Iris Doctor

By M. D. Faith

Subject: Rhizome or Bulb?

"We hear some members of the club calling rhizomes ‘bulbs’, as though the words were interchangeable. What is the difference between rhizomes and bulbs?" This question is from Celia in Little Rock, and it is a very pertinent question, since many people and a lot of home gardeners indeed very often use the terms interchangeably, and with some justification for that use.

Certainly members of a specialty plant society such as an iris society dedicated to the growing of iris, or members of a daffodil society who are equally dedicated to their favorite plant, all with some years of seniority in their respective fields, would not use the term in this loose broad general manner. Experienced gardeners would instinctively know and appreciate the difference between the terms, as well as the inherent nature of the underlying structures to which they refer, that is in particular the make up of a rhizome and a bulb.

A Daffodil grower of experience would no more refer to the underground structure that produces his or her favorite plant as a rhizome, as an iris grower would call a rhizome a bulb. The term ‘bulb’ has been used for centuries to refer to all the under ground storage structures that plants use to store nutrients for future use. Most reference sources use the term to denote a class of plants that produce underground storage structures. When used to denote a class of plants it should not be considered an incorrect usage of the term.

Merriam-Webster Collegiate Dictionary has it right, it defines the term first as: “ (a) a resting stage of a plant (as the lily, onion, hyacinth, or tulip) that is usually formed underground and consists of a short stem base bearing one or more buds enclosed in overlapping membranous or fleshy leaves (b) a fleshy structure (as a tuber or corm) resembling a bulb in appearance (c) a plant having or developing from a bulb”.

To me a bulb contains the complete life cycle of a plant with in itself. Amaryllis and Paper white narcissus demonstrate this succinctly by growing vigorously from stored reserves when being anchored in a non-nutrient containing medium and supplied moisture only. They bloom quite vigorously from the nutrients stored with in the bulb from the preceding year’s growth cycle. The nutrients were stored during the primary growing season, which mostly occurs after the bloom of the previous year. Bulbs forced in this way are truly spent after being forced to bloom without additional nutrients being supplied during the forcing period, and they should be discarded and not planted in the garden.

True bulbs are comprised of two distinct types, tunicate and imbricate bulbs.

Their tough paper like tunic distinguishes the tunicate bulbs, and this covering protects them from drying out, and thus the bulbs can be stored in a cool dry place for several months. From the bulb basal plate, from which the roots develop, there are concentric fleshy scales that surround the short stem and bud that arises from the base through the center of the bulb. The onion, daffodil, hyacinth, tulip, amaryllis, and other bulbs with a tunic or paper like covering, are well known examples. Iris growers are familiar with iris reticulata, iris danfordiae, and the Dutch and English irises, which are also tunicate bulbs.

Imbricate bulbs do not have the tunic and must be kept from drying out during the period of time that they are out of the ground before resetting into the garden. They are stored in a slightly moist medium during storage and shipment. Like the tunicate bulbs the roots develop from the basal plate, while the fleshy scales where the nutrients are stored are overlapped around the circumference of the bulb with the stem-bud rising through the center of the bulb, which is composed of these overlapping fleshy imbricate scales. The bulbs of the lily family are prime examples.

Corms are often lumped together, in the minds of some people, with tunicate and imbricate bulbs and referred to with the general term bulbs. There is quite a difference in the make up of the corm, though. The base of the corm from which the roots develop is topped with the short stem-bud, which is surrounded with a storage structure of undifferentiated substance covered with a thin brownish paper-like tunic. If you cut it in cross section you do not see the concentric rings or imbricate scales of which the true bulbs are composed. The new corm each year is formed just above the old spent corm with a lot of little corms around its base. This results in it becoming nearer the surface of the soil each year, if they are not reset on a regular basis. Crocus and gladiolus with which every one is familiar develop from Corms.

Tubers are a third type of underground structure often referred to with the general term bulb. They differ from the true bulbs, and from the corm by not having a stem-bud base from which the roots develop. Instead they have buds, which are scattered over their surface from which the plants sprout a shoot from which the roots develop. The familiar Irish potato, which we all know very well, along with the caladium, anemones, and oxalis, are good examples of tubers.

The tuberous roots of Dahlias differ from the other tubers by having the buds confined to that area at the top of the swollen elongated root where the stem from last year is attached. A portion of this stem base must be included in any division of the tubers to provide a place from which the new plant bud and shoot can develop. In tuberous rooted begonia the new buds all develop from the top of the round flat tuber. These nutrient storage structures are significantly different from rhizomes in that they are true roots, where as the swollen elongated rhizome’s storage structure is a true stem.

A rhizome, although it derives its name from the Greek term rhizomat or rhizoma meaning a mass of roots, is not a root, as all isarians should know, but a stem that grows along the soil surface. Either just below the soil surface or it, as we know, will crawl out and grow along the surface of the soil. Contrary to true roots which never have buds or shoots, the rhizome puts up true leaves and buds along the top of the stem (rhizome) into the lighted atmosphere above the soil with true roots growing into the moist nutrient rich soil of the rhizosphere below.

In an aside it might be worth noting, that the tendency to grow out upon the surface of the soil by the rhizome, may be a survival technique that evolved over time to protect it from the myriad of rot fungi and bacteria, which attack it with disastrous results on occasion. We are all aware of the beneficial effects of the open air and sunlight in stopping rot. It just may be that those on the surface survive better than those beneath the soil in adverse soil conditions.

Like the bulb, the rhizome does store plant nutrients with in it by elongating and swelling in diameter. These reserves are for the purpose of feeding the new sprouts until they can develop a rhizome and roots of their own, also supplying some of the nutrients for early spring growth and bloom. But unlike the true bulbs, I do not feel that the rhizome stores nearly the entire nutrients that are necessary for the spring bloom season. Experiments that I have made with Louisiana rhizomes indicate that a well-grown mature rhizome, supplied with water only, will bloom from the nutrients stored in the rhizome producing a fairly nice flower. The bloom thought, does not begin to approach the standard that the same plant exhibits when properly grown in a well-enriched soil.

Finally let us sum it up, the true bulbs, both tunicate and imbricate as well as the corm, have one thing in common with a rhizome, they are all consider to be modified stem structures, as opposed to the tubers and tuberous fleshy root which are modified true roots.

Bibliography: A practical book for the home gardener is “Bulbs, How to Select, Grow and Enjoy” by George Harmon Scott, Published by HP Books, Inc. PO Box 5367, Tucson, AZ 85703.