Certain plants have a swollen, underground plant stem called a "corm." The purpose of this stem is to store nutrients, food that the plant will use at a future time. Nutrients stored in the corm during one growing season will help in the production of roots, leaves, and flowers for the next growing season.
Difference Between Corms and Bulbs
To the horticulturist, the terms "bulbs," "tubers," "rhizomes," and "corms" all have distinct meanings. But when gardeners are speaking casually, they often fail to make a distinction between them. Instead, they loosely call all of these swollen, underground plant parts "bulbs." But if you want to lump them all together using one word, the correct umbrella term is geophytes. "Geophyte" is made up of the Greek words for earth and plant. It includes perennial plants with underground food storage organs. Shoots spring out of these organs, crack the surface, and develop the part of the plant that we see growing above-ground.
The crocus (Crocus vernus) is an example of a small plant that grows from a corm. Its rounded corm measures roughly 1 inch in diameter. A larger plant that springs from a corm is the snake lily (Amorphophallus konjac). The corm of a mature snake lily is hassock-shaped and can measure 10 inches at the widest point across, 9 inches front to back, and 6 inches tall. The cyclamen (Cyclamen persicum), a popular gift from the florist shop during the winter holiday season, also grows from a corm, as does the popular cut flower, Gladiolus.
We tend to lump Crocus vernus together with spring bulb plants (since it blooms at the same time and is hardy), but the following are examples of true bulb plants:
Like crocuses, these plants have bulbous, underground plant parts that can survive cold winters, and their flower stems push up through the ground in spring. But true bulbs are divided into layers (think of an onion), including a papery outer layer. A bulb is comprised of a plant's stems and leaves. The bottom of the bulb is a compacted stem, with roots growing from it. Layers of nutrient-filled leaves sit at the bottom of the bulb and surround a bud that eventually becomes a flower. Corms are not divided in this way; rather, they are solid units.
What Are Tubers and Rhizomes?
So what is the difference between tubers and corms? Tubers are formed from a stem or root. They store nutrients that allow the plant to grow the following season. Buds sprout from tubers (think of the "eyes" on a potato). Shoots grow upwards from many different places on the tuber. You can cut off individual hunks that have buds and plant them to get new plants, which is something you can't do with corms and bulbs. An example landscape plants that grows from tubers are the Dahlia and begonias. Tubers, like potatoes, multiply underground.
Still, other plants grow from rhizomes. Rhizomes are modified, swollen stem-like structures that grow horizontally, forming roots from the bottom and sending shoots upwards. They often appear as nothing more than roots. Buds form at different parts along the rhizome, not just the tip.
Powerhouse weeds such as Japanese knotweed (Polygonum cuspidatum) use rhizomes to full advantage, spreading to form monocultures that dominate a landscape. Leave just the smallest scrap of rhizome behind in the soil when trying to dig out such a weed, and it will defiantly return. Lily-of-the-valley (Convallaria majalis), Chinese lantern (Physalis alkekengi), and Canna lily are examples of landscape plants that grow from rhizomes.