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.
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.
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.