The short version
Starting indoors is worth doing for anything frost tender or slow to mature. The whole technique is warmth to germinate, then light the moment the seedlings appear. Getting those two the wrong way round is the single most common reason indoor sowings fail.
What you need
Seed compost
A fine, low-nutrient mix. Ordinary potting compost is too rich and too coarse for small seed.
Trays or modules with drainage
Modules disturb roots less at potting-on. Anything with holes in the bottom works.
A bright position or a grow light
A windowsill facing the sun does for most things. North-facing rooms need a light.
Labels
Two trays of unlabelled seedlings look identical for a fortnight.
How to do it
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Fill and firm
Fill to the rim, tap the tray down, and firm lightly. Water before sowing so the seed is not washed about.
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Sow at the right depth
The usual rule is two to three times the seed's own width. Fine seed is pressed onto the surface, not buried.
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Give it warmth
Most seed germinates fastest between 18°C and 24°C (64°F and 75°F). A propagator lid or a warm room does it.
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Move to light the moment they appear
This is the step people miss. A seedling held in dim light stretches within days and never recovers the loss.
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Pot on before they crowd
Once the first true leaves are up, move each seedling into its own pot. Crowded seedlings compete and all of them lose.
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Harden off over a week
An hour outside on day one, building to a full day. Plants moved straight out check badly and lose a fortnight.
When it goes wrong
Tall, pale, floppy seedlings
- Light
Not enough light, too warm, or both. Move to the brightest spot you have and drop the temperature.
Seedlings collapse at soil level
- Damping off
A fungal problem in wet, still, warm conditions. Water from below, give them air movement, and do not sow thickly.
Nothing germinates
- Temperature
- Depth
- Age
Usually too cold or buried too deep. Old seed is the third suspect: most vegetable seed loses vigour after three years.
Frequently asked questions
How early should I start?
Count back from your last frost by the weeks the guide gives. Starting too early is worse than starting late: the plant outgrows the pot with nowhere to go.
Do I need a heated propagator?
No. A warm room germinates most things. What you do need is somewhere bright for the moment they come up.
Can I reuse compost?
Not for seed. Used compost carries the fungal spores that cause damping off, and has no nutrition left.
