The many plants known as bulbs bring beauty to the garden in almost every season. The springtime glory of crocus, daffodil, and tulip is closely followed by the summer charm of iris, lily, dahlia, and daylily; more dahlias and crocus follow in autumn. In some climates, bulbs such as crocus and snowdrop bloom even in winter.

All these plants grow from underground structures that serve as storage organs, accumulating a reserve of nutrients to supply energy for growth and bloom in the year to come. Though gardeners typically call all such structures "bulbs," botanists divide them into five types: true bulb, corm, tuber, rhizome, and tuberous root.

True bulbs

A true bulb is an underground stem base that contains an embryonic plant complete with leaves, stems, and flower buds, ready to grow when conditions are right. Surrounding this embryonic plant are scales — modified leaves that overlap each other in a scalelike manner. At the bottom of the bulb is the basal plate, which holds the scales together and produces roots. Many true bulbs, including daffodil (Narcissus) and tulip (Tulipa), have a protective outer covering called a tunic. Lily (Lilium) bulbs lack a tunic; they're susceptible to drying and must be handled with care.

Corms

Like a true bulb, a corm is a stem base — but its tissue is solid, without scales. Roots grow from a basal plate at the bottom of the corm; the growth point is at the corm's top. Gladiolus and crocus are two familiar corms.

Each corm lasts for a single year. As it shrinks away after blooming, a new corm forms on top of it; many small increases, called cormels, may also be produced around the new corm's basal plate.

An individual bulb may live for many years. New, smaller bulbs, known as increases or offsets, grow from buds on the basal plate.

Tubers

A tuber, like a corm, is a swollen underground stem base. But it has no basal plate; roots grow both from its base and its sides. Multiple growth points are scattered over its upper surface.

An individual tuber can last for many years. Some (cyclamen, for example) continually enlarge but never produce offsets. Others, such as caladium, form protuberances that can be removed and planted separately.

Rhizomes

A rhizome is actually a thickened stem growing partially or entirely below ground. Roots grow directly from its underside. The primary growing point is at one end of the rhizome; additional growing points form along its sides. Bearded iris is probably the best-known rhizomatous plant; others are calla lily (Zantedeschia) and canna.

New plants are produced from the growing points, so a planting that starts with a single rhizome can spread horizontally into the surrounding soil.

Tuberous roots

While the other bulb types are specialized stems, a tuberous root is a true root, thickened to store nutrients. Fibrous roots for uptake of water and nutrients grow from its sides and tip. Tuberous roots grow in a cluster, with the swollen tuberous portions radiating out from a central point. The growth buds are at the bases of old stems rather than on the roots themselves. Dahlia and daylily (Hemerocallis) are well-known examples of plants with tuberous roots.

If it has a growth bud, an individual tuberous root can give rise to a separate, new plant.