Tomato Seedlings: When and How to Sow Them Successfully

The tomato is the indoor sowing par excellence: demanding on heat, greedy for light, but very reliable once three settings are respected. This guide details dates, temperatures, transplanting and hardening off — plus the mistakes that kill seedlings.

When to sow tomatoes?

• The rule: count back from planting out. Sow 5–6 weeks before the expected transplant date — gardeners commonly start indoors 4–6 weeks before the last frosts; the market-gardener method goes back 7–9 weeks for a production plan.
• In dates: between 15 February and late March under cover with bottom heat, or mid-March for a classic start. In a greenhouse or conservatory, a late-February sowing gives the best chance of ripe fruit by late June.
• Too early = trap: sowing too early in the year produces drawn-out, spindly seedlings if light is lacking early in the season. Early-January sowing without a lamp yields unsellable plants.

Temperatures: the setting that decides everything

• Germination: tomato seeds germinate best between 21 and 32 °C, with an optimum around 29 °C; at a steady 18–24 °C, expect emergence in about five days.
• Bottom heat: a substrate temperature of 21–27 °C is ideal, especially when heated from below — exactly the job of a heating mat, which lifts the compost 5–10 °C above ambient.
• After emergence, come back down: young plants grow best between 16 and 21 °C. A plant held at 25 °C day and night will etiolate for sure.
• Emergence time: 6–10 days under normal conditions, 8–11 days with gentle heat — noticeably longer if the substrate is cold.

The sowing move

• Depth: the reference range runs 5–12 mm — 6 mm, two seeds per pot, covered with fine sieved compost.
• Moisture until emergence: cover the container with a dome or clear film (or a plastic bag), then remove it as soon as seedlings appear.
• One real watering: moisten thoroughly before covering — often the only watering needed for the whole germination. Afterwards: keep the soil fresh, never soggy.

Light: where everything is decided after emergence

• Duration: 12–16 hours of light a day — a winter south window is not enough for proper growth.
• Lamp-to-plant distance: for fluorescent tubes, keep the source 2.5–5 cm above the foliage to avoid thin stems; for LEDs, the reference distance is 10–20 cm. The horticultural LED lamp adjusts as they grow.
• The diagnosis: the first sign of insufficient light is seedlings leaning toward the window with thin, long stems. Etiolation = more light, not more water.

Transplanting: the tomato's special feature

• When: transplant about two weeks after emergence, when plants show two or three good leaves — into 8–10 cm pots.
• Bury the stem — the tomato's strength: replant burying most of the stem, up to the first leaves; adventitious roots form along the buried section. A stretched plant catches up at transplanting this way.
• The extra trick: brushing the top of the plants daily once they reach 8 cm strengthens the stem and gives short, sturdy plants. Another tip: lay the pot on its side when the seedling is 10–12 cm tall to multiply roots.

Hardening off and planting out

• One to two weeks of transition: before planting out, stop fertiliser, reduce water and take the plants out progressively — half an hour in light shade on day one, a full day outside by day seven, bringing them in each evening.
• The safety threshold: soil must be at least 13–16 °C and nights should not drop below 7 °C — the traditional rule waits for mid-May, after the last frosts.
• The ideal plant: sturdy, flowerless, dark green foliage, 15–25 cm tall. Plant deep — up to the first leaf — spacing holes 40–50 cm (60–90 cm if left unstaked).
• Beware the cold snap: a newly planted tomato survives 3 °C but will never crop to full potential afterwards — surviving is not producing.

The two seedling killers

• Damping-off: a soilborne disease that kills young plants by constricting the stem just above the soil — fungi (Pythium, Fusarium, Phytophthora) favoured by waterlogged, nitrogen-rich substrate. Prevention: disinfect reused pots and trays, sow thinly to let air circulate, ventilate seedlings and measure watering.
• Etiolation: covered above — insufficient light + excessive heat. Leggy seedlings also become more susceptible to damping-off.

And your own seeds?

Good news: tomato seed has no dormancy and sprouts without pretreatment; its gelatinous coating disappears naturally through fermentation in the soil. To save your own: ferment the seeds 10 h at 30 °C (or 3–4 days at 18–20 °C), then dry them away from the sun around 20 °C. Viability: 4 years easily in good cool storage, up to a dozen years under controlled conditions.

Where to start?

The trio covering the three settings: the complete seedling kit (indoor greenhouse + LED + heating mat), the heating mat for bottom heat and the LED lamp for the daily 14–16 h. Related reading: the winter sowing calendar, seed stratification, and the pepper and eggplant guides, sown at the same time.

Growing calendar

	JAN	FEB	MAR	APR	MAY	JUN	JUL	AUG	SEP	OCT	NOV	DEC
Sowing
Planting
Harvest