How to Sow Tomato Seeds Indoors for Strong Seedlings

Why Start Tomatoes Indoors in the UK?

Tomatoes are warmth lovers from South America, and our British summers are, frankly, a bit of a gamble. Sowing indoors gives you a head start of six to eight weeks, so your plants are already flowering by the time the June sunshine arrives. Get the early stages right and you will be picking your first ripe 'Gardener's Delight' or 'Sungold' in July rather than September.

The trick is not simply to sow early. It is to sow into warmth, keep the compost evenly moist, and then move the seedlings into the brightest light you can manage. Weak, leggy, pale seedlings are almost always the result of too little light and too much heat once germination has happened. Fix those two things and you will raise plants that shrug off a cold snap.

When to Sow

Timing depends on where your tomatoes will eventually grow. As a rough guide for most of the UK:

• Heated greenhouse or conservatory: late February to early March.
• Unheated greenhouse: mid to late March.
• Outdoors in a sheltered, sunny spot or on the allotment: late March to early April.
• Cold northern gardens and exposed sites: delay by two to three weeks and choose a quick-maturing bush variety such as 'Red Alert' or 'Tumbler'.

Your target date is the last frost in your area, which ranges from mid-May in southern England to early June in parts of Scotland. Count back six to eight weeks from that date and you have your sowing window. Sowing too early is a common mistake; a plant that has been sitting on a windowsill since January will be stunted and root-bound long before it can go outside.

Choosing Compost and Containers

Tomato seeds are small and their roots are delicate, so use a fine, free-draining seed compost rather than a coarse multipurpose mix. A peat-free seed compost works well; just sieve out any lumps of bark first. Fill a clean seed tray or a 9cm pot to within a centimetre of the rim and firm it gently with a flat board or the base of another pot. You want a level surface, not a compressed brick.

Water the compost before sowing, using a watering can with a fine rose or standing the tray in a centimetre of water until the surface darkens. This avoids washing the seeds about afterwards. Sow seeds thinly on the surface, about 1cm apart, then cover with a fine layer of compost or vermiculite no more than 5mm deep. Tomato seeds need light to germinate well, so do not bury them deeply. Label the tray with the variety and the date — you will forget.

Warmth for Germination, Light for Growth

Tomato seeds germinate fastest at around 18–21°C. A warm windowsill above a radiator, an airing cupboard, or a heated propagator all work. Check daily, because germination can take as little as five to seven days at that temperature. Once the first green shoots appear, the rules change completely.

Move the tray immediately to the brightest spot you have. A south-facing windowsill is ideal, but you may need to supplement with a simple grow light if your windows are small or you are sowing in February. Cooler conditions with plenty of light — around 15–18°C — produce short, stocky plants. Warmth with poor light produces the spindly, pale seedlings that never recover properly.

Turn the tray a quarter turn each day so the seedlings grow straight rather than leaning towards the glass. Keep the compost damp but never soggy, watering from below where possible. Damping off, that frustrating collapse of seedlings at soil level, is usually caused by cold, wet compost and poor air movement. Ventilate the propagator lid once the seedlings are up.

Pricking Out and Potting On

When the seedlings have their first pair of true leaves — the jagged ones that appear above the initial rounded seed leaves — it is time to prick them out. Handle them by a leaf, never the stem, and use a pencil or a dibber to lift them with a good clump of roots.

• Pot each seedling into its own 9cm pot of multipurpose compost.
• Plant deeper than it was growing, burying the stem up to the seed leaves. Tomatoes will produce roots along a buried stem, giving you a stronger plant.
• Water in gently and keep out of direct midday sun for a day or two.
• Move on again into a larger pot once roots show at the drainage holes.

Keep the plants growing steadily rather than pushing them with feed. A weak liquid feed every fortnight is plenty before they are planted out. If your plants are ready to go in the ground but the weather is not, pot them on again rather than planting them out into cold soil. A tomato checked by cold will sulk for weeks.

Hardening Off and Planting Out

About ten days before your intended planting date, begin hardening off. Move plants into a cold frame, or outside during mild days and back in at night, gradually extending the time outdoors. This acclimatises them to wind, cooler nights and direct sunlight. Rush this stage and you will scorch the leaves and set the plants back.

Plant out only once night temperatures are reliably above 10°C and all risk of frost has passed. On the allotment, a cloche or fleece for the first fortnight makes a real difference. Space bush varieties about 45cm apart and cordon types about 60cm apart, and water them in well. From there, it is simply a case of tying in, feeding, and waiting for that first warm, home-grown tomato — which always tastes better than anything you can buy.