Garden & Greenhouse


Asexual Plant Propagation

Posted January 10th, 2008 by Garden & Greenhouse in

When it comes to easy… there is probably no easier method of plant propagation than through the use of bulbs, commonly known as flower bulbs. While often confused with being a sexual form of plant reproduction (i.e. seeds), bulbs, corms, rhizomes and tubers are all classic examples of asexual propagation.

Since seed development results from the fertilization of the ovules, or “eggs” by pollen produced in the stamens of the same or some other flower, reproduction of plants by seed is termed “sexual” as distinguished from the “asexual” processes of division, rooting cuttings, bulbs, etc.

A true bulb is essentially an encased leaf bud, or flower bud, and sometimes a combination of the two, which is surrounded by fleshy layers or scales. This bud is attached to a fibrous base from which the plants’ roots are produced. The common onion is a typical example of a true bulb. Some bulbs, such as the Hyacinth or Tulip, have the thickened fleshy layers wrapped all around them, while others, such as the Lily, have overlapping scales.

The decided advantage of planting true bulbs for flower production is the fact that the flower buds in mature, full-sized specimens are actually encased within the bulb at the time of planting and surrounded with a natural stored-up, time-release plant food. This is why some bulbs, like Hyacinths, Narcissus, and Amaryllis, need nothing more than to be placed in a container of water to bloom magnificently. Amazingly, some, like Colchicum bulbs, will bloom without any soil or water if the temperature and humidity are ideal.

The important thing to remember when working with bulbs is that the flower bud within a dormant bulb was created while the foliage was maturing the previous season. In other words, in all true bulbs, next year’s flowers are being produced while this year’s foliage is maturing. Therefore just as much care should be given to bulbs after they flower as before.

True bulbs are generally divided into two main groups – the first being generally termed “fall bulbs”, those that are dormant and should be planted in the late summer and fall; and the second being called “spring bulbs,” meaning those that are dormant over the winter and must be planted in the early spring. Typical fall bulbs include such well-known varieties as Tulip, Narcissus, etc. while the spring group includes Gladiolus and Dahlias.

The other important differentiation with bulbs is the division between “hardy” and “tender” varieties. When a bulb, like any other plant, is spoken of as being hardy, it means that it will survive the usual winter weather experienced in our temperate zone; tender bulbs will not do this and therefore must not be allowed to freeze – either in the ground or out of it. Included in the class of fall bulbs are many that are hardy and thus capable of remaining in the open ground during the winter. On the other hand, very few of the spring bulbs are hardy, which means that most of their group must come out of the ground in fall and placed in a frost-free area for storage during the winter.

Nearly all of the plants referred to in this article will increase naturally, the extent depending upon the locality and the conditions under which they are grown. The better care and attention they are given, the greater will be the increase, the larger the size and higher the quality of the bulbs.

A great many other floral subjects, because of their resemblance, are commonly confused with bulbs. These commonly include corms, tubers, and rhizomes.

A corm, of which the Gladiolus, Freesia, Caladium and Crocus are typical, is a solid object, usually quite hard, instead of being formed in layers or scales like a true bulb. The method of growth is also entirely different. A true bulb may live indefinitely as a single unit or may increase by splitting itself up, but a mature corm actually withers and dies after a year of growth, being replaced by a new corm or corms that form usually on top of the old one but sometimes beneath or alongside it.

The corm is actually a shortened, fleshy, erect underground stem with inconspicuous scale-like leaves. From the prominent terminal bud and smaller ones in the axils of its leaves, corms develop new plants, and often small, subsidiary corms known as cormels.

Tubers are shortened, congested or swollen parts usually, though not always, produced underground. They may be modified stems, in which case they bear leaf buds or “eyes” in a regular pattern over their surface (the Irish Potato is an example); or they may be swollen roots without eyes, as in the Dahlia and Sweet-potato, which sprout from buds on the stem end or “neck” of the tuber. The function of these fleshy tubers is to serve as reservoirs of plant food upon which the new shoots can subsist until their new rooting system is able to provide it by absorption throughout the root hairs.

A rhizome or rootstock is a thick fleshy root that usually grows horizontally and often quite near the surface of the growing media or soil. Containing multiple nodes, or joints, from which spring roots and stems of new plants, rhizomes can subsequently cut from the parent plant and treated as separate individual plants.

While some rhizomes are as slender as the over the ground stem, many are thickened by the storage of food material, which sustains the plant over winter. Unfortunately, the function of rhizomes as a means of propagation is especially apparent in the rapid underground spread of many invasive weeds and grasses, notably quack grass, it can also be a most valuable method of reproducing plants in the garden, as, for example, Solomon’s Seal and Sanguinary.

Unlike bulbs, which require continued care after flowering to prepare next year’s flowers, corms, tubers and rhizomes don’t share this characteristic. Sporting what can be termed a progressive flower embryo, the treatment corms, tubers and rhizomes receive during their early vegetative growth determines to a great extent that same season’s flowering results.

In both sexual and asexual propagation methods, gardeners and nurseries have relied largely on traditional rules that have proved only partially successful. Seeds of many plants have taken months or even years to germinate, and cuttings have failed altogether or have given poor results when handled by ordinary methods. However, thanks to experiments and investigations, many propagation problems have been solved in recent years and further improvements are clearly on the way. Alert plant propagators as well as home gardeners should strive to keep abreast of these new discoveries and become familiar with at least some of the methods worked out by professional horticulturists and proven in the trial greenhouses of different institutions.

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