For outdoor tomatoes in the UK, start seed indoors from late March to early April, then plant out only after frost risk has passed. Plants intended for a greenhouse can usually be started earlier, from late February to mid-March, if you can give them enough warmth and light. Count back roughly seven to eight weeks from the likely planting date rather than sowing everything in February.
A strong, compact young plant is more useful than an older seedling with a long, thin stem. Days are short at the end of winter, and warm rooms without supplementary lighting can produce leggy plants long before the soil is ready.
When should you sow tomatoes for an outdoor crop?
In much of England and Wales, late March and early April are practical sowing windows for unheated outdoor growing. In cooler Scottish or northern sites, outdoor planting can be later, and seed dates should follow. Local frost conditions matter more than a single national calendar date. A sheltered urban garden and an exposed rural plot can have very different planting times.
Plan to harden plants off before planting outside. A short run of warm spring weather is not proof that the last frost has occurred. Tender tomato growth can be damaged by cold nights, even after a sunny day.
Do greenhouse tomatoes need an earlier start?
Often yes. A heated greenhouse gives far more control over temperature than an unheated polytunnel. If plants will go into protected ground early, an earlier sowing can make sense. With no heating or grow lights, however, seedlings sown too early may outgrow their pots while cold weather keeps them indoors.
Light after germination matters more than extra heat
Tomato seed generally germinates reliably in warm compost, about 18-25°C. Once seedlings appear, take off covers and provide the brightest suitable position you can. A bright windowsill may work, but seedlings should not be left stretching towards a small patch of light in a hot room.
After emergence, a slightly cooler environment, often around 18-20°C during the day, can help prevent excessively soft growth when combined with good light. Avoid cold draughts and waterlogging; compost should be evenly moist rather than constantly saturated.
How to sow tomato seed
Fill clean modules or small pots with fine, free-draining seed compost. Water the compost lightly, sow seeds at the depth stated on the packet and cover gently. Label the variety and date. Keep warm until germination and then provide plenty of light.
Water carefully so the small root systems do not dry out, but do not keep the surface permanently soaked. Seedlings require air as well as water at the roots. Good hygiene and free drainage reduce avoidable problems with young plants.
When should tomato seedlings be pricked out?
Move seedlings once their first true leaves have developed and they are large enough to handle. Hold each plant by a leaf rather than squeezing its delicate stem. Tomatoes can be planted somewhat deeper in their new pots, often to just below the seed leaves, because buried stems can form additional roots.
If you sowed seeds singly into modules, you may simply transfer the entire rootball to a larger pot. That avoids unnecessary root damage. Give plants enough growing space so they do not shade one another.
How do you tell if you sowed too early?
Very long, thin stems, widely spaced leaves and plants leaning strongly towards a window suggest insufficient light in relation to warmth and time. Seedlings that have filled their pots weeks before planting weather arrives can also become difficult to water and manage.
Do not treat size as proof of readiness. What matters is a healthy root system, sturdy stem, good foliage and the ability to adapt gradually to outdoor light, wind and lower temperatures.
When can tomato plants go outside?
Over roughly seven to ten days, gradually expose plants to outdoor conditions, increasing the time as weather allows and protecting them from cold nights. Move them to the bed only once local frost risk is low and the soil has warmed. Plants going into a cold greenhouse may need additional protection during late frosts.
For a planned outdoor planting in late May, sowing around late March or early April often provides enough growing time. The best result still depends on weather, variety, available light and your growing setup.