How do I sow seeds indoors?

Germination and growth need different conditions. Seeds want warmth and moisture and do not need light; seedlings need very bright light immediately and less warmth. Most indoor sowing fails because the seedlings stay where they germinated.

What you'll need

• Seed compost
• Trays or pots with drainage
• A bright windowsill
• Propagator lid or clear bag (optional)
• Labels (optional)

Step by step

1. Use seed compost, not garden soil or general purpose.Seed compost is low in nutrients and fine in texture, which is what small seeds need. Rich compost burns seedlings and garden soil carries disease.
2. Fill and firm the tray gently, then water before sowing.Watering after sowing washes small seeds out of place. Moisten the compost first and let it drain.
3. Sow at about twice the seed's own depth.Small seeds barely covered, large ones deeper. Some — lettuce and snapdragons among them — need light and should sit on the surface.
4. Cover with a propagator lid or a clear bag.It holds humidity during germination. Remove it as soon as the seedlings appear, or they damp off.
5. Keep them at 18-22°C to germinate.Warmth matters more than light at this stage. An airing cupboard or a heat mat works well, but check daily.
6. Move to the brightest place you have the moment they emerge.This is the critical step. Seedlings left in warm dim conditions stretch and become weak within two or three days, and never recover.
7. Turn the tray daily if on a windowsill.They lean towards the light. Turning keeps them straight, and a piece of white card behind reflects light back.
8. Prick out into individual pots at the first true leaves.The first pair are seed leaves; the next pair are true leaves. Handle by a leaf, never the stem — a crushed stem is fatal.

Tips

• Sow fewer than you think. Twenty tomato seedlings is a great deal of tomato plants, and thinning is easier than finding homes for them.
• Label everything at the moment of sowing. Seedlings look almost identical and memory is unreliable three weeks later.
• Water from below by standing trays in shallow water for a few minutes. It avoids disturbing seedlings and reduces damping-off.

Common mistakes

• Leaving seedlings in a warm dark place after germinating — They stretch towards any light and become spindly within days. Weak seedlings never recover into strong plants.
• Sowing too early — Plants that outgrow their pots before it is warm enough to plant out become stressed and check badly. Later sowings often catch up and overtake.
• Overwatering seed trays — Damping-off fungus thrives in saturated compost and collapses whole trays of seedlings overnight.

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