How Do I Start Tomatoes Indoors from Seed?

Growing tomatoes from seed gives you access to hundreds of varieties unavailable as bought plants — heirlooms, unusual colours, prolific cherry types, and flavour-first varieties that supermarket growers ignore. Starting them indoors correctly gives you robust transplants that hit the ground running. Here is the complete process from sowing to planting out.

When to Sow Tomatoes Indoors

Sow tomatoes 8–10 weeks before your last expected frost date. In the UK this is typically late February to mid-March. In most of the northern US, February to early March is right for zones 5–7. Sowing too early (January) produces plants that outgrow their pots before it is safe to plant them out — resist the temptation.

Sowing Technique

Fill small pots or a module tray with moist seed compost. Sow two seeds per cell or pot at a depth of 5–6 mm. Cover with compost or vermiculite, firm gently, and place in a propagator or cover with cling film. Tomatoes need 21–24 °C (70–75 °F) at the compost for reliable, quick germination — expect seedlings in 5–10 days at this temperature. If using a windowsill, check compost temperature with a thermometer; night temperatures near glass can drop to 12–15 °C and significantly slow germination.

After Germination: Light and Temperature

As soon as seedlings emerge, remove the propagator lid and move to the brightest spot available. Tomato seedlings need 14–16 hours of light per day to stay compact — a south-facing windowsill in late winter rarely provides this. Grow lights set 15 cm above seedlings and running 16 hours per day produce noticeably stockier plants. Reduce night temperature to 14–16 °C after germination — this is a critical step that prevents legginess.

Potting On Stages

Prick out (if sown in a flat tray) or thin to one seedling per pot at the first true leaf stage. Move from a 7 cm pot to a 9–11 cm pot when roots appear at the drainage holes — typically 3–4 weeks after germination. If planting outdoors is still weeks away, pot on once more into a 13 cm pot. Each time, bury the stem slightly deeper than before; tomatoes root along their buried stem, making plants stronger.

Hardening Off and Planting Out

Begin hardening off 10–14 days before your intended planting-out date. Tomatoes should not go outside until all frost risk is past and night temperatures are reliably above 10 °C. Plant out into warm, well-prepared soil on an overcast afternoon, water in well, and protect with fleece for the first week if nights are still cool.