Sowing seeds indoors: a step-by-step guide

Sowing seeds indoors gives tender and slow crops a head start before the weather outside is warm enough. You fill clean pots or trays with fresh seed compost, sow thinly, keep them warm and just moist until they germinate, then give the seedlings as much light as possible. Most UK gardeners start in late February or March, but the right date depends on the plant, and starting too early is a far more common mistake than starting too late.

Indoor sowing is ideal for crops that need a long, warm season, such as tomatoes and peppers, for frost-tender herbs like basil, and for flowers such as cosmos and sweet peas. It also protects small seedlings from slugs and birds during their most vulnerable weeks.

When to sow

Work backwards from your last frost date. In much of southern England and coastal areas, the risk of frost has usually passed by mid to late May; in the north, Scotland and inland frost pockets it can be early June. Most tender plants want six to eight weeks indoors before they go out, so March and April are the main sowing months for many of them.

January–February: chillies and aubergines, which are slow. Onions from seed and sweet peas can go in now too.
March: tomatoes, peppers, lettuce for early crops, and half-hardy annuals. The March garden jobs page has the full list.
April: basil, courgettes, squashes, sweetcorn, French beans and sunflowers. These grow quickly, so there is no benefit in sowing earlier.
May: a second batch of courgettes and beans for late crops, and runner beans in pots.

A tomato sown in early January on a dim windowsill will usually be leggy and weak by May, while one sown in late March will often catch it up and overtake it. If in doubt, sow later.

Compost and containers

Use fresh, bagged seed compost, ideally peat-free. It is fine-textured and low in nutrients, which seedlings need at first. Do not use garden soil or your own garden compost for sowing; both can carry weed seeds and fungal spores, and the texture is too lumpy for tiny roots. Garden compost is brilliant for beds and borders, though; see making compost.

Wash old pots and trays in hot soapy water before reuse. Modules (trays divided into small cells) are excellent for crops that dislike root disturbance, such as courgettes and beans, and they save pricking out. Deep root trainers or cardboard tubes suit sweet peas and beans, which make long roots early.

How to sow

Fill your container with compost, level it and firm lightly with the base of another pot. Stand it in a tray of water until the surface darkens, then let it drain. Watering from below like this avoids washing seeds around.

Sow thinly. Large seeds such as beans and courgettes go in one per module, about 2cm deep; courgette seeds are best pushed in on their edge. Medium seeds like tomatoes can be spaced a centimetre or so apart over the surface and covered with a thin layer of sieved compost or vermiculite. Very fine seeds, including some flowers, are pressed onto the surface and left uncovered because they need light to germinate. Always check the packet.

Label every pot with the name and date before you forget. Then cover with a clear lid or slip the pot into a plastic bag to keep the humidity up.

Warmth and light

Most seeds need warmth to germinate, typically somewhere between 15°C and 25°C depending on the plant. Tomatoes and peppers like the warmer end; lettuce can fail to germinate if it gets too hot. A heated propagator gives steady bottom heat, but a warm room works for many crops.

Check daily. As soon as the first seedlings poke through, remove the cover and move them into the brightest light you have. This is the critical moment: seedlings left in the warm and dark for even a day or two stretch into pale, floppy stems. A south-facing windowsill is good; turn the pots every day so they do not lean. If your windows are dim, a simple LED grow light hung a short distance above the seedlings makes a big difference. Once germinated, most seedlings prefer slightly cooler conditions than they needed to sprout, which helps keep them sturdy.

Pricking out and potting on

When seedlings have their first true leaves (the second pair, which look like the adult plant), move them into individual pots of multi-purpose compost. Loosen the roots with a pencil or plant label and always hold seedlings by a leaf, never the stem, which bruises easily. Tomatoes can be planted a little deeper than before, as they root along their buried stem.

Pinch out the growing tip of sweet peas once they have three or four pairs of leaves to make them bush out. Start feeding with a dilute liquid feed after a few weeks in their new pots, when the compost nutrients run out.

Hardening off

Plants raised indoors are soft and need one to two weeks to adjust to wind, sun and cooler nights. Put them outside in a sheltered spot during the day and bring them in at night, then leave them out overnight when nights are mild. A cold frame makes this easy. Only plant tender crops out once all risk of frost has passed; a late cold snap can scorch or kill them, as described on our frost damage page.

Common mistakes

Sowing too early. Leggy seedlings and a crowded windowsill by April. Follow the packet and your frost date.
Overwatering. Soggy compost suffocates roots and encourages disease. Water from below and let the surface dry slightly between waterings.
Sowing too thickly. Crowded seedlings compete for light and are prone to disease. Sow fewer seeds than you think you need.
Not enough light. The most common cause of weak plants. Maximise light the moment seedlings emerge.
Skipping hardening off. Plants moved straight outside often stall or scorch.

Troubleshooting

Seedlings suddenly collapse at soil level. This is damping off, a fungal problem encouraged by cold, wet compost, crowding and dirty pots. Remove affected seedlings, improve air flow, water less and use fresh compost and clean containers next time.

Nothing has come up. The compost may be too cold, too dry or too wet, or the seed may be old. Check the germination temperature and how long the seed normally takes; parsley, for example, can take several weeks.

Seedlings are tall and floppy. Too little light or too much warmth. Move them to brighter, cooler conditions, and when pricking out, set them a little deeper.

There is green or white fuzz on the compost. Usually moss, algae or harmless mould from staying too wet. Water less and increase ventilation; if grey fuzz appears on the seedlings themselves, see grey mould.

Tiny insects are flying around the pots. Small dark flies are often fungus gnats, which breed in damp compost. Letting the surface dry between waterings usually sorts them out. Aphids on leaves can be wiped off by hand; our aphids page has more.