakly once the first true leaves appear (the second set of leaves, the ones that look like the plant).
For the germination stage in full detail — depths, soaking, paper-towel testing — see our guide on how to germinate seeds.
Potting On
When roots show at the drainage holes or seedlings have 2–3 sets of true leaves, move them into bigger pots (3–4 inch), burying tomatoes up to their first leaves — they root along the stem. Handle seedlings by a leaf, never by the stem: a torn leaf regrows, a crushed stem does not.
Hardening Off: The Step Beginners Skip (and Regret)
Indoor seedlings have never met direct sun, wind or cold nights. Moving them straight out can sunburn leaves white and set plants back weeks. Instead, over 7–10 days: start with 1–2 hours in shade, add an hour and more sun each day, bring them in on cold nights, and plant out only when they have had 2–3 full days outside. Watch the forecast — one late frost undoes everything, so keep covers handy for the first week after transplanting.
Troubleshooting Indoor Seedlings
| Problem | Cause | Fix |
|---|---|---|
| Tall, pale, leaning | Light too weak or too far | Lights 2–4 inches above, 14–16 hrs |
| Seedlings collapse at base | Damping off | Sterile mix, airflow, less water; restart affected cells |
| Purple leaves (tomatoes) | Cold room / phosphorus stress | Warmer spot; they recover outside |
| Nothing germinating | Cold soil or dead seed | Warmth + paper-towel viability test |
| Mould on soil surface | Still, humid air | Cover off, fan on, water less |
If nothing sprouts at all, work through our guide to why seeds fail to germinate — it pinpoints the cause in minutes.
Frequently Asked Questions
What month should I start seeds indoors? It depends entirely on your last frost date. For a mid-May last frost, peppers start in early March, tomatoes in late March, and cucumbers in late April. Count backwards from your own frost date using the table above rather than copying a calendar from a different climate.
Can I start seeds indoors without grow lights? You can, but expect leggy seedlings unless you have a very bright south-facing window and turn trays daily. A basic LED shop light hung close above the trays is the single upgrade that changes results the most, and it costs less than the plants it saves.
What temperature should my house be for seedlings? Warm for germination (most crops 70–85°F soil), then a cooler 60–65°F room grows the stockiest seedlings under light. Ordinary heated homes are fine; cold windowsills at night are the usual problem.
Which seeds should NOT be started indoors? Carrots and most root crops, beans, peas, corn and sweetcorn, and fast squash/cucumbers in most climates — they transplant poorly or grow so fast that direct sowing catches up. Sow these where they will grow.
How big should seedlings be before transplanting? Stocky matters more than tall: 2–3 sets of true leaves, a stem you cannot bend easily, and roots holding the root ball together. A short sturdy plant outgrows a tall floppy one within a week of transplanting.
