Starting plants from seed is rewarding and cheap — but germination fails more often from a few avoidable mistakes than from bad seed. This guide covers the principles that apply to most seeds, and the common problems and how to avoid them. Crucially, germination temperature, timing, depth and light needs vary by species — there is no single number that works for every seed, so always check the requirements for the specific plant you are sowing.
Germination basics
A seed needs the right combination of moisture, temperature, oxygen (and, for some species, light or darkness) to break dormancy and sprout. Get those right for that plant and most viable seed will germinate; get one badly wrong and it won’t.
Seed freshness & viability
- Fresher seed generally germinates better; viability declines with age and varies a lot by species.
- Store seed cool, dark and dry to preserve it.
- If unsure, a simple damp-paper-towel test shows roughly what percentage is still viable before you sow a whole tray.
Sowing depth
A common rule of thumb is to sow at a depth of about two to three times the seed’s own width. Fine dust-like seed is barely covered or surface-sown. Some seeds need light to germinate and must be sown on the surface; others need darkness — check the species (see light vs darkness below).
Growing medium
Use a light, free-draining seed-starting mix, not heavy garden soil (which compacts and can carry disease). See soil & growing media. A fine, clean medium lets delicate roots and shoots push through.
Moisture
Keep the medium consistently moist but never waterlogged — drying out at the wrong moment kills sprouting seed, while sogginess rots it and invites damping-off. Water gently (misting or bottom-watering) to avoid disturbing seed.
Temperature
Warmth drives germination, and the ideal range is species-specific — many warm-season crops want steady warmth while cool-season plants germinate at lower temperatures, and some seeds need a cold period (stratification) first. Consistent bottom heat speeds and evens germination for warmth-lovers; see heat mats. Do not rely on one universal temperature.
Light vs darkness
- Some seeds need light to germinate — surface-sow and don’t cover them.
- Some need darkness — cover to the correct depth.
- Most are indifferent at the germination stage. Check the species; once seedlings emerge, they all need good light (below).
Humidity
A humidity dome or cover keeps moisture steady until seeds sprout. Remove or vent it once seedlings appear to prevent stagnant, damp conditions that cause damping-off.
Air circulation
Gentle airflow once seedlings emerge strengthens stems and greatly reduces fungal problems. Stagnant, humid air is the main driver of damping-off.
Containers & trays
Seed trays, cells, pots or plugs all work — the key requirements are drainage holes and cleanliness. Reused containers should be cleaned to remove disease. See propagation equipment for trays, domes and kits.
Common germination problems
- No germination: old/non-viable seed, wrong temperature, sown too deep, or dried out.
- Patchy germination: uneven moisture or temperature.
- Rot before emergence: overwatering / poor drainage.
Damping-off prevention
Damping-off is a fungal collapse of seedlings at soil level. Prevent it with clean containers and fresh sterile medium, good air circulation, not overwatering, and removing the humidity dome once seedlings are up. Once it strikes, affected seedlings rarely recover — prevention is everything.
Leggy seedlings
Tall, pale, floppy seedlings mean not enough light. Give bright light as soon as seedlings emerge; a sunny window is often not enough indoors, so many growers use grow lights for seedlings kept close above the plants.
When seedlings need light
The moment they break the surface. Move them into strong light immediately to prevent legginess — germination in the dark is fine for some seeds, but seedlings are not.
When to transplant
Pot on or plant out once seedlings have their first true leaves (the second set, after the seed leaves) and are sturdy enough to handle. Hold them by a leaf, not the fragile stem.
Hardening off (the basics)
Seedlings raised indoors must be acclimatised gradually to outdoor conditions before planting out — increasing exposure to sun, wind and cooler temperatures over about a week. Moving them straight outside shocks and can kill them.
Differences between species
Temperature, timing, depth, light needs and any pre-treatment (soaking, scarifying, stratifying) all differ by plant. Treat this guide as the framework and the specific plant’s requirements as the detail.
Related guides
- Propagation equipment · heat mats · grow lights for seedlings
- Soil & growing media
- How to propagate plants from cuttings & division — the other way to make new plants
