Understanding bulbs and corms

Bulbs or bulb-like plants are usually considered perennial plants which have a period of growth and flowering, followed by a period of dormancy where they die back to ground level at the end of each growing season. The end of the growing season for spring flowering bulbs occurs in the late spring or early summer when they die back in preparation to grow and flower again the following growing season. Summer flowering bulbs flower in the summer and die back in the fall, then re-emerge and grow the following spring to flower again in the summer months. 

In general terms, spring flowering bulbs are referred to as hardy because they are planted in the fall before the ground freezes as they require a cold period, known as vernalization, before they can emerge and produce flowers. Spring bulbs are planted in the fall, generally before the first frost. Many hardy bulbs, such as daffodils, tulips, etc. and can be left in the ground to flower year after year and becoming reliable perennials. Summer flowering bulbs are considered tender, as these bulbs cannot survive harsh winter conditions and must be planted in the spring after the last frost. To enjoy these bulbs year after year, they must be dug up in fall and stored indoors over the winter and replanted every spring. One exception to this is the lily. Many summer-flowering lily varieties are quite hardy and can be planted in either fall or spring and up to Zone 3 can be left in the ground over winter, to produce blooms the following summer. When left in the ground the lily also becomes a reliable perennial.

The definition of a bulb is any plant that stores its complete life cycle in an underground fleshy storage structure. The generic term “bulb” can refer to any of the four main types of fleshy storage structures known as true bulbs, corms, tubers, and rhizomes, which store all of the nutrients the that the plant needs to sprout, grow, flower and produce seeds.

True Bulbs

Only some of the plants commonly called bulbs are actually bulbs. They are called “true bulbs”. Contained inside the bulb, is just about everything the plant will need to sprout and flower at the appropriate time. True bulbs are divided into two types: tunicate and imbricate bulbs. A tunicate bulb has a paper-like covering called a tunic that protects the scales from drying out and from physical injury while imbricate bulbs, like lilies, lack the papery covering.

All true bulbs have a basal centre portion of the bulb, where the leaves, known as scales cradle a baby bud. Scales are often white and have a juicy and meaty appearance within the bulb. In most species of true bulbs, the baby bud already has the appearance of a flower and the scales contain all the food the bulb will need to flower and thrive. The basal plate, located at the bottom of the bulb, holds the roots of the plant and anchors the scales and floral stalk which holds the bud. A thin outer skin, called the tunic, covers the outside and protects the entire bulb.


The differences between bulbs and corms are slight and the two look very similar. A corm is a swollen stem base that is modified into a mass of storage tissue. A corm does not have visible storage rings when cut in half which distinguishes it from a true bulb. Another main distinguishing trait is the method of storing food. In corms, most of the food is stored in an enlarged basal plate rather than the meaty scales, which in corms are much smaller. Corms generally tend to be flatter in shape than round, true bulbs.

Corms also contain a basal plate at the bottom of bulb from which roots will develop, a thin tunic and a growing point. Examples of plants that develop from corms include gladiolus, crocus, and autumn crocus.

Look for the second part of this article that covers off tubers and rhizomes in a future issue.