Starting seeds indoors gives you a head start on the season, a much wider choice of varieties than any garden center stocks, and a lot of plants for the price of one tray of seedlings. It isn't difficult, but seedlings are unforgiving about a few specific things — light, water and timing. This guide covers each step in order, with the mistakes that most often cause leggy or collapsed seedlings.
Step 1: Work out your timing
Everything is counted back from your area's average last frost date (local extension services and gardening sites list it by postcode or zip code).
| Crop | Sow indoors | Notes |
|---|---|---|
| Peppers, eggplant | 8–10 weeks before last frost | Slow to germinate; like extra warmth |
| Tomatoes | 6–8 weeks before | Very forgiving — a good first crop |
| Broccoli, cabbage, kale | 4–6 weeks before | Can go out before last frost once hardened off |
| Squash, cucumbers, melons | 3–4 weeks before | Grow fast and dislike root disturbance — or sow directly outside |
| Carrots, beets, radishes, peas, beans | Usually sown directly outdoors | Transplant poorly or grow fast enough without a head start |
Starting too early is the most common mistake. Seedlings that wait too long indoors get root-bound and leggy.
Step 2: Gather supplies
- Containers with drainage holes: cell trays, small pots, or recycled containers with holes punched in the bottom.
- Seed-starting mix: a fine, light, sterile mix. Don't use garden soil (it compacts and carries disease) or heavy potting compost with big chunks.
- A clear cover: a humidity dome or plastic wrap to hold moisture until germination.
- Light: a simple LED grow light or shop light. A windowsill alone is rarely enough (see Step 5).
- Labels. Every tomato seedling looks the same.
Step 3: Sow
- Moisten the mix before filling containers — it should feel like a wrung-out sponge.
- Fill cells and press lightly to remove air pockets, without packing hard.
- Sow 2–3 seeds per cell at a depth of about two to three times the seed's width. Tiny seeds (lettuce, many herbs) go on the surface, pressed in gently — some need light to germinate, so check the packet.
- Cover lightly, mist the surface, label, and put the clear cover on.
Step 4: Keep them warm until they sprout
Most vegetable seeds germinate best at about 21–27 °C (70–80 °F) soil temperature. A heat mat helps a lot with peppers and eggplant, which can sit for weeks in cool soil. Check daily; as soon as you see the first green loops, remove the cover and move the tray under light. Leaving the cover on is an invitation to mold.
Step 5: Give them real light
Seedlings need around 14–16 hours of light a day. Even a sunny windowsill in early spring usually isn't enough: seedlings stretch toward it, becoming tall, pale and floppy ("leggy"). Under LED lights, keep the light a few centimeters to about 15 cm above the leaves (follow the light's guidance) and raise it as the plants grow. A cheap timer makes this automatic. A small fan on low for an hour or two a day strengthens stems.
Step 6: Water from below
Set trays in a shallow tray of water for 15–30 minutes, then let excess drain. Bottom watering keeps the surface drier and the stems clean. Water when the top of the mix feels dry and the cell feels light — not on a fixed schedule.
Damping-off — seedlings suddenly collapsing at the base — is a fungal problem encouraged by cold, soggy mix and still air. Prevent it with sterile mix, bottom watering, good airflow and by removing covers after germination. Collapsed seedlings can't be saved; remove them.
Step 7: Thin and pot up
When seedlings have their first set of "true" leaves (the second set, which look like the plant's adult leaves), keep the strongest one per cell and snip the others at soil level — pulling disturbs the roots of the one you keep. Fast growers like tomatoes usually need moving to a larger pot after 3–4 weeks. Always handle seedlings by a leaf, never the stem. Tomatoes can be buried deeper than before; they'll grow roots along the buried stem.
Seedlings in seed-starting mix need feeding once they have true leaves: a diluted liquid fertilizer at quarter to half strength every week or two.
Step 8: Harden off before transplanting
Indoor seedlings have never felt wind or direct sun. Over 7–10 days, put them outside in a sheltered, shaded spot for an hour or two, gradually increasing time and sun exposure, and bring them in at night when it's cold. Skipping this step is how healthy seedlings get scorched or stunted in their first week outside. Transplant on a cloudy day or in the evening, water well, and wait until nights are reliably above the crop's minimum (tomatoes and peppers prefer nights above about 10 °C / 50 °F).
Choosing seeds
Seeds Now lists more than 770 products across vegetables, herbs, flowers, sprouts and cover crops, and describes its seeds as heirloom, organic, non-GMO, open-pollinated and untreated. They come in waterproof, resealable packs — useful because most packets hold more seeds than one season needs — and the shop also sells organic seed-starting soil pods and offers free grow guides. Whatever you buy, read the packet: it lists sowing depth, days to germination and whether a crop is better started indoors or sown direct, and those details beat any general rule.
Common problems at a glance
| Problem | Likely cause |
|---|---|
| Nothing sprouts | Too cold, too deep, dried out, or old seed |
| Tall, pale, floppy seedlings | Not enough light, or light too far away |
| Seedlings collapse at the base | Damping-off: overwatering, cold, poor airflow |
| White fuzz on the soil | Too wet with the cover left on; remove the cover, add airflow |
| Yellowing lower leaves | Needs feeding, or roots are crowded — pot up |
Conclusion
Starting seeds indoors comes down to a few non-negotiables: sow at the right time for your last frost date, use a sterile seed-starting mix, keep seeds warm until they sprout, then give seedlings strong light for 14–16 hours a day, water from below and keep the air moving. Thin with scissors, pot up before roots crowd, and harden plants off for a week or more before they go in the ground. Start with an easy crop like tomatoes, keep notes, and next year's timing will be even better.
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