Few vegetable seeds germinate as soon as they mature--most have an inert period. This is an advantage for wild plants, since it allows the seeds to be dispersed to distant locations and wait for favorable conditions. The seeds of most cultivated plants have inert periods as well, though most domestic plants have been bred for a fairly short inert period.
Dormant or Quiescent?
Technically, a mature seed that is not germinating may be dormant or quiescent. A quiescent seed will germinate as soon as conditions are appropriate (for example, when the temperature is warm or the environment is moist). Truly dormant seeds require both appropriate conditions and some kind of special treatment specific to the species. In the wild, dormant seeds from the same species do not germinate all at once. This spreads out the germination period, making it more likely that the seedlings will survive--even after disasters. It is useful for gardeners to know which seeds require special handling for successful germination.
Most vegetable seeds are quiescent. They can be planted as soon as environmental conditions are right, usually the following spring. Favorable conditions can also be supplied artificially, as in a greenhouse. Quiescent seeds should be planted within a year of maturation for the highest germination rate. In the second year, a smaller percentage of seeds will be able to germinate successfully, and in following years the viability of the seed drops dramatically.
Some dormant seeds rely on the seed coat to restrict germination. These seeds should be prepared for planting in a way that mimics the conditions the plant experienced in the wild. Many plants from dry areas have particularly tough coats, which let them survive long periods of drought. These seeds should be soaked before planting. Seeds that were dispersed by being eaten and passing through an animal's or bird's digestive system usually need scarification, a slight nick in the seed coat. Seeds that required freeze/thaw cycles or fire for germination should be prepared for planting by roughening or nicking the seed coat.
Some types of seed are dormant because the embryo is not yet developed. In nature, these seeds are usually very sensitive to light. Some species require light to germinate, and other species require the absence of light. Light sensitivity often interacts with temperature requirements.
Deep Physiological Dormancy
These seeds normally require a cold period before they can germinate. This is an obvious benefit for plants in climates with cold winters, since it delays germination until the following spring. Gardeners provide "stratification" for these seeds, a process of placing the seed in soil and then forcing cold or freezing temperatures for a period of time. The temperature and time period are varied according to species requirements.
Some seeds are dormant because of seed coats and underdeveloped embryos. These require mixed treatment--perhaps nicking the coats and making sure the seeds get light, for example.