Understanding bulbs, corms, tubers...

By Melanie Mathieson
Gardening Guru

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 center 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.
Corms
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.
Tubers
A tuber differs from the true bulb and the corm by not having a basal plate from which roots develop and not having a protective tunic covering. As a result, tubers and root tubers are easily distinguished from bulbs and corms. A tuber is really just enlarged stem tissue found under the ground that is engorged or swollen and is used by the plant to store food and nutrients. Tubers come in a variety of shapes, from cylindrical, to flat, to just about any combination you can imagine. Tubers usually only last for one growing season but new tubers sometimes form shoots the next year. Examples of plants that develop from tubers include caladiums, oxalis and anemones, and the common vegetable, the potato.
There are two different kinds of tubers, the stem tuber and the root tuber. Root tubers are different from stem tubers because they actually grow on the root itself, not the stem. Root tubers, such as sweet potatoes, grow under the ground, like stem tubers, but can be found at the end of the root or even in the middle. They function the same as the stem tubers in that they also store nutrients and are used to keep the plant alive during the non-growing season. They are different from stem tubers also because they have the structure of roots and will have additional small fibrous roots coming from them that if left alone, will spread out underground and produce more roots and tubers. If you pull up a sweet potato from the ground, you will see the hairy looking new roots growing off the surface. Root tubers are designed to produce new plants when the growing season comes around again.
Rhizomes
Rhizomes are distinguished from other bulbs because they are really underground stem tissue that sends out roots and shots from its nodes. Rhizomes can also be referred to as creeping rootstalks or rootstocks.
Rhizomes differ from other storage structures by growing horizontally under the surface of the soil and on some plants this type of rooting structure can be very invasive. If a rhizome is separated into pieces, each piece may be able to give rise to a new plant. This is a process known as vegetative reproduction and is often used by farmers and gardeners to propagate certain plants. Examples of plants that are propagated this way include hops, asparagus, ginger, irises, lily of the valley, cannas, and sympodial orchids.
It is obvious that depending on the type of bulb, some require more attention from the gardener than others as far as maintenance, reproduction and overwintering methods. Understanding what type of bulb you have can assist you in planning your perennial garden and scheduling the appropriate time to plant your bulbs.