The temptation to start sowing in January is strong — the seed catalogues arrive, the gardening enthusiasm peaks and the idea of getting ahead of the season is appealing. But most seeds sown too early produce weak, leggy plants that struggle when they finally go outside. Timing matters.
This guide covers the key indoor sowing dates for UK gardeners, with a focus on avoiding the most common timing mistakes.
If you're in the north of England, Scotland or at altitude, add 2–3 weeks to any outdoor planting dates. If you're in the south west or a sheltered coastal area, you may be able to start slightly earlier.
January
Very little should be sown in January unless you have a heated greenhouse or propagator. The light levels in a UK January are simply too low for healthy seedling growth — most seedlings started in January become leggy and weak because they can't get enough light.
Exception: onions and leeks can be started in January on a very bright windowsill or under grow lights. They're slow-growing and benefit from the early start. Sweet peppers and chillies can also be started in late January in a heated propagator — they need a long season.
February
Still early for most crops, but more viable as light levels improve through the month.
Both need a long growing season — 16–20 weeks from seed to first fruit. Start in a heated propagator (20–25°C) and pot on as they grow. They'll be ready to go outside after the last frost in late May/early June.
Similar timing to chillies. Need warmth to germinate (25°C+) and a long season. Worth starting early if you have a propagator.
March
March is when indoor sowing really begins for most UK gardeners. Light levels are improving and the range of crops you can usefully start increases significantly.
The most important indoor sowing date for most vegetable growers. Sow in late March for planting out in late May/early June. Earlier than this and you risk leggy plants; later and you lose fruiting time. Use a propagator or warm windowsill — tomatoes need 18–21°C to germinate.
Fast germinators that don't need a long head start. Sow 4–6 weeks before last frost date. Sowing too early produces plants too large to keep on a windowsill comfortably before it's safe to put them outside.
Both need a long season and are slow to get going. Sow thinly on the surface of moist compost — the seeds need light to germinate so don't cover them.
April
April is the peak indoor sowing month. Many crops can be sown either indoors or directly outside in April depending on conditions.
Needs warmth to germinate and sulks in cold conditions. Start on a warm windowsill in April. Don't rush it outside — basil hates cold and should stay indoors until the weather is reliably warm.
Fast germinators with large seeds — easy for beginners. Sow 3–4 weeks before last frost. One or two plants will produce more squash than most families can use.
What can go direct outside
Not everything needs to start indoors. Many crops are better sown directly where they'll grow: peas, beans, beetroot, carrots, parsnips, salad leaves, spinach, radishes and most brassicas can all be sown directly once the soil has warmed (usually from March/April depending on your location).
Use the GrowGuide Seed to Harvest tool to find the exact sowing and planting windows for any crop you're planning to grow.
Exact sowing dates for every crop
The GrowGuide Seed to Harvest tool shows the full growing timeline for any vegetable — from germination through to harvest.
Seed to Harvest Timeline →