Understanding Bulbs, Corms, Rhizomes and Tubers
Paul James explains the difference between the plants that are sometimes collectively called "bulbs."
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Bulbs are among the easiest plants to grow, and spring-flowering bulbs in particular, such as daffodils and tulips, provide weeks of color at a time when few other plants are in flower, all with little to no fuss.
Botanically speaking, bulbs are geophytes, which are herbaceous plants with underground storage organs. Geophytes don't just include true bulbs, but also those that we collectively refer to as bulbs, which are in fact corms, rhizomes and tubers.
A true bulb, such as an onion, consists of fleshy layers of leaves that store food for the
developing plant. The roots at the bulb's base anchor the plant in the soil and absorb nutrients. The central tip at the top of the bulb is the bud from which leaves eventually emerge. Other examples of true bulbs include daffodils, tulips, lilies and garlic.
Corms, such as gladiolus, contain a solid mass of stem tissue, rather than concentric rings of leaves. Crocosmia, crocus, freesia and bananas are corms.
The fleshy portion at the roots of a canna is called a rhizome, which is a general term for a stem that grows horizontally. Some of the best known rhizomes are ginger, bamboo and many irises.
Then there are the tubers, the most well-known of which is the potato. A potato is technically a stem tuber, meaning that it's actually a swollen stem, or more correctly, the swollen tip of a rhizome.
Shoots develop from the buds or eyes on the potato, and much like a rhizome, you can cut the tuber into pieces. Each piece will develop into a mature plant so long as the cut piece has at least one eye. You can do likewise with dahlias; they are examples of root tubers, which develop from the root rather than the stem.
All geophytes - whether true bulbs, corms, rhizomes or tubers - have one thing in common: They all require a dormant period. Depending on the geophyte, that period will be either summer or winter.
Understanding the dormant period of bulbs and other geophytes is the key to growing them successfully. Unless the climate in your area is similar to the bulb's native environment, you'll have to take extra steps to grow them in your garden. That's why, for example, northern gardeners have to dig up and store tender bulbs during the winter, and southern gardeners have to pre-chill bulbs to trick them into blooming.