Seed Sowing and Seedling Raising

Choosing a compost

When raising seeds of flower and vegetable plants indoors always use trays or pans of good seed compost. Never be tempted to go out into the garden and scoop up ordinary soil for seed raising. Unless your garden soil is exceptional, its physical structure will be poor compared with seed compost and not conducive to the successful raising of fine-seeded varieties. It is also likely to be host to all manner of pathogens which will cause problems later on.

Unlike potting composts, seed composts have few nutrients in them. The lack of nutrients ensures that there is unlikely to be any fertilizer damage to tender seedlings and that the growth of moss and liverworts is impaired. Soil-based seed composts are best for those plants which are going to take several weeks to germinate, while rapidly emerging vegetable and bedding plant seeds are best sown in a soil-less organic mixture.

Seed sowing - indoors

It is a general rule when seed sowing that seeds are covered by about their own depth of compost. Some seeds like sweet peas and tomatoes which are large and easily handled are sown individually in small pots (sweet peas) or spaced out in a seed tray (tomatoes). Others are broadcast thinly and then covered with compost. Fine seeds can be very difficult to handle and so add a little dry sand to the packet, give it a gentle shake to re-distribute the seeds, and then sow the sand along with the seeds. This not only assists in the distribution of the seeds, but shows the area over which they have been sown.

Pricking out 

Once the seedlings have germinated they can be pricked out. They should ideally have their seed leaves fully expanded and the first true leaf showing before transplanting. Care must be taken with handling as they are very delicate and often brittle. Never hold a seedling by its root or stem, always by the tip of the seed leaf. Individual seedlings are lifted from the clusters in which they germinated and spaced out in a pan or seed tray.

It is usual to prick out seedlings slightly lower in the compost that in the pan or tray in which they germinated, in most cases burying the stem up to the level of the seed leaves. This only applies if the seedlings are short, strong and healthy. Burying a lanky seedling will result in its collapse. It is usual to prick out seedlings into a potting compost.

Hardening off

The most critical time in the raising of half-hardy seedlings is the weaning period. This is the time when the young plants are eased out of their comfortable greenhouse or kitchen window ledge atmosphere prior to planting in the garden. A cold frame is invaluable for weaning, for in cold weather the frame top can remain in place, whereas if the weather turns warmer it can be removed entirely. What must be achieved is a gradual tolerance of lower temperatures over a period of two or three weeks without giving the plants a check.

Seed sowing - outdoors

Preparation of soil that has been weathered should be left until just prior to sowing. If a tilth is prepared too much in advance rain may compact the surface and spoil its structure. However, a successful seed bed outdoors does depend upon the soil being firmed before sowing and this is best achieved by shuffling across the bed with the feet after the soil has been knocked roughly level. A shallow tilth is then created with a rake.

Annual flower seeds which are going to be part of a bed or border arrangement are usually broadcast and covered with soil, the seedlings being thinned afterwards. To distinguish emerging seedlings from weeds, sow in straight narrow drills, the plants eventually merge together. Vegetables are usually sown in flat bottomed drills (peas and beans) or V shaped drills (carrots, lettuce etc.) and these are taken out by a swan neck hoe.

Freezing seeds

Some seeds require freezing in order to germinate freely. If the conditions for germination are not amenable when a seed matures an embryo inhibitor acts to render the seed dormant until winter has passed. This applies to many alpine plants. To create an artificial winter, sow the seeds in the normal way and place in their pans in a deep freeze for a week or ten days, subsequently removing them to a warm light environment where germination will quickly follow.