A homegrown tomato begins as a tiny seed that can grow into a two-metre vine by the end of summer. The temptation is to sow as early as possible, but that often leaves gardeners with tall, pale plants desperate for space and light while outdoor temperatures remain too cold. A slightly later sowing in good light usually gives more manageable plants.
For a UK garden, tomato seeds are normally started indoors in warm conditions. The Royal Horticultural Society recommends different sowing windows for greenhouse and outdoor crops, so plan backwards from where the plants will eventually grow.
Decide whether the tomatoes will grow outside
For tomatoes destined for a greenhouse, the RHS advises sowing from late February to mid-March. For plants that will grow outdoors, the recommended window is late March to early April.
These are useful guidelines, not an instruction to ignore weather. If you have no heated greenhouse and your home gets little natural light in February, an unnecessarily early sowing may create more work than benefit.
Choose a cultivar suited to its final position. A compact bush tomato for a sunny patio is easier to accommodate than a vigorous greenhouse cordon that will eventually need tall support.
Choose a variety you actually want to eat
Cherry tomatoes often produce small fruit suitable for snacking and salads. Plum types can be excellent for cooking. Beefsteak tomatoes may need a long growing season and strong support.
Read the seed packet for growth habit, expected crop and disease resistance. Bush (determinate) and cordon (indeterminate) tomatoes are trained differently after planting, so it's worth understanding that distinction early.
Use seed from a reputable supplier when predictability matters. Saved seed from some hybrids may grow into plants that differ from the original variety.
Gather your sowing equipment
You'll need a clean seed tray or small pots, suitable peat-free seed compost, labels, a watering can or mister and access to a warm, bright position.
You don't need a large electric propagator if your home can provide an appropriate temperature, although a thermostatically controlled unit makes warmth more consistent.
Wash reused pots before planting. Old compost debris and algae can carry problems into a fresh sowing, particularly when seedlings will be crowded together.
Fill pots without compacting the compost
Use fresh seed compost and fill modules or small pots almost to the rim. Tap gently to settle the mix rather than pressing it into a solid block.
The growing medium should be evenly moist before sowing. If it is very dry, water it carefully and let excess drain so seeds don't wash into the bottom of the pot.
A fine, open compost allows delicate roots to establish. Large wood pieces and compacted lumps make sowing small seeds more difficult.
Sow at the right depth
Tomato seeds are small. The RHS advises sowing them in modules, small pots or seed trays and covering with a thin layer of compost, generally around 0.5cm.
Place one or two seeds in a module, or sow sparingly in a tray if you plan to prick out seedlings later. Don't pour a whole packet into a small pot simply because you want several plants.
Cover gently and label immediately. Different tomato cultivars may look nearly identical as seedlings, but their eventual growth and care needs can differ considerably.
Give seeds enough warmth to germinate
The RHS recommends a germination temperature of around 18°C. A warm windowsill may be suitable; a heated propagator can help where the room is cooler.
Cover trays appropriately with a clear lid or suitable plastic to retain moisture while seeds germinate, making sure the equipment doesn't overheat in direct sun.
Check daily. As soon as seedlings emerge, give them plenty of light and ventilation rather than keeping them in warm darkness.
Move seedlings into strong light
Tomato seedlings stretch quickly if they receive warmth without adequate light. Long pale stems and tiny leaves are signs that the growing position may be unsuitable.
Use a bright windowsill or suitable grow light. If using supplementary lighting, follow the equipment's safety and distance instructions.
Turn pots periodically so the plants don't all lean strongly towards the window. Keep conditions fairly steady rather than moving seedlings between a hot radiator and cold glass every day.
Water gently and check the compost
Seedlings need a moist root zone, but saturated compost can encourage losses from damping-off and root problems.
Water with a fine rose, from beneath where appropriate, or use a gentle pour beside the stem. Avoid washing the seedling sideways with a strong jet.
Allow excess water to drain. A little pot can dry out quickly in warm light, while one in a cool room may remain moist for much longer. Check rather than following a rigid timetable.
Thin crowded seedlings early
If both seeds germinate in a module, decide whether you want to keep the strongest and snip the other at soil level, or gently move it when large enough.
Overcrowding encourages weak growth and makes roots difficult to separate later. A few sturdy plants are more useful than a dense forest of seedlings in one tray.
Handle seedlings by a leaf rather than the fragile stem when moving them. Damaged young stems are harder to recover than a lost leaf.
Prick out into individual pots
Once seedlings have their first true leaves and are large enough to handle, move them into individual pots if they were sown together.
Prepare each new pot with peat-free multipurpose compost and a hole large enough for the roots. Lift the seedling carefully with a dibber and keep as much root material intact as possible.
Tomatoes can produce roots along buried stems, so a slightly leggy seedling can often be planted deeper than before. Leave healthy leaves above the compost rather than burying them in damp soil.
Grow plants on without rushing to the garden
Keep the young tomatoes in bright, frost-free conditions as they gain size. Repot into a slightly larger container when roots begin filling the old pot.
Support a growing plant with a small cane if it starts leaning. Don't feed heavily simply because you want rapid height; steady healthy growth is better.
If several seedlings are nearing windowsill height in April but outdoor nights remain cold, maintain them under suitable shelter. This is why sowing later often makes life easier.
Harden off before outdoor planting
Tomatoes grown indoors have tender leaves accustomed to little wind and filtered sunlight. Before planting outdoors, expose them gradually to sheltered daytime conditions.
Increase the time outside over a week or two and bring them under cover if cold nights are expected. Sudden direct sun can scorch leaves, while frost can kill tender growth.
Plant into the final position once frost risk has passed, usually late May or June for outdoor tomatoes in many UK regions. Conditions matter more than an exact date on the packet.
Know what to do when the plants mature
Bush varieties may remain fairly compact and generally aren't pruned like cordons. Cordon types need strong support and a regular system of side-shoot management.
Choose a good-sized container or properly prepared garden bed. Tomato plants need warmth, sufficient light and dependable watering to produce sound fruit.
Don't squeeze ten seedlings into one small growbag because every seed germinated. Give away the surplus or plant fewer so each has room to thrive.
Why tomato seeds sometimes fail
No germination may reflect old seed, cold compost, excessive wetness or allowing the surface to dry out. Check conditions before sowing another packet.
Seedlings that collapse at soil level may be suffering from damping-off or other problems associated with hygiene, moisture and ventilation. Remove affected material and improve growing conditions.
A pale stretched seedling isn't necessarily hungry; it may need more light. Avoid overfeeding before you have corrected the environment.
The most productive approach is to start at the right time, sow shallowly, provide warmth for germination and plenty of light afterwards. That gives tomato plants a better start than racing to plant the first seeds of the year.