Propagation: Sowing Seeds
By Alex N •
Many different plants can be raised from seed, sown in containers in a greenhouse: summer bedding plants, flowering pot plants, and vegetables like tomatoes, lettuce, celery, cucumbers and marrows. But the techniques of sowing and subsequent care are very similar.
Early spring is the main period for most sowings; more precise timing is usually given on the seed packets. A heated greenhouse, or a propagator, is necessary for germination (that is, starting seeds into life).
Seed trays and pots
Seed trays, approximately 5-6cm (2 in) deep, are available from all good garden centres and DIY stores. Most are plastic, which last for many years, if well looked after, and are easy to clean. Hygienic conditions are important if you are to raise healthy seedlings, so always clean your seed trays thoroughly before use.
For very small quantities of seed use plastic pots 9 or 13cm (3 or 5 in) in diameter. These are also recommended for very large individual seeds, such as marrows and cucumbers. Again, wash all pots carefully before use.
Types of compost
Ordinary garden soil is not the best medium in which to grow seedlings as it is full of weed seeds and harmful organisms, and it may not provide the correct conditions required by the seed for successful germination. Instead, buy one of the ready-mixed seed-sowing composts, the most popular being John Innes Seed Compost, which contain a mix of loam, peat, sand, superphosphate and ground chalk.
Alternatively there are many brands of seed compost which consist only of peat with added fertilizers; these are known as ‘soilless’ composts because they do not contain loam. When using soilless compost you have to be especially careful with watering, because if it dries out, it can be difficult to moisten again; Conversely over-watering may saturate it and cause the seeds to rot. With a little care, however, soilless compost gives excellent results.
Building in drainage
Pearlite or sharp sand can be added to improve drainage
Be sure that surplus water is able to drain from all containers. When using John Innes’ composts it is essential to place a layer of crocks (broken clay flower pots or stones) at least 15mm (3/4 in) deep in the bottom of your pots. If you use seed trays, crocks are not needed.
Soilless compost can be used without any crocking - unless it is going in clay flower pots, in which case you must cover the large hole at the bottom with crocks.
Once you have added the drainage material add the compost to about 15mm (3/4 in) below the top of the tray or pot, lo allow room for watering. Firm it gently all over with your fingertips, paying particular attention to the sides, ends and corners of seed trays. Make sure that the surface is level by pressing gently with a flat piece of wood that just fits into the tray or pot. Soilless compost should not be pressed hard but merely shaken down by tapping the container on a hard surface or lightly firming with the wood.
Very tiny seeds (like lobelia and hegonia) should be sown on a fine surface. So before pressing down, sieve a layer of compost over the surface using a very small-mesh sieve. Alternatively you can sprinkle a thin layer of silver sand over the compost before sowing. Do not use builder’s sand as this contains materials toxic to plants.
Water the compost lightly, using a fine rose on the watering can, before you sow.
sowing the seeds
Seeds must be sown thinly and evenly otherwise the seedlings will be overcrowded and you will find it difficult to separate them during pricking out (transplanting). They will also have thin, weak stems and be prone to diseases like ‘damping off’.
Small seed is usually sown broadcast (scattered) over the surface of the compost. Take a small quantity of seed in the palm of one hand - just sufficient to sow a tray or pot. Hold your hand about 30cm (12 in) above the container and move it ‘to and fro’ over the surface, at the same time tapping it with the other hand to release the seeds slowly. If you move your hand first backwards and forwards and then side to side this will help to spread the seeds evenly. You may find it easier to hold the seeds in a piece of paper, instead of in your hand.
It is difficult to sow very small seeds evenly, some being as fine as dust, but if you mix them with soft, dry, silver sand (using 1 part seeds to 1 part sand) this helps to bulk them up and makes them easier to see and handle.
Large seeds, which are easily handled, can be ‘space-sown’ - that is placed individually, and at regular intervals, on the surface of the compost. Tomato seed, for instance, can be treated in this way.
Very large seeds, such as cucumbers, peas and various beans, are best sown at two per 9cm (3 in) pot. If you use peat pots, they can later be planted, complete with young plant, into the final pot or open ground. When they have germinated, remove the weaker seedling, leaving the stronger one to grow on.
Pelleted seeds
This term describes seeds that are individually covered with a layer of clay which is often mixed with some plant foods. They are easily handled and can be space-sown in boxes or pots. The compost around pelleted seeds must remain moist as it is moisture which breaks down the coating and allows the seeds to germinate.
After sowing
Seeds should be covered with a layer of compost equal to the diameter of the seed. It is best to sieve compost over them, using a fine-mesh sieve. However, do not cover very small or dust-like seeds with compost as they will probably fail to germinate.
If you use John Innes or another loam-containing compost the seeds should then be watered, either using a very fine rose on the watering can or by standing the containers in a tray of water until the surface becomes moist. (This latter method is not advisable for loam-less composts as they tend to float; moisten them well before sowing the seed.) Allow the containers to drain before placing them in the greenhouse.
A good, or even better, alternative to plain water is a solution of Cheshunt Compound, made up according to the directions on the tin. This is a fungicide which prevents diseases such as damping off attacking seedlings.
Aids to germination
Place the pots or trays either on a bench in a warm greenhouse or in an electrically- heated propagator. Most seeds need a temperature of 15°-18°C (60-65°F) for good germination. The containers can be covered with a sheet of glass that, in turn, is covered with brown paper to prevent the sun’s warmth drying out the compost. Turn the glass over each day to prevent excess condensation building up on the inside. Water the compost whenever its surface starts to become dry. As soon as germination commences remove the covering of glass and paper, for the seedlings then require as much light as possible if they are to grow into strong, healthy plants.
Pricking out
Once the seedlings are large enough to handle easily prick them out into trays or boxes to give them enough room to grow. Generally, standard-size plastic or wooden seed trays are used that are 6cm (2^ in) deep; there is no need to put drainage material in the base. The trays are filled with compost in the way described for seed-sowing, again leaving space for watering. A suitable compost would be John Innes Potting Compost No. 1 which can be bought ready-mixed. It consists of loam, peat, coarse sand, John Innes base fertilizer and ground chalk. Alternatively, use one of the soilless potting composts that contains peat, or peat and sand, plus fertilizers. Make sure the compost is moist before you start pricking out.
You will need a dibber for this job - either a pencil or a piece of wood of similar shape. With this lift a few seedlings at a time from the box or pot, taking care not to damage the roots. Handle the seedlings by the seed leaves - the first pair of leaves formed. Never hold them by the stems which are easily damaged at this stage.
Spacing out
The number of seedlings per standard- size box will vary slightly according to their vigour. Generally 40 per box is a good spacing (5 rows of 8). For less vigorous plants you could increase this to 54 per box (6 rows of 9).
Mark out the position of the seedlings with the dibber before commencing, ensuring equal spacing each way. Next make a hole, with the dibber, which should be deep enough to allow the roots to drop straight down. Place the seedling in the hole so that the seed leaves are at soil level, and then firm it in by pressing the soil gently against it with the dibber.
If only a few seeds have been sown in pots each seedling could be pricked out into an individual 7cm (3 in) pot. But if you have single seedlings, such as marrows, already started in 9cm (3| in) pots, these will not need to be moved.
After pricking out, water in the seedlings (with a fine rose on the watering can) preferably using Cheshunt Compound. Then place them on the greenhouse bench or on a shelf near to the glass, as maximum light is essential. Continue to water whenever the soil surface appears dry.
Windowsill propagation
If you do not have a greenhouse, heated frame, or propagator, you can still raise seedlings in the house. Ideally the germination conditions should be as similar as possible to those which are recommended for greenhouse cultivation. Windowsills are the best places for raising seeds, and if they are wide ones you can use standard-size seed trays.
However it is usually possible to fit a few pots onto the narrowest of windowsills. For best results use trays or pots that are fitted with propagator tops. The temperature on the sill must not drop below the average room temperature and south- or west-facing sills are obviously best.
Make sure the seedlings are never deprived of daylight or allowed to get cold at night. Never draw the curtains across between the plants and the warm room air on cold nights, if necessary bring them into the room. Finally, to maintain strong and even growth, turn all pots and trays around every day.
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