Home » Gardening Information » Indoor Gardening
Chapter 2: Bulbous Botany
by Steven E. Clements
Botanists are notorious for changing the names of plants, but they are very precise about the use of descriptive terms such as bulb. To a gardener, a bulb is any perennial that overwinters as a fleshy, underground structure. To a botanist, bulb means something much more specific: a true bulb is a fleshy, underground bud. It is not a corm, rhizome, tuberous stem or tuberous root, all of which gardeners group together loosely as bulbs; these non-bulbs are in fact swollen roots or stems.
The botanical term for the gardener's bulb is "geophyte" -- literally "earth-plant" -- a term coined by Danish botanist Christien Raunkiaer. Raunkiaer devised a plant classification system based on where the dormant buds are located, and thus where new growth occurs. In geophytes, new growth begins below ground, while in other plants new growth occurs at or above ground level. In this chapter geophyte refers to the gardener's bulb and bulb refers to the botanist's true bulb. In other chapters, the term bulb will be used more loosely.
Where Geophytes Grow
Geophytes come from almost every corner of the globe, but most of the ornamental species are native to the so-called Mediterranean regions. For example, alliums, irises and tulips come from the Mediterranean basin (extending east to Iran and eastern Turkey); Camassia and Triteleia are native to our own Pacific Northwest (from California to British Columbia); Habranthus comes from southern Chile; and Agapanthus, Babiana and Boophane come from southern Africa. A number of less frequently cultivated species are found in western and southern Australia. These areas, particularly the Mediterranean basin and California, have cool, wet winters, short springs and dry, hot summers. Geophytes can reproduce during short springs and survive long, dry summers, giving them a leg up in these climates.
Types of Geophytes
It's a good idea to be able to distinguish the different types of geophytes from fibrous-rooted plants and from each other because this will help you determine how to plant them. One way to tell them apart is to group them according to where the starches and sugars used for food are stored.
Bulbs
A true bulb is really just a typical shoot compressed into a shortened form. Food is stored in a number of small, fleshy "scale" leaves. Most bulbs are egg-shaped, with a stem "plate" at the wider end. Attached to this stem are the storage leaves, forming concentric circles surrounding the growing tip. (If you cut a bulb in two you'll see the concentric rings typical of an onion cut in half.) From the lower part of the stem new roots form, growing downward. So it's important to plant the bulb with this broader, root-forming end facing down toward the bottom of the pot.
Many of the commonly cultivated geophytes are bulbs, including tulips, alliums and lilies.
Corms
In a corm, food is stored in stem tissue. Many corms look a lot like bulbs because they often have the same egg shape. But, unlike a bulb, if you cut a corm in half you'll see that it does not have the concentric rings of fleshy leaves. Instead, it is one mass of homogenous tissue -- that is, stem. In both bulbs and corms, roots grow from the wider end, and therefore a corm should also be planted with the wide side facing down. Buds poke out of the pointy end.
Gladiolus, crocus and freesia are examples of corms.
Rhizomes
In rhizomes, as in corms, food is stored in the stem. Indeed, a rhizome is a general term for a stem growing more or less horizontally below ground level. Thus you should plant it horizontally in a pot. Rhizomes tend to be thick, fleshy or woody, and bear nodes with scale or foliage leaves and buds. Growth occurs at the buds on the ends of the rhizome or nearby nodes. You can distinguish rhizomes from roots by the presence of scale leaves or the scars where old leaves have fallen off.
Irises, cannas and lily-of-the-valley are all rhizomes.
Stem Tuber
As the name implies, in these plants, like the two above, food is stored in the stem. It's often hard to tell a stem tuber from a rhizome; they're both swollen, horizontal, usually underground stems. But stem tubers usually form at the ends of a rhizome and will give rise to new rhizomes the following year. Growth starts from one or more nodes or buds called "eyes" at the base of the older stem. Like rhizomes, stem tubers should be planted horizontally in a pot, with at least one growing eye attached to each division.
The potato is the most easily recognized stem tuber, but some anemones and cannas produce them as well.
Root Tuber
Root tubers are swollen roots in which food is stored. Being roots, they lack nodes, leaf scars and buds. To be viable, they must include a portion of the stem with one or more buds. This portion of stem is usually planted facing upward.
Dahlias are the best-known root tubers; gloriosa lilies are also examples of this type of geophyte.
The Life Cycles of Geophytes
You must understand a plant's life cycle to grow and care for it successfully. Armed with this knowledge, you will know when to water your plants, when to withhold water and when to repot and begin watering again to coax them back into bloom.
Geophytes, whether bulbs, corms, tubers or rhizomes, have one thing in common: they have a dormant period. During adverse weather -- either hot and dry summers or cold and snowy winters -- many geophytes shed their foliage and live off the nutrients they stored up during favorable conditions. "Dormant" is really a misnomer for this phase: though they appear to be resting, most geophytes continue to develop, but the changes take place out of sight, underground, fueled by food stored during the previous growing season. Geophytes are best transplanted at this time when they are not reliant on light or water.
When environmental conditions once again become more favorable for growth, the plant is stimulated to put out roots, leaves and flowers. Many geophytes adapted to cold respond to a rise in temperature; those adapted to hot and dry conditions are tuned to an increase in moisture. And geophytes are tuned finely enough that they will usually not respond to just any rise in temperature or increase in moisture -- a freak warm spell in December or a rain shower in July won't get a rise out of the properly conditioned geophyte. This mechanism protects them from sending out vulnerable shoots that will be killed off when more typical seasonal weather again prevails.
Geophytes often start their growing season with a rush, producing flowers and leaves at the same time or sometimes producing flowers before leaves. This effort usually exhausts the geophyte's store of nutrients and the plant either dies after fruiting or attempts to accumulate enough nutrients for the next flowering season. For this reason, you should not cut off the leaves after flowering; they are working to build up the food stores through photosynthesis.
Some dry-climate species (amaryllis, for example) may keep growing if you don't simulate the drought spell they would experience in their native habitats, but they probably will never produce flowers. Stop watering after the plant has had a chance to build up its food reserves, and let it rest for a few months.
Propagating Bulbous Plants
Many geophytes are easy to propagate. Bulbs and corms often form daughter bulbs (bulblets) or daughter corms (cormels) which you can simply cut off the parent and transplant. Often rhizomes and stem tubers only need to be cut into pieces, or divisions.
The most important thing to remember is that most new growth comes from buds, and so you need to make sure that each division or "propagule" includes a bud. To ensure successful propagation, learn where the buds are in each type of geophyte. In bulbs, for example, the bud is enclosed in the numerous scale leaves. In a corm the bud is opposite the broad end where the roots emerge. In rhizomes and stem tubers the buds are spread out along the length of the organ, and each is associated with a leaf scar, where the old leaf was once attached. In a root tuber the bud or buds are found on the small portion of stem at the top end.
Steven E. Clemants is Vice President of Science at the Brooklyn Botanic Garden.