When sowing in heat is recommended, a simple window sill propagator in a sunny window should be adequate. The seedlings will need as much light as possible to prevent them growing too leggy. If you are using a heated propagator, then you should reduce the heat and grow them on at room temperature, once the seeds have germinated and also increase the ventilation. The main cause of leggy seedlings is too much heat and not enough light. The natural light levels available from January to March are really quite low. Some people augment the natural light by using special “growing lamps”, which are designed to mimic natural sunlight. Because of the extra costs involved, it is only worth it in special situations.
Remember to read the sowing instructions on the seed packet carefully!! Some seeds need to be covered with a thin layer of compost to keep out the light. Other seeds may require to be sown on the top of the compost and covered with a thin layer of vermiculite, to allow some light to reach the seed. Some other big seeds, such as squashes and melons, should be just pushed into the compost on edge, to cut down the danger of rotting.
The choice of compost is important for the easy germination and continuing rapid growth of the seedlings. The most usual choice of compost for vegetable seedlings is a peat based compost, specially formulated with added fertilizer and with the peat finely milled for seedling growth. You may be tempted on ecological grounds to go for a peat-free compost, but most independent tests still show that they are not as good as the peat based ones. The peat-free ones seem to be largely based on composted bark, and they just do not seem to have the right consistency for good seedling growth.
I would not recommend the use of sieved garden soil, as unless it has been sterilised before use, there is a big danger of garden bacteria and pests attacking your seedlings before they get started. It is possible to sterilise soil by using Jey’s Fluid according to the Manufacturer’s instructions.
Correct watering is vital during the early growth of a seedling, as they have very small root systems, and cannot cope with a drought on a sunny windowsill. They will also die if they are drowned in water. You really have to test the dampness of the compost daily with your finger.
Some seed trays, like that shown on the left, have a built-in water reservoir with a piece of capillary matting to transfer just the correct amount of moisture to the compost. You may find that the capillary matting works better if you soak it first before setting up the propagator.
The correct ventilation of the seed tray is also important to cut down on the danger of damping-off disease affecting the seedlings. Some transparent covers have adjustable ventilation openings on the top of the transparent cover, to prevent excessive humidity building up in the propagator.
Difficult to germinate seeds. Some seeds, such as beans and peas, can have very tough outer layers which prevent moisture reaching the inner layers and cause poor germination.
Hot water treatment is an effective way to overcome this problem. Pour boiling water over the seeds in a small container. Give a quick shake and pour off the hot water and then pour cold water over them. In tests done by Garden Organic, this was the most effective treatment.