Sowing Seeds Indoors
Many of the plants that fill our gardens through the summer, including outdoor varieties of tender vegetables and a few herbs, are not frost hardy and require a longer growing season than would be possible if their seeds were not sown until conditions outside were suitable for germination. For this reason their seeds need to be sown indoors in a greenhouse or conservatory, on a windowsill or, in some cases, in a cold frame. True indoor and greenhouse plants are, of course, also sown 'under glass' as are some hardy annuals, perennials and vegetables where an early start is desirable.
As young seedlings are very fragile and, in the first days after germination, vulnerable to disease, cleanliness is all important. All pots, trays, propagators, labels etc., if not new, should be well scrubbed, using a garden disinfectant, and then thoroughly rinsed.
Compost and containers - For almost all seeds not sown directly in the garden a moist peat-based or other soilless seed sowing compost is recommended. General purpose composts can also be used although it may be necessary to sieve out larger lumps. John Innes seed compost is preferred by some, is best for a small number of seeds which take a long time to germinate and has the advantage of being sterilised. Compost brought in from outside on a cold day should be given time to warm up before being used.
Most seeds are sown in seed trays or small plastic pots or half pots although other clean plastic containers can be used as long as they are provided with drainage holes. It is better not to use larger sized normal pots as the seedlings are not in them long enough to need the extra depth. For sowing larger seeds a variety of plug trays, cell inserts which fit inside seed trays and extra deep cells called rootrainers are now available.
Cubes of a fibrous material known as stone wool and plugs of peat/coir mix called Jiffy7s can also be used. All of these allow seedlings to be later transplanted without root disturbance and are ideal for seedlings, such as sweet peas and runner beans, which quickly develop deep root systems.
Fill the tray or pot you are going to use to the rim and sweep off any surplus with your hand or a suitable implement such as a large label. Then tap it sharply on the bench or table to settle the compost and give a final gentle firming with the bottom of another tray or pot. This should leave a large enough gap at the top of the tray or pot that it will not be filled completely to the rim even after any covering of the seed has been carried out. It is important not to over-firm, though, as compost which is too compacted may not drain sufficiently and may make it more difficult for seedling roots to penetrate.
Sowing - After filling, trays and pots should be watered well. The best way to do this is from the bottom so stand them in about an inch of tap water and leave them until the surface is wet. Then remove them and allow to drain for at least half an hour. Now sow the seeds thinly and evenly over the surface. Larger seeds can be sown one per cell in plug trays, cell inserts etc., or in individual small pots. If they are not sown thinly the resulting seedlings will be overcrowded and will grow poorly due to being in competition with each other. They will also be more difficult to transplant and more prone to disease problems.
Large seeds can easily be positioned with the fingers but small ones are more of a challenge and various seed sowing devices are now on the market to assist with this task. In the absence of such a device, the end of a damp matchstick can be used to pick up and position individual seeds. Alternatively, a simple but very effective aid to scattering small seeds in a controlled fashion can be made by folding a stiff sheet of paper to form a groove. The seeds are tipped into this groove and the paper is then inclined and tapped gently and repeatedly to dispense the seed. Even when using such a sowing aid, it is easier to sow and see where you have sown very small seeds if they are first mixed with a carrier such as fine dry sand. As soon as the seeds have been distributed on the compost, very fine ones should be pressed gently into the surface but otherwise left exposed. Seeds which require light should be treated similarly or (larger ones) can be covered with a thin layer of seed sowing grade vermiculite which lets light through.
Care After Sowing - Once they have been sown examine your seeds at least once every day. In many cases further watering will not be required before germination but if the surface of the compost shows any sign of drying out, and particularly if the seeds are sown at or near the surface, water from below using luke-warm tap water. Stand the pot or tray in about an inch of water until the surface starts to glisten. Large seeds can be watered using a fine rose or mist sprayer.
As soon as the first signs of germination are seen, remove any paper cover and, if the tray or pot concerned is in a dark place, transfer it to a location where it is in full light but still equally warm and not in direct sun. Shortly afterwards, when germination is well under way, remove the glass or plastic cover so that air can circulate around the seedlings. It should be remembered that not all types of seeds germinate at the same rate and that, when several different varieties are sown, not all of the seedlings will start to appear at the same time. When most seeds have germinated temperatures are best reduced somewhat by moving the trays or pots to a slightly cooler location or turning down the propagator. Trays or pots on a windowsill should be turned daily to compensate for the seedlings' inclination to grow towards the light.
Be sure to keep the developing seedlings moist at all times but wait until the compost surface starts to dry slightly before carefully watering again so as to avoid over-watering. No feeding is needed at this stage.
Growing on - As soon as seedlings not already in cells or plug trays are large enough to handle they should be transplanted to other seedtrays, cells or small pots to give them more space to grow. This is generally referred to as 'pricking out'. The fresh trays, cells or small pots should be filled, as before, using general purpose compost and a pot/cell size or spacing should be chosen that is appropriate to the vigour of the seedlings concerned. The right time for pricking out is usually when seedlings have developed their first set of true leaves but some plants, such as petunias, which have very small seedlings may need to develop more than one set before they are large enough to manage. Seedlings should never be left beyond the point at which they can first be handled, however, as their rapidly growing rootsystems very quickly become difficult to disentangle and relocate without major damage.
Lift the seedlings carefully from their tray or pot using a suitable implement such as a pencil or an old fork from the kitchen. If, despite sowing thinly, they are too crowded to be lifted singly, lift them in clumps and then gently tease them apart with as little damage to the roots as possible. A few, such as lobelia, can be left in small clumps. Always hold them by their leaves, never by their stems as, if you crush their stems, they will die.
Drop the roots of each seedling into a prepared hole in the fresh compost, firming it in gently afterwards. In trays, space the holes about 3cm or a little over an inch apart. To ensure it is well enough held by the compost that it doesn't flop over when watered, it is normally best to plant each seedling a little deeper than it was previously. This is particularly true of seedlings, such as tomatoes, which tend to have a long stem below the first leaves (the seed leaves or 'cotyledons') but in all cases the lowest leaves should still be above the compost surface.
For growing on, return the pricked out seedlings to a position where the temperature and light are similar to previously. Keep moist but, as fresh general purpose composts contain a certain amount of fertiliser, no feeding should be necessary until they are well established. After that, for optimum growth they will need regular feeding with a balanced liquid fertiliser at recommended rates. If they become excessively large or crowded before they can be planted out don't be afraid to 'pot them on' again into larger cells or pots. Most seedlings developing from seeds sown individually in plug trays or cells will also need potting on at some stage when they outgrow their accommodation.