How to Start Seeds Indoors: Sowing, Germination and Seedling Care

Starting plants from seed is cheap, satisfying and far more reliable than it looks, once you know what a seed is actually asking for. It needs moisture, the right warmth, air in the compost and, the moment it breaks the surface, light. Get those four right and almost everything comes up. Get one wrong and you see the classic problems: nothing appears, seedlings stretch into pale threads, or a healthy tray collapses overnight. This guide covers each step, from choosing a medium and sowing depth to temperature, light, watering and potting on, so your tomatoes, chillies, herbs and salads get off to a strong start.

What a seed actually needs

A seed carries its own food store, so it does not need feeding to germinate. What it needs is a signal that conditions are right to grow. Water is the first part of that signal: the seed absorbs it, swells, and switches on. Warmth is the second, and it sets the pace. Too cold and the chemistry inside the seed runs so slowly that it can sit in wet compost for weeks and rot before it ever sends out a root.

The third need is the one people forget: air. A germinating seed is respiring hard, and it needs oxygen in the compost around it. Compost that is kept soaking wet has its air spaces full of water, and the seed suffocates. That is why the aim is always evenly moist, never wet.

Light is the fourth need, but it only matters at two moments. A few small seeds, lettuce among them, use light as part of the signal to germinate, which is why they are sown on the surface. And every seedling needs strong light the instant it emerges, because its food store is almost used up and from that point on it has to make its own.

What to sow into

Seeds want something fine, light and free-draining that holds moisture without becoming soggy. Ordinary potting compost is too coarse and usually too rich for small seed. There are three good options, and the right one depends on how you will grow the plant on.

Seedling compost

A fine, lightly fertilised mix sown into a tray or cell tray is the traditional route and suits anyone growing on in soil. It holds water well, roots move through it easily, and the light feed it carries is enough for the first couple of weeks. Fill the tray, tap it down gently to settle it, and water it before you sow rather than after, so the seed is not washed around.

Plugs and cubes

Peat pellets, organic propagation cubes and ready-filled plug trays give every seed its own cell. They are clean, quick and very hard to overwater, and the seedling goes into its next pot without its roots being disturbed. That makes them especially good for crops that dislike root disturbance, such as cucumbers and courgettes. Peat pellets need to be soaked until they have fully expanded before you sow.

Rockwool

Rockwool is inert, so it carries no food at all. It is the natural choice if the plant will grow on in hydroponics, because the cube drops straight into the system. It needs soaking in pH-adjusted water before use, exactly as our cuttings guide describes, and because it holds a lot of water it should be squeezed gently so it is damp, not dripping.

Our growing media guide compares soil, coco and hydro in more depth if you have not yet decided how the plants will be grown on.

How to sow

1. Start clean. Use new or scrubbed trays and fresh compost. Old compost and dirty trays are where damping off comes from.
2. Moisten the medium first. Water the tray, or soak the plugs, and let any excess drain away before the seed goes in.
3. Get the depth right. Cover each seed to about twice its own width. Fine seed goes on the surface and is pressed in lightly; tomato and chilli seed sits under a few millimetres; large seeds such as courgettes and beans go in around 2cm.
4. Sow thinly. One or two seeds per cell, or spaced out across a tray. Crowded seedlings compete for light and stay damp, and both cause trouble later.
5. Label as you go. Write the crop, variety and date. Seedlings of different crops look alike for the first week or two.
6. Cover and warm. Put the lid on to keep the surface from drying out and stand the tray where it can hold the temperature the crop needs.

Temperature: the usual reason nothing comes up

When seeds fail to appear, cold compost is the most likely cause. Each crop has a range it germinates best in, and the figure that matters is the temperature of the compost, not the air in the room. A tray on a windowsill can be several degrees colder than the room behind it, especially at night.

Two things follow from that table. First, the warm-season crops, chillies, peppers, aubergines and tomatoes, are very hard to start in a British house between January and April without some help, which is why heated propagators and heat mats exist. Second, lettuce goes the other way: put it on a heat mat beside your chillies and it may refuse to germinate at all.

A heat mat under an unheated propagator is usually the best value, because the mat works with any tray and moves under pots later. A built-in heated propagator is tidier. Whichever you choose, a thermostat stops it overshooting in a room that is already warm, and our propagator guide covers how to set one up and how to read the temperature at the compost rather than under the lid.

When they come up: lid off, light on

Check the tray every day. The moment the first seedlings break the surface, the job changes completely. Until now you were keeping the seed warm and the surface damp. From now on you are growing a plant, and a plant needs light and moving air far more than it needs a humid lid.

Open the vents straight away and take the lid off over two or three days, so the seedlings adjust to drier air gradually. If some cells have not come up yet, it is better to move the emerged seedlings out from under the lid than to leave the whole tray covered waiting for the stragglers.

Once the seedlings are up you can also let the compost run a few degrees cooler than it was for germination. Seedlings kept very warm in poor light grow soft and leggy, and a slightly cooler, brighter spot gives a stockier plant.

Light: why seedlings stretch

A seedling that does not get enough light puts everything into growing upwards to find it. The result is the familiar long, pale, floppy stem that cannot support the leaves. It is the most common seedling problem of all, and a south-facing windowsill in January simply does not provide enough light to prevent it.

A small propagation light solves it. Seedlings want gentle light rather than intense light, around 100-300 µmol/m²/s, because they have very few roots to replace the water that bright light drives out of the leaves. Our light height guide sets out how far to hang each type of fixture to hit that. Run the light for 14-18 hours a day on a timer, and raise it as the seedlings grow so it stays at the right distance.

If you are using a windowsill anyway, turn the tray a half-turn every day so the seedlings do not all lean towards the glass, and move them back from the window on cold nights.

Watering and feeding seedlings

The aim is compost that is evenly moist all the way through, and then allowed to dry back slightly before the next drink. The easiest way to manage that is from below: stand the tray or cells in a shallow tray of water for ten or fifteen minutes until the surface darkens, then lift them out and let them drain. Watering from below keeps the surface and the stems drier, which helps prevent damping off, and does not wash small seedlings over.

Before the seedlings emerge, a fine mist from a spray bottle is enough to stop the surface drying under the lid. After that, judge it by weight just as you would a larger pot: a freshly watered cell tray feels noticeably heavy, a dry one light. Our watering guide covers the same principle for bigger pots.

Feeding depends on the medium. A seedling compost carries enough for the first couple of weeks, so the plant usually needs nothing extra until it is potted on. Rockwool and most plugs carry little or no food, so once the first true leaves have opened, start a seedling-strength feed at the weakest rate on the label. Little and gentle is the rule. Young roots burn easily, and our nutrient burn guide shows what too much looks like.

Damping off

Damping off is the name for seedlings that collapse at compost level, as if someone had pinched the stem. It is caused by soil-borne fungi that thrive in cold, wet, stagnant conditions, and it can take out a whole tray in a day or two. Once a seedling has gone over it cannot be saved, so prevention is everything.

• Clean trays and fresh compost every time. Our cleaning guide covers washing kit between batches.
• Sow thinly so the air can move between the seedlings.
• Water from below and let the surface dry slightly between drinks.
• Take the lid off once the seedlings are up.
• Keep a little air moving over the tray with a small fan on its lowest setting. A gentle breeze also makes the stems thicker and sturdier.

If a few seedlings collapse, remove them and the compost around them straight away, ease off the watering and improve the airflow. Our fungus gnats guide is worth a read too, because the same over-wet compost that encourages damping off is where gnats breed.

Thinning, potting on and hardening off

The first pair of leaves a seedling opens are seed leaves, simple and rounded. The next pair are its first true leaves, which look like a small version of the adult plant's leaf. That is the point to thin out: where two seedlings came up in one cell, snip the weaker one at compost level with scissors rather than pulling it, which would disturb the roots of the one you keep.

Pot on when roots start to show at the bottom of the plug or cell. Go up to a small pot, around 7-9cm, rather than straight into a large one: a small root system in a big pot of wet compost sits cold and soggy, which is exactly what our pot size guide warns about. Always hold a seedling by a leaf, never by the stem. A torn leaf will be replaced; a crushed stem will not. Tomatoes are the exception to the depth rule: plant them deeper than they were, up to the seed leaves, because they grow new roots along the buried stem.

Plants that will finish outdoors or in a greenhouse need hardening off first. Over a week or two, put them outside for a few hours on mild days, building up gradually, and bring them in at night until the nights are reliably frost-free. Plants that will grow on in a tent or indoors skip this step.

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Eight seed-starting mistakes to avoid

1. Sowing too deep. A small seed buried under a centimetre of compost runs out of food before it reaches the light.
2. Cold compost. Chillies and tomatoes in a cold room may take weeks, or rot first. Add heat.
3. Soaking wet compost. Seeds need air as well as water. Damp, never wet.
4. Leaving the lid on. Once seedlings are up the lid does more harm than good.
5. No light after emergence. Even a day or two in the dark is enough to make seedlings stretch.
6. Sowing too thickly. Crowded seedlings stretch, stay damp and invite damping off.
7. Feeding too early or too strong. Seedling compost already has food in it; inert media need only a weak feed.
8. Potting on into a huge pot. Go up in steps so the compost dries back between drinks.

Quick answers

What temperature do seeds need to germinate?

It depends on the crop, and it is the temperature of the compost that counts, not the air. Lettuce and brassicas germinate well at 15-20°C, tomatoes and basil at 20-25°C, cucumbers and courgettes at 20-30°C, and chillies and peppers want 25-30°C. Below those ranges seeds are slow or rot before they emerge, which is why a heated propagator or heat mat makes such a difference in a British winter or spring.

How deep should I sow seeds?

Cover a seed to roughly twice its own width. Very fine seed such as lettuce goes on the surface, pressed in lightly, because many small seeds need light to germinate. Tomato and chilli seed sits just under a few millimetres of fine compost or vermiculite, and large seeds such as courgettes and beans go in around 2cm deep.

Should I keep the propagator lid on after seeds sprout?

No. The lid is there to stop the surface drying out while the seed germinates. Once the seedlings are up, open the vents and take the lid off over a few days. Leaving it on traps damp, still air around the stems, which is exactly what damping off needs.

Why are my seedlings tall and floppy?

They are stretching for light. Seedlings that come up on a dim windowsill, or that sit in the dark for even a day or two after emerging, grow long, pale stems that cannot hold them up. Give them light the moment they appear, keep the light at the right height, and do not keep them warmer than they need once they are up.

What is damping off and how do I stop it?

Damping off is a fungal problem that makes seedlings collapse at the base, as if pinched at compost level. It thrives in wet, cold, stagnant conditions. Use clean trays and fresh compost, sow thinly, water from below, take the lid off once the seedlings are up, and keep a little air moving over the tray.

When should I pot on seedlings?

When roots start to show at the bottom of the plug or cell, usually once the first one or two true leaves have opened. Move them into a small pot rather than a large one, and handle them by a leaf, never by the stem. Tomatoes can go in deeper than they were, because they root along the buried stem.

When should I sow chillies and tomatoes indoors in the UK?

On a windowsill, chillies and aubergines are usually sown from January to March because they are slow to grow, and tomatoes from February to April. Under a propagation light in a heated room or tent you are not tied to the season at all, and can sow whenever you have space.

Does this work for herbs, salads and bedding plants too?

Yes. Every seed needs the same four things: moisture, the right warmth, air in the compost and light as soon as it emerges. Only the temperature and sowing depth change from crop to crop, and the seed packet will usually tell you both.