Seed: sowing indoors

Ideal for seeds that need warmth to germinate and grow, such as tender and half-hardy flowers and vegetables, sowing indoors allows you to get plants off to an early start and give them the protection they need

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Sowing seed indoors. Credit: Tim Sandall / RHS

Quick facts

Suitable for: Annuals, bedding plants, vegetables, tender perennials
Timing: Usually spring
Difficulty: Moderate

Why sow seeds indoors

Sowing seeds indoors is easy and fun, but do bear in mind that the seedlings will need regular care for several months. You’ll also need space in a greenhouse or on a bright windowsill to keep them until they’re ready to be moved outdoors.

Reasons to sow indoors include:

  • Frost-free conditions - Some plants need warm conditions to germinate and grow, such as tender and half-hardy flowers and vegetables
  • Safeguarding - To provide additional protection, perhaps from slugs or damp weather
  • Faster, more reliable germination
  • Head start - If you want earlier crops or flowers, sowing seeds indoors gives you a head start
  • Additional time - Sowing indoors gives slow-growing plants the additional time they need to flower or crop
  • Ensures success - Particularly if you only have a few seeds, or they are rare or expensive  

You can sow a wide range of seeds indoors, including:

  • Tender crops - such as tomatoes, chillies and courgettes
  • Half-hardy annuals - such as cosmos and nasturtiums
  • Hardy annuals and veg - to give them a head start, such as sunflowers and broccoli
  • Annual climbers - such as morning glory and sweet peas
  • Perennials - such as delphiniums and echinacea, although these may not flower in their first summer
  • Tender herbs - such as basil
  • Slow-growing crops - such as celeriac

Where to sow seeds and grow on

You can sow into small pots, seed trays or modular trays, as well as recycled containers such as fruit punnets, juice cartons or yoghurt pots with holes cut in the base, as well as pots made from strips of newspaper or toilet roll middles. If you choose to re-use seed trays or pots from last year, make sure they are thoroughly cleaned out.

Seed packets will give details of whether seeds should be sown indoors or outside.

Seedlings of Sweet peas in paper tubes and a modular tray

Top tip

Large seeds can be sown individually in modules or small pots, while smaller seeds are best sown in shallow seed trays.

After sowing, place the pots or trays in a greenhouse (heated or frost-free), on a bright windowsill, in a conservatory, an enclosed porch or any other frost-free location that gets plenty of sunlight. A heated propagator can also be useful, either for seeds that need a lot of warmth or a specific temperature to germinate, and to speed up germination.
Seed packets will give details of the temperature needed for germination.

Greenhouses: choosing

Greenhouses: choosing

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RHS guide to go plastic free

When to sow seeds

Sowing times vary depending on the plant – check the seed packet for recommended sowing months.

As a general guide:

  • Sow in spring – most tender and half-hardy flowers and vegetables, for planting out in late spring and early summer
  • Sow in summer – biennial flowers, such as foxgloves, and fast -growing vegetables, such as salads
  • Sow in autumn – winter and early spring salads and vegetables, for cropping over winter indoors or planting out in spring
  • Sow in late winter – tender and half-hardy flowers and vegetables that need a long growing season, such as chillies

Top tip

If you have old packets of seeds, check their ‘use by’ date before sowing. Germination rates deteriorate over time, so you may get disappointing results from old seeds and end up having to start all over again with fresh seeds at a later date.

You can also sow fast-growing microgreens and sprouting seeds (such as mung beans, cress and fenugreek) at any time of year on your kitchen windowsill – see our guide to microgreens.

Growing cress

Growing cress

Growing microgreens

Growing microgreens

How to sow seeds

Sowing seeds and pricking out
  1. Fill a seed tray, modules or small pots with seed compost or sieved multipurpose compost, and firm down gently to just below the rim 
    Top tip

    Smaller seeds prefer a seed compost as it has a finer texture than multipurpose. It also containers fewer nutrients, which suits young seedlings.

  2. Smaller seeds should be scattered as thinly as possible on the surface, while larger seeds can be pressed into the compost individually, at the spacing recommended on the packet.
  3. Most seeds should then be covered with a fine layer of compost, vermiculite or perlite. Check the seed packet to see whether and how to cover the seeds you are sowing
  4. Water gently using a watering can fitted with a rose (sprinkler head), to avoid dislodging the seeds. Alternatively, stand the container in a tray of water to soak up moisture from below, until the compost is thoroughly damp 
  5. Label the container, so you remember what you’ve sown and when
  6. Cover the container with a clear polythene bag to maintain a humid atmosphere, or place it in a propagator with a clear lid. (Most seeds need a temperature of about 18°C (64°F) for germination, unless the seed packet states otherwise) 
  7. Water regularly, aiming to keep the compost just moist at all times
  8. Remove the lid or bag as soon as seedlings appear to increase ventilation. Germination usually takes two to three weeks
  9. Ensure the seedlings get plenty of sunlight and water regularly to ensure they grow steadily and evenly

It's worth noting that whilst most seeds can be sown direct from the packet, a few with tough seed coats may need to be pre-soaked, scraped or nicked (with sandpaper or a knife) to aid germination – seed packets will provide details. A few others need a period of cold or warmth before they will germinate – see our guide to sowing tree seeds for details

Top tip

Keep your seed packets to hand after sowing, so you can refer to them later. They contain useful information such as how long germination takes, when to transplant outside and recommended planting positions.

How to look after seedlings

Once your seedlings have at least two pairs of leaves, move them into individual pots or modules. Known as pricking out, this ensures they don’t get overcrowded and gives them space to keep growing strongly.

Top tip

Seeds sown singly in modular trays don’t need pricking out – simply move each seedling into a 9cm (3½in) pot once its roots have filled its module.

Don’t delay pricking out, as overcrowded seedlings are more prone to fungal diseases, such as damping off. Their roots become intertwined too, making it more difficult to separate them without damage. They can also become spindly and weak (leggy) as they strive upwards for more light and space.

To prick out seedlings:

  1. Fill several 9cm (3½in) pots or modular trays with multipurpose compost
  2. Use ‘dibber’ (a stick, pencil or similar) to loosen the compost around the first seedling
  3. Lift it out of the compost, holding it carefully by a sturdy leaf, not the delicate stem, while supporting the roots with the dibber. Try to keep as much compost around the roots as possible
  4. Use the dibber to make a hole for the seedling in its new pot of compost, and lower the seedling in place
  5. Firm the compost gently around the seedling and water in, taking care not to damage or dislodge the seedling

Pricking out

Top tip

If a seedling has become leggy (with a long, weak stem), bury it slightly deeper (up to the first pair of leaves) in its new pot.

Growing on your seedlings:

  • Water seedlings regularly, aiming to keep the compost slightly damp and never letting it fully dry out. Seedlings can soon die if short of water, so it’s important to keep a close eye on them
  • To encourage strong growth, feed fortnightly with a balanced liquid fertiliser. If seedlings develop pale lower leaves, they may be short of nutrients.
  • With flowering plants, pinch out the tips of long shoots to encourage branching – a bushier plant will produce more blooms. If plants grow tall and straggly (leggy), it’s usually a sign they need more light

How to plant out

By April or early May, after the last frost in your area, and once your seedlings have grown strong and sturdy, it’s time to plant them into their final positions in the garden:
  1. Harden off - a week or two before planting out, start to acclimatise them to outdoor conditions – see our guide to hardening off. This ensures they continue growing strongly and should settle in well
  2. Get your planting site ready – check the seed packet to find out whether the plants need sun or shade. Weed the area thoroughly – see our guide to weeding
  3. Water well - an hour or so before planting, water your plants so the compost is thoroughly damp
  4. Dig a hole - make it a similar size to the plant’s container, and water the base
  5. Place the plant - carefully tip the plant out of its pot and stand it in the hole. Make sure the surface of the compost is level with the surface of the soil
  6. Fill any gaps - with more soil, fill any gaps around the rootball, firm it in gently, then water generously
  7. Plant the rest - repeat with your other plants in the same way, at the spacing given on the seed packet. This will vary depending on the plant’s ultimate size
For more on caring for your annuals, see our Growing Guide.

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