Ask most people what tulips, daffodils, crocus, and dahlias have in common and they’ll tell you that they are all bulbs. They’d be half right. Only the tulips and daffodils are true bulbs. Crocuses are really corms and dahlias are tuberous roots.
True BULBS are really miniature plants in a neat little package. They contain fleshy, modified leaves which act as food reserves, held together by a basal plate, a disc of thickened stem tissue surrounded by roots. Most bulbs have a covering of dry papery leaves called a tunic. Inside this remarkable package lies the embryo of the flower. Hyacinths, lilies, amaryllis, and even onions are all bulbs.
CORMS, on the other hand, are a mass of modified stem tissue with a basal plate at the bottom, and one or more “eyes” or growing points at the top. Corms, too, are covered with a dry papery tunic. During the growing season, the old corm is completely used up, but will form new ones above or beside the old. Crocus, gladioli, and freesias are all corms.
A TUBER is a swollen mass of stem tissue. It differs from bulbs and corms in that it contains no basal plate or papery covering. Usually tough-skinned, tubers generate roots from many parts of the surface, but will grow shoots only from knobby projections called “eyes.” Begonias, anemones, and potatoes are all well-known tubers.
Dahlias and ranunculus are TUBEROUS ROOTS. Tuberous roots differ from tubers in that the storage tissue is not a modified stem but is actually part of the root. The fat, fleshy roots act only as nutrient storage chambers. During active growth, the plants will produce small fibrous roots to take in moisture. New growth will appear only at the base of the old stem where it joins the tuberous roots. Never separate these plants unless each segment contains a piece of the growing stem.
RHIZOMES, sometimes called rootstock, are thickened stems that grow horizontally at or just beneath the soil surface. Roots grow from the base of the stem while leaves grow from the top and/or sides. Some rhizomes, like Lily-of-the-Valley, produce small detachable shoots called pips which can be removed and stored for later planting. German-bearded iris and Gloriosa lilies are rhizomes.
To the average gardener it doesn’t really matter whether a plant is a bulb, a corm, or a tuber. What matters is that all of them have developed food storage capabilities to carry them through adverse conditions. Therefore, most need some sort of dormancy or rest period. This may be induced naturally by cold winter weather or summer drought. In the case of bulbs grown in pots, like amaryllis, it is up to the grower to enforce resting by withholding water.