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Indoor seed sowing

Sowing seeds indoors allows you to start off tender plants, especially half-hardy annuals, with the protection they need from cold outdoor conditions. You can also sow seeds of hardy plants indoors, as this will produce plants sooner than sowing outside, and so extends the sowing/growing season.

Most plants are started off indoors in late winter or spring, for putting outside in late spring or early summer when the risk of frost has passed.

Although it is possible to sow and grow on the young plants on a warm windowsill or in a warm room, you will get better results by using a heated propagator or a heated greenhouse or conservatory.

A propagator produces an even temperature, which is important since seeds and young seedlings don’t like widely fluctuating temperatures.

Seed sowing

Small seeds are usually sown in small pots or seed trays and pricked out (transplanted) individually when large enough. Seeds can be sown into cell trays, which limits transplant shock when planting out and is especially useful for plants that dislike root disturbance. Large seeds can be sown individually into 7.5-9cm pots. When using cell trays and pots it is usually a good idea to sow two seeds and then remove the weakest seedling if both seeds germinate.

Sowing techniques

Some seeds need particular sowing treatments, such as light exclusion, so check for specific instructions first.

Small seeds are scatter-sown thinly over the surface of the compost. Very small seeds can be mixed with fine, dry sand before sowing to obtain an even distribution. After scattering, sift a thin layer of compost or fine vermiculite over the seeds.

Larger seeds can be station-sown, pressing each seed individually into the surface of the compost 13-25mm apart. iIf using a cell tray, sow one seed per cell or two seeds and later discard the weaker one.

Step by step

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  • For best results, the compost surface should be smooth, flat and level. Sieve very coarse compost to remove large lumps, which will affect the overall structure of the compost and reduce germination.
  • Fill the container with compost until it is above the rim, tap the container or tap it on the work surface to help settle the compost. Gently firm the compost with your fingertips to help eliminate air pockets, but don’t overfirm it. Add more compost if necessary, and tap again to settle it.
  • Level off the excess compost to the rim of the container with your hand, compost tamper or container of the same size.
Indoor seed sowing | Copyright GreenPlantSwap Ltd
  • Sow half the seeds as evenly as possible all the way across the compost surface; keep your hand low to prevent the seed bouncing.
  • Turn the container through 90 degrees and sow the remaining seeds evenly.
Indoor seed sowing | Copyright GreenPlantSwap Ltd
  • Cover the seeds to no more than half their own depth with sieved compost. Keep the sieve low over the container.
  • Label the container with details of the plant and sowing date.
Indoor seed sowing | Copyright GreenPlantSwap Ltd
  • Water the seeds with tepid water. Either water overhead with a watering can fitted with a fine rose, keeping the watering can close to the container, or stand the container for a few minutes in a bath of shallow water.
  • Cover the container with a sheet of glass or Clingfilm, which can be left in place until the seeds germinate, or place in a propagator.
  • Stand the container in a warm place or set the thermostat on the propagator, depending on the germination temperature needs of the particular plant; a temperature of around 18-21C suits most plants.
  • As long as viable seeds are used and the correct conditions are provided, germination can take place in as little as five to seven days.
  • Even temperatures are important for good germination and growing on. If the temperature fluctuates widely then the germination will be erratic and seedling growth affected. The easiest way to ensure an even temperature is to use a thermostatically controlled propagator.
  • The higher the compost temperature the quicker the germination occurs, providing it does not reach critical temperatures. Forcing germination in very high temperatures can lead to weak, leggy seedlings, especially if light levels are too low. As there is a correlation between temperature and light levels for strong growth, in these situations it is better to reduce the germination or growing on temperature.
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