Read the packet and check the seed
A seed packet tells you the crop and variety, sowing window, method, depth, spacing, expected germination time and often temperature or harvest information. “Sow under cover” means give seedlings shelter; it does not always mean they need heat. Match the instructions to your region and conditions rather than treating a printed month as a guarantee.
Check the use-by or packing information and store leftover seed cool, dry and labelled. Viability varies by crop and storage, so an old packet may germinate unevenly. If the seed matters, sow a small test batch before relying on it.
Compost, containers and sowing
Use clean containers with drainage holes and fresh, fine seed compost for small seeds. Peat-free mixes vary in texture and can dry on the surface while remaining moist below; check beneath the surface rather than watering by appearance alone. Larger seeds can often be sown into modules or small pots, which reduces root disturbance later.
Moisten compost before sowing, fill without hard compaction, and follow the packet depth. A useful general rule is to cover a seed to roughly its own depth, but some seeds need light and should be left uncovered; the packet takes precedence. Sow thinly or place one or two seeds per module to avoid crowding. Label crop, variety and date.
Germination: warmth, light and moisture
Different crops germinate at different temperatures. Warmth-loving crops such as tomatoes, peppers and cucumbers need more warmth than hardy brassicas or many salad crops. A propagator can help warmth-demanding seed, but an unheated windowsill may be enough for some crops. Once shoots emerge, remove covers and move them into bright light; warm, dim conditions produce weak, stretched growth.
Keep compost evenly moist, not saturated. A fine rose or careful bottom watering helps avoid washing small seed away. Ventilate a covered tray and remove excess condensation. Seeds can rot in cold, waterlogged compost, while tiny seedlings can dry rapidly in small cells.
After germination: thin, prick out and pot on
Thin crowded seedlings by snipping extras at compost level when they are large enough to handle; pulling can disturb the plant you keep. Prick out seedlings once they have seed leaves and are sturdy enough to handle, often as the first true leaves begin to appear. Lift gently with a dibber or label and hold a leaf rather than pinching the fragile stem. Set each seedling into a small pot or module at a suitable depth and firm lightly.
Pot on when roots fill the container or growth slows, before plants become pot-bound. Move into a slightly larger pot and a suitable peat-free multipurpose compost. Avoid feeding very young seedlings heavily: fresh compost may already contain nutrients, and tender roots can be damaged by excess fertiliser.
Harden off and transplant
Indoor-raised seedlings need gradual exposure to outdoor light, wind and cooler temperatures. Over roughly 7–14 days, place them in a sheltered spot for increasing periods by day and bring tender plants back under cover at night. Reduce protection gradually; delay the process during cold, windy or very wet spells. Tender crops still need frost protection after hardening off.
Transplant into moist, workable soil at the spacing on the packet or crop guide. Water in, label the row, and protect vulnerable seedlings from slugs, birds or other local pests with suitable barriers. Water the root zone while plants establish.
Troubleshooting and common mistakes
- Leggy seedlings: usually too little light or too much warmth after emergence. Move to brighter conditions and avoid overcrowding.
- Damping off: seedlings collapse at soil level in damp, stagnant conditions. Use clean containers, fresh compost, sensible watering and ventilation; discard affected seedlings and compost safely.
- Poor germination: check seed age, sowing depth, compost temperature and moisture, and whether the crop needs light or darkness. Do not keep a tray saturated while waiting.
- Uneven seedlings: sow more thinly, keep containers at an even temperature and rotate them if light comes from one side.
- Yellow or stalled seedlings: check drainage, root crowding, light and watering before assuming they need fertiliser.
- Common mistakes include sowing too early for available light, burying tiny seed too deeply, leaving covers on after emergence, and planting out tender seedlings before local frost risk has passed.