When to sow tomato seeds, and when it's warm enough to plant out

In a typical UK garden, sow tomato seeds indoors from late February to late March. That gives you a sturdy young plant ready for an unheated greenhouse in early May, or for planting outdoors from late May into early June, once the frosts are over.

The dates here assume a last frost around 20 April, which is typical for lowland England and Wales. In Scotland, the north of England and frost pockets, push everything a week or two later. In the south-west, near the coast and in cities, you can usually go a little earlier.

Why late February to late March

Tomatoes are killed by any frost, and they sulk in cold soil even when it isn’t freezing. So the whole timetable works back from the day they can safely go outside or into the greenhouse.

A tomato takes roughly 6 to 8 weeks from sowing to a good-sized plant with its first flower truss showing. Count back from early May and you land in March. Sow much earlier than that and you end up with tall, leggy plants on the windowsill, waiting for the weather to catch up. Light is the limit in February, not warmth.

Matching the sowing date to where they’ll grow

Where the plants will crop	Sow indoors	Plant out
Unheated greenhouse or polytunnel	early to mid-March	early to mid-May
Outdoors, in a bed or pots	mid to late March	late May to early June

Outdoor plants are the ones most often sown too early. They can’t go out until the nights stay above about 10 °C, which in much of the UK isn’t until late May. A late March sowing is ready at about the right time. If you have a heated greenhouse you can sow in late February and plant a little earlier, but most people don’t, and the dates above assume you don’t.

How to sow

1. Fill a small pot or tray with seed compost, firm it lightly and water it.
2. Scatter a few seeds on top and cover them with about 0.5 cm of compost or vermiculite.
3. Keep them at around 18 to 21 °C. A propagator or a warm windowsill both work.
4. Seedlings usually appear in 7 to 14 days. Move them somewhere bright and slightly cooler as soon as they’re up, or they’ll stretch towards the light.

You only need a handful of plants for most gardens, so sow a few more than you need and pick the strongest.

Potting on

When the seedlings have their first true leaves (the jagged ones that look like tomato leaves, after the two smooth seed leaves), move each one into its own 7 to 9 cm pot. Hold them by a leaf, never the stem. Tomatoes grow roots along any buried stem, so plant them deeper than they were, with the seed leaves just above the compost.

Pot on again into a 12 to 15 cm pot if they fill their pots before it’s warm enough to plant out. A tomato that’s been starved in a small pot for weeks is slow to get going.

Hardening off

Plants raised indoors need a week or two to get used to wind, sun and cooler nights. Put them outside during the day and bring them in at night for the first few days, then leave them out in a sheltered spot overnight once the forecast is mild. A cold frame makes this easier: open it by day and close it at night.

Greenhouse plants need less of this, but a few days of an open door and vents helps.

Planting out

Plant tomatoes out once frost has passed and the soil has warmed, which for most of the UK means early May in an unheated greenhouse and late May to early June outdoors. Pick a sunny, sheltered spot. Space them 45 cm apart in rows 60 cm apart, and put a cane or string in at the same time so you don’t spear the roots later.

Keep a bit of fleece handy for the first couple of weeks. A late cold night in May is common enough to catch people out. VegLog’s frost alerts are worth having on for exactly this stretch.

Once they’re growing, pinch out the side shoots on cordon types each week, and start feeding with tomato food when the first fruit sets. Expect the first ripe fruit from mid-July, with plants cropping for about ten weeks after that, until the end of October in a greenhouse.

Common mistakes

• Sowing in January or early February without grow lights. The plants get leggy and weak.
• Cold, wet compost. Seeds rot rather than germinate. Keep them warm and only just moist.
• Planting outside too soon. Even without a frost, cold nights in early May check growth.
• Skipping hardening off. Leaves turn white or purple from sun and cold, and the plant stalls.