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What is a Bulb?

When spring arrives bulbs like tulips and narcissus come up and bloom everywhere. As summer approaches you watch for bulbs like gladiolas and lilies. And in autumn you might see fall blooming bulbs like autumn crocus. But are all of these plants really bulbs? The bulb catalogues and gardening books treat them all as bulbs, and they all have a fleshy underground structures that look nothing like the network of roots of most of the plants in the garden. Some really are bulbs but others are actually corms, rhizomes, or tubers.

Onion (true bulb)

So what is a true bulb? It is an underground bud with fleshy leaves modified into scales forming rings and a shoot consisting of a developing flower and leaf buds. A flat modified stem (the basal plate) produces roots. Think about an onion, especially one you have had too long and it has sprouted. The “onion rings’ are the scales. Some true bulbs (like onions and tulips) have a paper-like covering that protects it from injury and drying out. Others, like lilies, do not have a papery covering and care must be taken to keep them moist. Some bulbs, like narcissus, produce new bulbs at their base called offsets. When the offsets become too crowded, the flowers diminish in size, and the bulb can be dug and the offsets harvested to produce new plants. Some varieties of bulbs require a cold period to produce flowers and can not successfully be grown in southern parts of the United States unless given an artificial cold period.

Gladiolus corm

A corm is the base of a stem that has been modified into a fleshy mass of storage tissue. The storage tissue is not in visible rings like that of a true bulb, but like a bulb it has a flat bottom (basal plate) that produces roots, a thin protective skin, and a growing point. Plants that produce corms include gladiolus, some iris, freesia, liatris, and crocus.

Ginger rhizome

A rhizome is a horizontal stem that is usually found underground but may grow along the surface of the soil and sends its roots into the soil below. Bearded iris, Peruvian lily and ginger are a good example.

Dalhia tubers

Large, fleshy, underground stems with eyes that can sprout and produce new plants are tubers. Potatoes, dahlias, Eranthis, and ranunculus are examples.

Tuber, rhizome, corm, or true bulb, all are similar in appearance and act as storage organs of food for developing plants. We eat some like onions and potatoes but others we want to have bloom year after year so we must be sure that the food in the storage organ is replenished each year. This means that the leaves, the food producing organs, must be left in place with maximum surface area exposed to the sun so they can do their job and we can have large, showy flowers year after year.