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Bulbs and Similar StructuresSometimes gardening terms can become confusing and lead to wrong cultural practises. For example, the term bulb is used to apply to a number of different plant structures which are planted and propagated differently. Although all of these structures are modified stems, they go by different names depending on their location and make-up. Above ground bulblike structures are called crowns, and the below-ground ones are bulbs, corms, rhizomes and tubers.Crowns are found on such plants as strawberries, daylilies,and African violets. Crowns are compressed stems having leaves and flowers on short internodes. On most ornamental plants the crowns gradually thicken to include many stems in close proximity. Rhizomes grow underground rather than above ground. Some rhizomes are compressed and fleshy such as Iris; they can also be slender with elongated internodes such as bentgrass. Johnsongrass is an insidious weed principally because of the spreading capability of its rhizomes. Below-ground stem variations such as the potato tuber, the tulip bulb and the iris rhizome are underground stems that store food for the plant. The tuber, like any other stem, has nodes that produce buds. The eyes of a potato are actually the nodes on the stem. Each eye contains a cluster of buds. Tulips, lilies, daffodils, and onions are plants that produce true bulbs which are shortened, compressed, underground stems surrounded by fleshy scales (leaves) that envelop a central bud located at the tip of the stem. In November, if you cut through the center of a tulip or daffodil bulb, you can see all the flower parts in miniature within the bulb. Many bulbs require a period of low temperature exposure before they begin to send up the new plant. Both the length of this treatment and the temperature are of critical importance to commercial growers who force bulbs for holidays. Easter lilies are especially tricky to time since the date of Easter may vary by six weeks. Corms have shapes similar to bulbs, but do not contain fleshy scales. A corm is a solid, swollen stem whose scales have been reduced to dry, scale-like leaves. Some plants produce a modified stem that is referred to as a tuberous stem. Examples are tuberous begonia and cyclamen. The stem is shortened, flattened, enlarged, and underground. Buds and shoots arise from the top or crown and fibrous roots are found on the bottom of the tuberous stem. In addition, some plants such as the dahlia and the sweet potato produce an underground storage organ called a tuberous root which is often confused with bulbs and tubers. However, these are roots, not stems, and have neither nodes nor internodes. All of these modified stems are useful for propagation. Crowns can be divided with a spade or sharp knife; rhizomes can be divided into pieces; bulbs form small bulblets at the base of the parent bulb; cormels are miniature corms that form under the parent corm; and tubers can be cut into pieces containing eyes and nodes. The key to success with propagation is to make sure that each plant part used, contains a node with buds from which new stems can sprout. Tuberous roots do not have nodes so care must be used in dividing them to make sure that part of the stem is included as a source of buds.
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