Tulip Fields, Keukenhof, The Netherlands

The bulbs are typically planted around late summer and fall in well-drained soils, normally from 4 inches (10 cm) to 8 inches (20 cm) deep, depending on the type planted. In parts of the world that do not have long cool springs and early summers though, the bulbs are often planted up to 12 inches (300 mm) deep. This provides some protection from the heat of summer and tends to force the plants to regenerate one large bulb each year instead of many smaller non-blooming ones. This can extend the life of the plant in warmer areas by a few years, but it does not stave off degradation in bulb size and the eventual death of the plant.
Propagation
Tulips can be propagated through offsets, seeds or micropropagation. Offsets and tissue culture methods are means of asexual propagation for producing genetic clones of the parent plant, which maintains cultivar integrity. Seed-raised plants show greater variation, and seeds are most often used to propagate species and subspecies or to create new hybrids. Many tulip species can cross-pollinate with each other, and when wild tulip populations overlap with other species or subspecies, they often hybridize and create mixed populations. Most tulip cultivars are complex hybrids and actually sterile. Those plants that do produce seeds most often have offspring dissimilar to the parents.
Growing salable tulips from offsets requires a year or more of growth before plants are large enough to flower. Tulips grown from seeds often need five to eight years of growth before plants are flowering size. Commercial growers usually harvest the tulip bulbs in late summer and grade them into sizes; bulbs large enough to flower are sorted and sold, while smaller bulbs are sorted into sizes and replanted. Holland is the main producer of commercially sold plants, producing as many as 3 billion bulbs annually.