Bulbs and Tubers and Rhizomes, O My!

June 9, 2021

By Linda Summers, Master Gardener —

This spring boasted a profusion of flowers, such as daffodils, tulips, hyacinth, crocus and iris. They all differ in appearance, but there is a common thread that binds all of them together. That shared thread lies beneath the soil in bulbs, a generic term for true bulbs or bulb-like organs, such as corms, tubers, rhizomes and tuberous roots. Despite the differences between them, all of these share one thing in common. Bulbs are storage organs for reserves of food necessary for keeping plants alive from one growing season to the next. This article describes some of the characteristics and examples of each type of bulb.

True bulbs are underground buds or stem bases, containing an embryonic plant already complete with stems, leaves and flowers just waiting to grow when the conditions are right. Surrounding this embryonic plant are scales or overlapping, modified leaves that give the bulb somewhat of a pear shape. The basal plate at the base of the bulb holds the scales together and that’s what produces the roots. Examples of true bulbs are onions, but also narcissus (daffodils) and tulips. You’ll notice that these bulbs also are covered by a thin, papery skin called a tunic, which serves a dual purpose of protecting the bulb from injury as well as dehydration. Other bulbs, such as the lily (Lilium), do not have this tunic and more care must be taken with them in handling. They will also dry out when exposed to air for an extended period of time. True bulbs over time may produce new, smaller bulbs from their basal plate.

Corms, like true bulbs, grow from a stem base, but they are solid tissue rather than having overlapping, modified leaves. Roots grow from the basal plate and corms have a principal growth point at the top of the corm. Examples of corms are gladiolus and crocus. Some corms are also covered with a tunic, but this tunic is formed from the previous season’s leaves. Each corm lasts just one growing season, but a new corm may form on the top of the one shrinking away or around the new corm’s basal plate.

Tubers are similar to corms in that they are also swollen underground stems. A tuber does not have the structure that a true bulb or corm has. There is no tunic or basal plate and roots grow from the tuber’s base and sides, and sometimes even from the top. Instead of one or just a few growing points, the tuber has multiple growth points along its upper surface. Each growth point is actually a leaf with a growth bud in its axil (the angle between the upper side of the leaf or stem and the supporting stem). Some of the best examples of tubers are sweet potatoes but also begonias, caladium or cyclamen. With cyclamen, tubers enlarge but do not produce offsets. Caladiums form protuberances that can be removed and planted separately to grow new plants.

Rhizomes are thickened stems, growing horizontally and partially or entirely underground. Gardeners easily identify the tall bearded iris as a rhizomatous plant, but others include the calla lily and cannas. There is no basal plate or tunic and roots grow from the rhizome’s underside. The primary growing point is at one end of the rhizome, but additional growing points form along the rhizome’s sides or on the upper surface. Growth usually proceeds in a straight line and, over time, a single rhizome can produce many more plants and extend into a larger area.

Finally, the tuberous root is the only bulb type that is actually a true root. The roots are thickened to store nutrients and they grow in a cluster, surrounding a tuberous portion in the center. The growth buds lie at the top, either on the bases of old stems or on the crown where the roots come together. Well-known examples of tuberous roots are dahlias or daylilies. A tuberous root can be divided from a cluster, as long as it has a growth bud attached to its neck (daylilies) or to the base of an old stem (dahlias).

Some bulbs are hardy here in southern Illinois and stay in the same place flowering for years. Other bulbs are tender and must be removed from the garden and stored in the fall before they freeze. With careful planning, bulbs can be grown with other perennials for season-long color. For more information on bulbs or any gardening topic, please contact your local University of Illinois Extension Office or any Master Gardener.

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