Sowing tomatoes: from seed to transplant-ready plant

Choose where the crop will grow, then count back to an indoor sowing date. In the UK, the RHS suggests sowing indoors from late February to mid-March for greenhouse crops, or late March to early April for outdoor crops. Those windows are British reference points; your local frost risk, shelter and light determine when seedlings can move outside.

When should you sow for your growing space?

Decide first whether the plants will grow in a greenhouse or outdoors. The RHS tomato guide recommends around 18°C (64°F) for germination and planting out once temperatures are consistently above 16°C (about 61°F). These figures describe different stages: warmth to start seed and conditions for establishing plants.

For a colder North American example, the University of Minnesota Extension recommends sowing indoors five to six weeks before outdoor planting in Minnesota, after frost risk and once the soil has warmed. Its calendar is regional. In other areas, count back from the local safe planting date rather than adopting Minnesota or UK months.

The French SNHF tomato sheet gives another sequence: sow in pots, keep at 20°C for four to five weeks, then plant out once frost risk has passed.

How should tomato seed be started indoors?

Here, “potting on” means moving a seedling into a larger container; “transplanting” means planting it in its final growing position. Use clean pots or modules with drainage and a fine seed-starting mix. Moisten the mix before sowing so it is evenly damp rather than waterlogged. Minnesota Extension specifies a sowing depth of ¼ inch (about 0.6 cm) and, in its method, a heat mat at 75–85°F (24–29°C) until emergence. This is a Minnesota germination setup; the RHS gives about 18°C (64°F) for germination, while its above-16°C (about 61°F) figure is for planting out. Follow one source's method and your packet rather than combining temperature settings.

Label the variety and date. If you want one plant per pot, sow one seed in each; if you sow extra as insurance, cut surplus seedlings at soil level instead of pulling around the roots you keep. Once shoots appear, move them promptly into bright light. Heat helps seed germinate, but it cannot compensate for dim conditions after emergence.

What care do seedlings need after emergence?

Keep the mix lightly moist and let excess water drain. Turn pots if light reaches them from one side, and give leaves room so seedlings do not crowd one another. If stems are becoming long and weak, check light and warmth before adding water. Pot on when roots fill the small container or the plant needs more room, handling the root ball gently.

Potting on is not planting out. At final planting, the SNHF advises setting the root ball at an angle and burying the stem up to the first leaves. Choose the final spacing using a guide suited to your variety and growing system; the tomato plant spacing guide covers that separate decision.

How do you harden plants before transplanting?

Plants raised indoors need a gradual transition. Put them outside in a sheltered place for short periods, increase their exposure over time and protect them from cold or strong wind. Check local night temperatures and soil conditions before planting. A warm day alone is not proof that the risk has passed.

If the safe window has not arrived, keep seedlings in a bright sheltered place and pot on if necessary. The guide to when to plant tomatoes covers the planting-date decision; tomato plant spacing helps you prepare the bed before moving seedlings.