Starting seeds off indoors

NOTE TO SELF: Think before you sow. Is it essential to sow indoors or would a later sowing or a later outdoor sowing be just as good?

Starting seeds off indoors is more labour intensive and needs regular time spent checking and observing for issues. Seedlings require potting on and usually hardening off before planting out. It requires enough space for all these stages. Do I have it?

Why start off indoors?

  • UK climate is notoriously variable even within the country; last frost dates can vary significantly. This means that for some more tender vegetables and flowers the growing season isn’t long enough.
  • Temperature requirements for germination are not high enough to get them started for summer fruiting or flowering if we sow outdoors. Some seeds require specific germination temperatures but can then have lower growing-on temperatures.
  • You may have bought a few expensive seeds and need to nurture them more closely.
  • You may not know what the seedling leaf looks like and have a better chance of recognising in a controlled environment rather than broadcast.
  • Easier to thin seeds that are in smaller pots. Or if you sow similar germination requirements and times together in trays. 
  • Protection from pests. Peas and bean seeds can be lost to hungry mice if sown outdoors, protect until established by sowing indoors and then planting out.


  • Check instructions for sowing times and germination requirements.
  • Clean seed trays and pots. 
  • Use fresh, clean, seed sowing compost, preferably peat free. 
  • To improve drainage mix compost with vermiculite or perlite if needed.
  • To reduce fungus gnats/sciarid flies a suggestion from Kew Gardens quarantine section on BBC Gardeners question time is to freeze the compost before you start. (Make sure it is back to temperature before you sow your seeds).
  • Fill the seed tray or pot and moisten from bottom, by standing in water and allow the compost to drain well.
  • Sow seed on surface of compost. If light is needed for germination do not cover. Otherwise cover with fine compost or vermiculite to a similar depth to the size of the seed.
  • Cover with glass plate, plastic bag or propagator dome to keep the seed moist for germination.


  • Plants that need a long season to grow and fruit such as peppers and tomatoes
  • Tender flowers that you want to flower earlier in the first year, for example begonias and petunias.
  • Early crops of brassicas, beans and runner and climbing french beans

Note: Root vegetables are best sown outside where they are to grow as their taproots do not like being disturbed.


Your seeds need warmth and moisture to germinate so check your seeds requirements. If you have a heated propagator steady temperatures should be less of an issue. Warm but do the seeds need light? Propagators, either controlled and uncontrolled temperatures available. Lids designed to keep moisture in for germination and to vent once seedlings start to emerge.

Window sill is often a good place to germinate seeds but has pros and cons. Pros - often by radiators so warm, can gain warmth if in the sun. close by, and has light available. Cons - night time temperatures may be too low. Window ledges re a good way of giving seedlings light, however if the window ledge is south facing there can be problems on sunny days with the seedlings  getting too hot.

Seedlings are phototropic - turning toward the light, You may need to turn your window ledge seedlings once or twice a day to help reduce legginess. Legginess gives an elongated stem which is more fragile and lacks the strength to support growth. Alternatively grow lights can be positioned at a close distance over the seedlings which helps prevent etiolation The young seedling is likely to need support than one that has grown more slowly. 

Boilers can be good for bottom heat. Airing cupboards can be a good source of steady heat for germination for those seeds that don’t require light to germinate. Move seeds to light area as soon as they start to germinate, even if they haven’t all come through yet.

What ifs?

Regular observation for moisture and pests is important. 

A few years ago I had to look after my mother for a week and came home to an infestation of aphids on chitting potatoes which had also spread to some seedlings and I needed to re-sow from scratch.

Aphids - even with good cleaning can be harboured in window cracks and curtains and blinds and neighbouring plants and pots.. Be observant and remove as soon as any are seen otherwise they will multiply very quickly. Washing the plants can help to clean them off the fragile leaves and growing tips. Also be aware that at certain times in their lifecycle aphids drop off the plant and can be harboured in the soil/compost.

Damping off - Caused by compost that is too moist which allows fungal growth. 

Fungus gnats/Sciarid flies which are more of a nuisance to us but can cause issues with larvae eating small seedling roots

Watering - mostly advised from the bottom. Allow to drain so that the roots of the emerging seedlings are not waterlogged.

The great thing about nurturing from seed is the success!

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