A Beginner's Garden: What Are Tubers, Bulbs, Corms, Rhizomes?

Tubers, bulbs, corms, and rhizomes are all geophytes, the parts of plants that store food during dormancy for energy the following growing season. Annual plants make seeds for this purpose: each seed contains a tiny embryonic plant. Perennials also make seeds. However, the most efficient way some perennials can ensure longevity is by storing their own food by one of these geophyte methods. The word "bulb" is often used generically to refer to all geophyte structures. It seems easier to use one umbrella term when discussing these plants, but it is not correct in all cases.

If you take a true bulb and cut it in half vertically, you will see that it consists of layers, and it contains leaves and flowers. An onion is a true bulb, as is a tulip, narcissus and lily. True bulbs are usually flat or rounded on the bottom root end, and the top is somewhat pointed. We interrupt the development of onion bulbs, harvesting them when they are the best size for storage and eating. A tulip or other spring flowering bulb that is planted in the fall sends out roots before the ground freezes hard. The roots hydrate the bulb, keeping it ready to grow as soon as spring temperatures are warm enough. The same bulb stores new food each year, and the bulb itself matures and grows larger, often forming bulblets which can separate and develop into new bulbs.

A crocus grows from a corm, and so does a gladiolus, elephant ear and anemone (windflower). A corm is actually the enlarged base of a stem. It serves the same food storage function as a bulb, but it does not contain leaf or flower components. A corm grows new roots, leaves and flowers each year. Some corms are not cold hardy. For example, in a northern garden, elephant ears and gladiola corms must be dug up and stored over the winter in a cool, dry place where they will not freeze. Crocus and windflower corms are freeze tolerant in most areas of the US, and they can remain in the ground over the winter. Each year, a new corm develops from the stem of that year's plant.

A rhizome is a horizontal stem that enlarges and thickens to store food. New leaves appear vertically every few inches along a rhizome. Each group of new leaves will root and become a new plant, sending out its own network of rhizomes. Irises are common plants that spread by rhizomes, as well as many grasses such as zoysia. The plants growing along a rhizome can be cut apart with a section of the rhizome, and separated to make new plants. This is also the method used for thinning out crowded beds. The same rhizomes continue to grow from year to year, often forming a matted web.

Tubers are enlarged places that develop on roots or underground stems to store food. The most familiar tuber is a potato. A potato grows on an underground stem, not on the plant's fibrous roots. Examples of flowers that form tubers are dahlias, tuberous begonias and gloxinias. A tuber has eyes, from which new stem/leaf shoots grow. Tuberous plants use the energy stored in the tuber for the initial leaf and stem growth, then roots take over as the tuber's energy store is depleted. New tubers develop each year.

Bulbs, tubers and corms are planted completely underground. Rhizomes are planted so that the roots are below the soil surface, the leaves are above. In some cases the rhizome is only partly buried, and in others the rhizome is just beneath the surface of the soil.

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Source: Personal Experience