How to grow tomatoes from seed in the UK

By Sophie Sparkes · Updated September 2026 · 7 min read

A packet of tomato seeds costs a couple of pounds and can give you kilos of fruit all summer. Growing from seed also opens up hundreds of varieties you'll never see in a supermarket. Here's the full journey, timed for the UK, from sowing on a windowsill to picking your first ripe tomato.

Short answer: sow indoors from late February to March if the plants will go into a greenhouse, or March to early April if they are going outside. Do not sow earlier than that. Leggy, root-bound plants sitting around waiting for warm weather crop worse than younger ones planted at the right moment.

When to sow tomato seeds in the UK

• Late February to March: if you'll grow them in a greenhouse or polytunnel.
• March: early April: if they're going outside. Don't sow too early: leggy, root-bound plants waiting for warm weather do worse than younger ones planted at the right time.
• Missed the window? Garden centres sell young plants well into June. You can join the process at the planting-out step below.

Step 1: Sow indoors

Fill a small pot or seed tray with peat-free seed compost, water it, then sow seeds about 2cm apart and cover with a fine layer of compost or vermiculite. Tomatoes germinate best at 18-21°C: a bright windowsill above a radiator is ideal. Keep the compost just moist and you'll see seedlings in 7-14 days.

Step 2: Prick out into pots

Once seedlings have their first pair of "true" leaves (the serrated ones that appear after the initial rounded pair), move each into its own 7-9cm pot. Handle them by a leaf, never the stem, and bury them slightly deeper than they were. Tomatoes grow new roots along any buried stem, which makes a sturdier plant.

Step 3: Pot on and keep them growing

Keep them somewhere bright and turn the pots every couple of days so they don't lean towards the light. If roots start showing at the drainage holes before it's warm enough to plant out, move them up to a slightly bigger pot. Feed weekly with a general-purpose liquid feed once they're 15cm tall.

Step 4: Harden off

UK tomatoes can't go outside until all risk of frost has passed: mid-May in the south, late May to early June further north. For 7-10 days before planting, put your plants outside during the day and bring them in at night. This "hardening off" toughens the leaves and prevents the shock that stalls unacclimatised plants.

Step 5: Plant out

Choose the sunniest, most sheltered spot you have. Tomatoes want 6-8 hours of sun. Plant deeply (up to the first leaves), 45-60cm apart, in rich soil or 30cm+ pots and growbags. Push in a sturdy cane or spiral support at planting time; doing it later damages roots.

Summer care: water, feed, pinch

• Water consistently. Irregular watering is the main cause of split fruit and blossom end rot. Little and often beats drought-then-drench.
• Feed weekly with a high-potash tomato feed once the first flowers appear.
• Pinch out cordon side shoots weekly, and stop the plant (snip the growing tip) after 4-5 trusses of fruit outdoors so the plant ripens what it has.
• Remove lower leaves below the first truss as fruit develops. Better airflow means less blight risk.

Harvesting

Pick fruit as soon as it's fully coloured: regular picking encourages more. At the end of the season, ripen any stubborn green tomatoes indoors in a drawer with a banana (the ethylene it releases speeds ripening), or turn them into chutney.

Common problems

• Blight: brown patches on leaves and fruit in warm, wet spells. Water at the base, keep foliage dry, and pick off affected leaves fast.
• Blossom end rot: black sunken patches on the fruit base. It's a calcium shortage caused by erratic watering, not the soil. Water more evenly.
• Yellowing leaves: often hunger or overwatering. See our guide to why leaves turn yellow.

Tomato seed questions

When should I sow tomato seeds in the UK?

Late February to March for greenhouse or polytunnel plants, and March to early April for anything going outdoors. The timing works backwards from your last frost: plants want to be about six to eight weeks old when it is finally safe to put them out, which is mid-May in the south and late May to early June further north.

Can I sow tomato seeds directly outdoors in the UK?

Not reliably. The UK season is too short and too cool, so seed sown straight into the ground germinates late and the plants run out of summer before the fruit ripens. Start them indoors on a windowsill and plant out after the frosts instead.

What temperature do tomato seeds need to germinate?

Around 18-21°C is ideal, which is why a bright windowsill above a radiator works so well. Below that they are slow and patchy rather than doomed. Once the seedlings are up, move them somewhere bright and slightly cooler, because too much warmth with too little light produces tall, floppy plants.

How long do tomato seeds take to germinate?

Usually 7-14 days at the right temperature. If nothing has appeared after a fortnight, the compost was probably too cold or was allowed to dry out. Keep it just moist rather than wet, because soggy seed compost rots the seed before it sprouts.

Is it too late to sow tomato seeds?

Once you are past April, sowing is a gamble in most of the UK. The better move is to buy young plants, which garden centres sell well into June, and pick up the process at the planting-out stage. You lose the choice of unusual varieties but you keep the crop.