A previous column outlined the difference between bulbs and corms and this column will tackle tubers and rhizomes. Like bulbs and coms, tubers and rhizomes are also 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
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.
