Growing tomatoes from seed means sowing indoors from late February to early April, keeping the seedlings warm to germinate and then bright to stay sturdy, pricking out once the first true leaves appear, and hardening off before they go outside in early June. One packet gives you dozens of plants for a couple of pounds.
A single tray on a warm windowsill is enough to fill a greenhouse, a row of pots or a balcony with tomato plants that cost a fraction of the garden-centre ones. The trick is less about green fingers than about timing and a couple of decisions made in the right order. This guide takes you from the seed packet to the point where the plants are ready to go in the ground; for everything that happens after that, see our full growing guide.
Key takeaways
- Sow late Feb to mid-March for a greenhouse crop, or late March to early April for plants going outside.
- Warmth germinates, light grows. Seeds need about 18°C to sprout; the moment they do, they need the brightest spot you have or they go leggy.
- Prick out at the first true leaves — the second, jagged-edged pair — holding each seedling by a leaf, never the stem.
- Harden off for 7 to 10 days before planting out in early June, once the last frost has passed.
- One £2 packet yields dozens of plants, against £4 to £6 each for ready-grown ones. No heated propagator required.
When to sow tomato seeds in the UK
The single most common mistake is sowing too early. Tomatoes are tender, frost-hating plants from the warm Andes, and a seedling sown in January spends months indoors going pale and weak before it can possibly go out. Match the sowing date to where the plant will finish its life.
| Where it will grow | When to sow | Why |
|---|---|---|
| Heated greenhouse | Late February | You can keep frost off, so an earlier start buys an earlier, longer crop. |
| Unheated greenhouse or polytunnel | Late February to mid-March | The glass holds enough warmth to plant out in May, a few weeks ahead of outdoors. |
| Outdoors (windowsill-sown) | Late March to early April | Plants go out only once frost has passed in late May or early June, so a later sowing avoids root-bound, leggy seedlings. |
The Royal Horticultural Society puts the windows at late February to mid-March for a greenhouse crop and late March to early April for outdoor plants. Work back from your last expected frost: most of the UK is clear by the end of May, the far north and exposed spots a fortnight later. Sow roughly six to eight weeks before that date and the plants will be the right size, neither pot-bound nor weedy, when the weather turns.
Warmth gets the seed up. Light keeps it sturdy. Confuse the two and you grow a leggy disappointment.
What you need to sow tomato seeds
The kit is modest, and you can sow a respectable crop with nothing more than a yoghurt pot and a sunny sill. Here is the short list, with the honest version of what matters and what does not.
- Seed. One packet holds 20 to 50 seeds and costs around £2 to £3. Buy a cordon variety such as ‘Gardener’s Delight’ or ‘Sungold’ for a long greenhouse or staked crop, or a compact bush type such as ‘Tumbling Tom’ for pots and baskets. The cordon-versus-bush choice is made here, at the packet, because it changes how you grow the plant later.
- Containers. A seed tray, a half-dozen 7cm pots, or cell modules. Modules are tidiest because each seedling lifts out with its roots intact; a shared tray works perfectly well if you prick out promptly.
- Seed compost. A peat-free seed or multi-purpose compost. Seed compost is low in nutrients on purpose, which suits tiny roots; ordinary multi-purpose is fine if that is what you have.
- Warmth. A heated propagator is pleasant but not essential. A warm windowsill, an airing cupboard for the first day or two, or the top of the fridge will all reach the 18°C tomatoes want to germinate.
- Light. The brightest windowsill in the house, ideally south-facing. This is the one thing you cannot fudge.
Do you even need a greenhouse?
No. Tomatoes will crop happily outdoors against a warm, sunny wall, or in a sheltered spot in pots and growing bags. A greenhouse earns its keep by giving an earlier start and a longer, more reliable harvest, especially in the cooler and wetter north and west. If you do not have one, sow a little later, choose a quick, hardy variety, and grow against the sunniest wall you own.
Where to buy seed and young plants: Gardening Direct stocks vegetable seed and starter plants delivered to the door, which is handy if you miss the sowing window and want to skip a stage.
Disclosure: the Gardening Direct link is an affiliate link. We may earn a small commission if you buy through it, at no extra cost to you. We only link to merchants we would use ourselves.
How to sow tomato seeds, step by step
The whole job takes ten minutes and one washing-up bowl. Here is the method.
- Fill and firm. Fill your tray or pots with seed compost to about a centimetre below the rim and firm it gently flat with your hand or the base of another pot. Water it before you sow, ideally by standing the container in a tray of water until the surface darkens, then let it drain.
- Sow thinly. Scatter seeds across a tray about 2cm apart, or drop two or three into each module or small pot. Tomato seed is cheap, but crowded seedlings compete and stretch, so resist the urge to empty the packet.
- Cover lightly. Sieve or sprinkle a scant 5mm (a generous ¼in) of compost or vermiculite over the top. Tomato seed needs only a shallow covering; bury it deep and it sulks.
- Keep it warm and humid. Put the tray in a heated propagator, or cover it with a clear plastic bag or a sheet of glass to hold the moisture, and stand it somewhere that holds about 18°C. The airing cupboard is fine for the first day or two, but check daily.
- Move to the light the moment they appear. Seedlings usually break the surface in 5 to 10 days. As soon as the first loops of stem show, take off the cover and move the tray to your brightest windowsill. This handover, from warmth to light, is the hinge of the whole job.
Light, and how to stop leggy seedlings
A leggy seedling is one that has shot up tall, thin and pale, flopping under the weight of its own leaves. It is the commonest failure in raising tomatoes, and the cause is almost always the same: plenty of warmth and not enough light, so the seedling stretches towards the window looking for more.
How to prevent legginess
- Give the brightest spot you have from the moment of germination, not after. A south-facing sill beats a north-facing one by a wide margin.
- Turn the tray daily. Seedlings lean towards the light; a quarter-turn each day keeps them upright and even.
- Drop the temperature a little once they are up. Cooler, brighter conditions grow a stockier plant than warm, dim ones. There is no need to keep them as warm as the germinating seed.
- A reflector helps. A sheet of white card or kitchen foil propped behind the tray bounces window light back onto the shaded side.
Rescue: my seedlings are already leggy
All is not lost. Tomatoes have a useful party trick: they grow roots from any buried stem. When you prick out a leggy seedling, replant it more deeply, so the first pair of leaves sits just above the compost surface. The buried length of stem then roots, and you end up with a sturdier, better-anchored plant. The RHS confirms this works for stretched tomato seedlings, which is why a little legginess is forgivable in a way it would not be for many other plants.
Pricking out: moving seedlings to their own pots
Pricking out means lifting each young seedling from its crowded tray and giving it a pot of its own, with room to grow. Get the timing and the handling right and the plants barely notice the move.
When to prick out
Wait for the first true leaves. The first pair a seedling unfurls are the seed leaves, or cotyledons: smooth, rounded and rather plain. The next pair are the true leaves, jagged-edged and recognisably tomato. Once those have opened, usually a couple of weeks after sowing, the seedling is ready to move.
How to prick out
- Water the tray first so the compost holds together around the roots.
- Loosen from below with a dibber, a pencil or a lolly stick, easing the rootball up rather than pulling from the top.
- Hold by a leaf, never the stem. A crushed leaf is replaced; a crushed stem kills the seedling. Support the rootball with your other hand.
- Pot into 9cm pots of multi-purpose compost, one seedling each. Make a hole with the dibber, lower the roots in, and firm gently. Plant any leggy ones deep, to the seed leaves.
- Water in and keep bright. Stand the pots back on the brightest sill at no less than 16°C, and water whenever the surface dries.
Potting on and growing to planting size
After their move to 9cm pots, the plants spend about a month bulking up. As the roots fill the pot, shift each plant on into a 1-litre or larger pot so it never becomes pot-bound and checked. You will know the roots are ready when they start to show at the drainage holes.
Keep them in good light, water steadily and turn the pots if they lean. A faint purple tinge to the leaves usually means they are cold or hungry; move them somewhere warmer and brighter and they green up. By the time the plants are 20 to 25cm tall, stocky and dark green, with the weather warming outside, they are ready for the final stage before going out.
Hardening off before planting out
Hardening off means gradually acclimatising indoor-raised plants to life outside, over 7 to 10 days, so the move does not shock them. A tomato plant that has only known a warm windowsill will be scorched by wind and chilled by night air if it is planted straight out; a hardened-off one shrugs both off.
- Days 1 to 3: stand the plants outside in a sheltered, part-shaded spot for two or three hours in the warmest part of the day, then bring them back in.
- Days 4 to 7: lengthen the time outside by an hour or two each day, and move them into more sun and a little more wind.
- Days 8 to 10: leave them out all day, and on the last mild nights leave them out overnight too, as long as no frost is forecast. A cold frame, if you have one, makes this easier: open it by day, close it by night.
Only once they are fully hardened off, the last frost has passed and the soil is reliably above 16°C should the plants go into their final home. In most of the UK that means late May to early June. When you plant out, set them deep, with the lowest leaves just above the soil, so the buried stem makes extra roots.
What next: from seedling to harvest
Planting out is where this guide hands over. From there it is a matter of feeding, watering, supporting and, for cordon varieties, pinching out the side shoots, all the way to a kitchen windowsill full of ripening fruit. For the full grow-on, from variety choice to harvest, read Growing Tomatoes: A Beginner’s Guide to a Bumper Crop.
If you are planning where everything will go, our guide to raised beds for vegetables is a good companion, and the month-by-month plans for what to plant in June and July show where tomatoes sit in the wider growing year. Short on space? Our small garden ideas include windowsill and balcony tomatoes.
Frequently asked questions
When should I sow tomato seeds in the UK?
Sow late February to mid-March if the plants will grow in a greenhouse, or late March to early April if they will go outdoors. Outdoor plants cannot go out until the last frost has passed, usually late May or early June, so sowing them too early only leaves you with leggy, pot-bound seedlings waiting indoors.
How long do tomato seeds take to germinate?
Usually 5 to 10 days at about 18°C. Below that they are slower and patchier; much above 25°C and germination drops off. A heated propagator or a warm windowsill both work well.
Why are my tomato seedlings so tall and thin?
They are leggy, which means too much warmth and not enough light, so they have stretched towards the window. Move them to the brightest spot you have, turn the tray daily, and let them grow a little cooler. When you prick them out, plant them deeper, to the seed leaves, and the buried stem will root and steady the plant.
What are true leaves on a tomato seedling?
The first pair of leaves a tomato seedling produces are the seed leaves: smooth, rounded and plain. The true leaves are the next pair, which are jagged-edged and look like miniature tomato leaves. Once the true leaves have opened, the seedling is ready to prick out into its own pot.
Do I need a heated propagator to grow tomatoes from seed?
No. A heated propagator speeds and evens up germination, but a warm windowsill, an airing cupboard for a day or two, or the top of the fridge will all reach the temperature tomato seed needs. The part you cannot skimp on is light once the seedlings are up.
Can I sow tomato seeds straight outside?
In most of the UK, no. The season is too short and cool to sow tomato seed directly into the ground and still ripen a crop. Start them indoors from late February to early April, raise them in pots, and plant the young plants out once the frosts have finished.
Words by Linda Garrould. Updated 15 June 2026. Horticultural timings checked against the Royal Horticultural Society. General gardening information, not specific advice for your own plot.
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