Sowing and aftercare in protected environments

The ideal conditions for raising plants from seed can be achieved in a protected environment such as a glasshouse or cheaper alternatives such as polythene tunnels or cold frames.

Most seeds grown in protected culture are sown into containers (see Figure 12.2):

• seed trays
• half trays
• pots (as deep as wide)
• pans (‘half pots’)
• long toms.

These must have adequate drainage to allow excess water out or for the water in the capillary matting to pass into the compost. Square shapes utilize space better, but are harder to fill properly in the corners. Although more expensive, rigid plastic is easier to manage. Rims on containers give more rigidity and make them easier to stack. Gardeners can make use of plastic food containers so long as they are given sufficient drainage holes. All containers should be clean before use (see hygiene). There are also disposable pots made of compressed organic matter, paper or ‘whalehide’ through which roots will emerge, which makes them useful for the planting out stage.

For production horticulture there is a wider range of materials, including polystyrene, for once only use; cost and presentation of the plants becomes the main consideration. Too large a container is a waste of compost and space whereas one that is too small can lead to the seedlings having to be spaced out before they are ready; if left they become overcrowded and susceptible to damping off diseases.

Seed composts are commonly equal parts peat and sand mixes with lime and a source of phosphate. Potting composts into which seedlings are transferred and young plants established tend to have a higher proportion of peat with lime and a full range of nutrients. Many advocate the use of sterilized loam which makes the compost easier to manage and increasingly alternatives to peat are being utilized.

Sowing seeds. The container is generously overfilled with seed compost. Care is taken to ensure that there are no air pockets by tapping on the bench and the corners are fully filled. With a sawing action across the container top, the surplus compost is ‘ struck off ’ using a straight edge. The compost is then lightly firmed to just below the rim of the container using an appropriately sized presser board. Seeds are then sown on the surface at the rate recommended. Many advocate that when using trays, half the seed is sown then, to achieve an even distribution, the other half is sown after turning the container through 90 degrees. The seeds are then covered with sieved compost or fine grade vermiculite to their own thickness. Finer seeds are often sown in equal parts of fine dry sand to help distribution, lightly pressed into the surface and then left uncovered. Larger or pelleted seed tends to be ‘space or station sown’ i.e. placed at recommended distances in a uniform manner.

The seeds are then labelled with name of plant and the sowing date. The compost is then watered either from above gently, with a fine rose, or by standing the container in water. A fungicide can be added to the water to protect against damping off diseases. The moist conditions around the seed must be maintained and this is most easily done by covering with a sheet of glass, clear plastic or kitchen film. The container should be kept in a warm place (approximately 20°C). If necessary, a sheet of paper can be used to shade the seeds from the direct sunlight or to minimize temperature fluctuations. There are advantages in placing the seed containers in a closed propagator. Covers on the container should be removed as soon as the seedlings appear and they must now be well lit to avoid etiolating, but not exposed to strong sunlight. Watering must be maintained, but without waterlogging the compost. In production horticulture much of the work is done by machines. Pots are rarely filled by hand and increasingly the whole process is automated, including the seed sowing.
Pricking out. When the seedlings are large enough to be handled, they should be transplanted into potting compost prepared as for the seed tray. Each seedling is eased with a dibber, lifted by the seed leaves, dropped into a hole made in the new compost, gently firmed and watered in. The seedlings are normally planted in rows with space, typically 24 to 40 per seed tray, for them to grow on to the next stage.

Many will be planted out when ready, but others will continue in pots and as they need more space they are ‘potted off’ (moved into another container). Those continuing in containers are ‘potted on’ when they outgrow their container. As they are moved to the next size container, the compost, especially the nutrient level, is chosen to suit the stage reached (see JIP).

Hardening off is required to ensure that the seedlings raised in a protected environment can be put out into the open ground without a check in growth caused by the colder conditions, wind chill and variable water supply. As the pricked off seedlings become established they are moved to a cooler situation, typically a cold frame, which starts the process of hardening off by providing a closed environment without heat. After a few weeks the cold frame is opened up a little by day and closed at night. If tender plants are threatened by cold or frosts they can be given extra protection in the form of easily handled insulation such as bubble wrap or coir matting put over the frame. Watering has to be continued and usually the plants will use up the fertilizer in the compost and need applications of liquid fertilizer. The hardening process then continues with the frame lid (‘light’) gradually being opened up more to allow air circulation day and night. Ideally the plants have been fully exposed to the outdoor conditions by the time it is ready to plant them out.

The young plants are very susceptible to fungal diseases while in the frame because of the high density of planting and the difficulty with keeping the humidity level right. The need to maintain air circulation is essential as the opportunity arises. Excessive feeding with high nitrogen fertilizers should also be avoided because it can create soft growth which makes them vulnerable to disease and excessively soft and vigorous plants can be checked on planting out.

Bedding plants (see Figure 12.1) are raised as described above with the sequence geared to producing the plants ready to plant out at the right time. For this, the time when required and the growth rates of the selected plants needs to be known. Usually the seeds are sown into seed trays or pans with very lightly firmed peat compost. Care should be taken to avoid waterlogging in shallow containers which leads to the seeds rotting off, poor seedling development, attack from sciarid fly or fungal diseases. There are methods of raising plants from seed without containers by using blocks of compressed peat. Larger seeds can be sown into rockwool modules to create ‘plug plants’.