When should I sow tomatoes, chillies, sweet peppers and aubergines?

February is the usual answer for sowing tomatoes, chillies, sweet peppers and aubergines in the UK, but the date matters less than the light. These are warmth-loving crops with a long season, so they need an early start indoors, and the thing that decides whether that start works is how bright a windowsill you can give them the moment they come up.

You do not need to turn the house into the tropics. Too much warmth combined with poor light is the fastest route to thin, floppy seedlings, and it is the single most common reason a promising sowing comes to nothing.

• When to sow, and when to wait
• Germination temperatures
• How deep, and how many per pot
• Windowsill or heated propagator?
• Why seedlings go leggy
• What actually helps
• Frequently asked questions

When to sow, and when to wait

Late winter into early spring is the window. Within it, judge by your setup rather than the calendar.

• Bright windowsill in a warm room: late winter to early spring is ideal. You can sow in late January if you are keen, though February is early enough for most.
• Low light, north-facing window, gloomy stretch of weather: wait a fortnight. Two weeks later with good light beats two weeks earlier with none.

The rule that covers every case: sow when you can give the seedlings plenty of light as soon as they emerge, not before.

Germination temperatures

These four crops are lumped together in most sowing guides, but they want noticeably different warmth to germinate.

• Tomatoes: 18 to 24 degrees
• Sweet peppers: 22 to 28 degrees
• Chillies: 22 to 30 degrees
• Aubergines: 24 to 30 degrees

Tomatoes will come up on an ordinary warm windowsill. Chillies and aubergines are the ones that sulk without extra heat, which is why they get a reputation for being difficult when the real problem is a cold kitchen.

Once they are up, drop the temperature. Tomatoes grow sturdiest at around 16 to 20 degrees, and peppers, chillies and aubergines at around 18 to 22 degrees, provided the light is good. Cooler and brighter beats warmer and darker every time.

How deep, and how many per pot

The rule of thumb for any seed is to sow at about twice its own thickness. For all four of these that works out at roughly 5mm.

• Fill pots or modules with compost and firm gently, so the surface is level but not compacted
• Sow one or two seeds per pot or module
• Cover lightly with compost or fine vermiculite
• Water gently, so you do not wash the seed out of position

Sowing thinly matters more than it sounds. Crowded seedlings shade each other from day one, and that shading is the start of the legginess problem below.

Windowsill or heated propagator?

A windowsill is enough, particularly for tomatoes. What it must be is bright, because a seedling on a dim sill leans towards the glass and stretches within days.

A heated propagator is genuinely useful for chillies, aubergines and sweet peppers, given the temperatures above, and for chillies it can be the difference between germination and nothing at all. It is not essential if the warmth is coming from somewhere else, such as an airing cupboard for the germination period only.

One caution with airing cupboards: get them out the moment the first shoots appear. A seedling in the dark will stretch several centimetres overnight and never recover properly.

Why seedlings go leggy

Long, floppy seedlings are almost always a light problem, usually made worse by warmth. The usual causes are a dim windowsill in late winter, seedlings set too far back from the glass, a radiator underneath, or sowing too thickly so they crowd each other out.

• Use the brightest spot you have. A south or west-facing sill is ideal
• Keep pots close to the glass, and turn them daily so they grow straight rather than leaning
• Once germinated, cool them down. Cooler and brighter, not warmer and darker
• Prick out and pot on promptly, before they start competing

If tomatoes have already gone leggy, they can usually be rescued by potting on and burying the stem deeper, because tomatoes root readily along a buried stem. Peppers, chillies and aubergines do not do this, so with those three there is no rescue and you need to get the light and temperature right from the start.

What actually helps

Very little kit is needed for this, but three things make indoor sowing less of a chore, and doing it little and often is what produces good plants.

A coir block instead of a bag of compost. The Coir Vital Grow potting mix is a peat-free block made from coconut husk fibre. A 650g block expands to around 10 litres with water, so there is no half-open bag drying out in the shed, and the texture is light and even, which fine roots find easy to work through. It also solves the storage problem, which is the real reason indoor sowing gets put off. It is in Garden Supplies.

Labels, which are not optional with these crops. Tomato varieties look identical as seedlings, and so do chillies, which matters a great deal when one of them is a habanero. Write your initials, the date, the crop and the variety in pencil or indelible pen. The Sting In The Tail wooden labels come with a pencil, so they can be rubbed out and reused, and they eventually rot down. Those and the rest are in Garden Accessories.

A table brush and pan. Unglamorous, and the thing that decides whether you are allowed to keep sowing on the kitchen table. The Rex London set has plant-based bristles soft enough for a worktop and stiff enough to gather compost crumbs.

For chillies specifically, a grow kit takes the guesswork out. The Plant Gift Company's eco kit includes Red Habanero, Jalapeño and Orange Cayenne seeds with biodegradable pots, soil discs, markers and a reusable tin planter, so all you supply is the warmth.

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Frequently asked questions

When should I sow tomato seeds in the UK?

February suits most UK gardeners, with late January possible if you have a bright, warm windowsill. Tomatoes need a long season to reach cropping size, so an early indoor start matters, but the limiting factor is light rather than the calendar. If your only windowsill is north-facing or the weather has been persistently dull, waiting a fortnight will give you better plants than sowing on time and watching them stretch. Sow when you can give the seedlings good light the moment they emerge.

What temperature do chilli seeds need to germinate?

Chillies want 22 to 30 degrees to germinate reliably, which is warmer than most kitchens. That is why they get a reputation for being difficult, when the real issue is simply that the seed has been sown somewhere too cool. Sweet peppers need 22 to 28 degrees and aubergines 24 to 30, while tomatoes are content at 18 to 24. A heated propagator earns its place for the first three. Once they are up, grow them on cooler, at around 18 to 22 degrees, with plenty of light.

How deep should you sow tomato and pepper seeds?

About 5mm, which follows the general rule of sowing a seed at roughly twice its own thickness. Fill the pot or module with compost, firm it gently so the surface is level rather than compacted, sow one or two seeds, then cover lightly with compost or fine vermiculite. Water gently so the seed is not washed out of position. Sowing too deeply is a common cause of poor germination with these crops, since the seedling exhausts itself before reaching the light.

Why are my seedlings tall and floppy?

Not enough light, usually combined with too much warmth. A dim sill, pots set back from the glass, a radiator underneath, or seedlings sown so thickly that they shade each other will all produce the same stretched growth. Move them to the brightest spot you have, keep them close to the window, turn the pots daily so they grow straight, and lower the temperature now they have germinated. Leggy tomatoes can be saved by potting on and burying the stem deeper. Peppers, chillies and aubergines cannot.

Do I need a heated propagator?

Not for tomatoes, which germinate happily on a warm windowsill. For chillies, sweet peppers and aubergines it is genuinely useful, because they need 22 degrees or more and most homes do not hold that at a windowsill in February. If you would rather not buy one, an airing cupboard works for the germination period, but check daily and move the pots into the light the moment the first shoots show. A seedling left in the dark stretches overnight and never quite recovers.