What is a bulb ?
A bulb is an underground vegetal organ made up of a bud surrounded by close-knit fleshy leaves. These leaves are filled with nutritive ressources which allow the plant to regenerate its aerial parts every year.
To be more exact, the bulb is made up of a short, thick stem and scale-like leaves which protect the embryonic roots of the flower. These leaves which are situated all around the bud may be tight-knit (amaryllis, tulips) or loose (fritillaries, lilies). They contain all the nutritive ressources needed by the plant.
Their appearance and means of reproduction vary according to the types of bulbs.
The dormant period of bulbs
Before every period of activity preceeding flowering, the bulb is in hibernation, resting. Its stops or slows down all vital activity and shows a greater resistance to its surrounding conditions. This may be provoked by a drop in temperature and by a decrease in the time it is exposed to sunlight, and also by internal factors of the bulb. The interruption or "lifting" of this sleeping period is brought about by the change in temperature, in light conditions, by chemical substances, etc....
Planting bulbs in Spring or Summer
Bulbs which flower in Spring like the crocus, the tulip, the jacintha, and the narcissus, are planted between September and November, or even in December. Other bulbs like the fritillaires, begonias, lilies, cannas and gladiolii are planted after the last frosts, in Spring. These bulbs are planted in sunny conditions, with a few exceptions ( like the renunculus and the tuberous begonia).
Generally speaking, bulbs are planted at a depth of twice their height, whether in Spring or in Summer.