Like most of the other gardening arts, raising plants from seed is a matter of giving the plant what it needs to grow. Seeds need a certain environment to germinate, and seedlings need slightly different conditions to grow into strong transplants. If you provide the right conditions, the rest happens automatically.
Temperature, moisture, and oxygen supply are the three most important influences on germination. In some cases, light constitutes a fourth factor. Most vegetable seeds will tolerate quite a bit of variation in these factors; however, as extremes are approached (above or below optimum), the germination rate slows, increasing the number of abnormal seedlings and reducing total germination.
Temperature affects vegetable seed germination both directly and indirectly. In soil that is too cold, seeds germinate slowly or not at all. In soil that is too hot, they die quickly.
Most seeds that require cool soil to germinate should be sown directly into the garden in early spring. Seeds that must have warm soil should not be sown outdoors until the danger of frost has passed and the soil temperature is between 50 and 60 degrees. When starting these seeds indoors, bottom heat may be necessary to warm the growing medium. Even though the germinating room may be the correct temperature, the medium will be cooler because of evaporation. Bottom heat can be achieved by setting seed flats on a warm surface, such as the top of a refrigerator, water heater or television set, or by using soil heating cables.
The food stored in a seed is in a very concentrated, complex form. Before it can be used, a series of chemical reactions must take place, and for this to occur, moisture must be available. Water serves two functions: it triggers the necessary series of reactions within the seed and it softens and weakens the seed coat, thus permitting the growing embryo to break through.
Since a seed is in a state of suspended animation, its energy requirements are low. As its growth begins, triggered by the presence of moisture, oxygen combines with the stored food to produce energy.
Some seeds, particularly squash, pumpkin, cucumber, gourds, and related plants, are more sensitive to low oxygen than others. On the other hand, celery seed can germinate even if it is completely immersed in water.
Seedlings need strong light to grow into stocky, healthy plants. Indoors, they require a sunny window or fluorescent lights. The warmer the location, the brighter the light need be. The best situation for raising most vegetable seedlings is a slightly cool temperature (below 75 degrees) and full sun (or bright fluorescent light) for at least six hours a day.