The term ‘bulb’ is used loosely by homeowners to define any underground roots plants use to store nutrients. Corms, rhizomes, tubers, and tuberous roots are often mistaken for true bulbs. That they all share similarities in growing habit and maintenance makes it easy to confuse them or simply lump them all under the term ‘bulb.’ This article will differentiate between these, giving examples of common plants of each type.
So what is a bulb? A bulb is a plant structure consisting of a stem and roots which emerge from its base. Attached to the base and surrounding the growing plant are modified leaf structures known as scales. The scales do not grow or emerge as leaves; rather, they serve to store nutrients that the plant uses when breaking dormancy. Examples of bulbs are narcissus and tulip; one bulb we are very familiar with is the onion.
Corms, much like bulbs, consist of a base, from which the roots descend while the plant grows upwards from. What makes a corm different from a bulb is that it lacks fleshy scales; rather it is the base of the plant stem that has become swollen to store the nutrients. Familiar corms include those of the crocus as well as those of Gladiolus, a plant often dug up and stored indoors in areas with colder winters.
Rhizomes are stems that usually grow underground or at the surface, almost always horizontally. Roots emerge from the rhizome, as do stems. In most cases, the stem is found at the end of the rhizome, slowly growing forwards and extending the rhizome. Examples of rhizomes include German iris, ginger, and bamboo.
Tubers are underground food storage stems. Tubers are devoid of a base as well as any protective husk, but they have the ability to put forth roots and shoots. Also known as stem tubers, these storage systems usually deplete over the course of the growth season as the plant establishes itself. In order to survive as a perennial, the plant will set forth new tubers for storing nutrients before winter, when all the plant dies except the new tubers. A few plants, like the tuberous begonia, have tubers that last longer. An example of a tuber would be the common potato.
Tuberous roots, unlike the others, are actual roots instead of stems; the food supply is stored in the actual root tissue as opposed to in leaves, stems, or extensions from the stem. Tuberous roots lack the ability to produce buds; instead the buds develop from the crown of the plant. Should this part be severed, the tuberous root will not grow a plant. Daylilies and Dahlias are examples of plants that have tuberous roots.
So these are the fundamental differences between bulbs, corms, rhizomes, tubers, and tuberous roots. Thus, what is ambiguously referred to as ‘bulbs’ has a wide difference in storage, division, and planting requirements due to this diversity.