Pseudobulbs are thickened stems at the base of each growth in a sympodial orchid, usually an epiphyte. They serve as storage organs, primarily for water, and are more likely to be present in orchids that experience drought in their natural habitats. They may last only one year, or for several years, depending on the type of orchid. They differ botanically from true bulbs (like in tulips), which are actually masses of thickened leaves or petioles around a short stem. Corms and tubers also have technical differences.
Conspicuous pseudobulbs on a Lycaste

Each of the growths emerging from the end of a sympodial orchid's rhizome has a single stem (which may be thickened and bulbous), some leaves, and usually blooms once before diverting its energy to producing the next year's growth from its base. Inflorescences may emerge from any node on the stem. Lycaste orchids have flower stems emerging from the base of each storage organ, in Laelias they emerge from its top, and in some Dendrobiums they may appear at many nodes along the thickened stem.

Pseudobulbs differ widely in appearance among the many types of orchids that have them. In Dendrobium they take the form of a long thickened cane, with leaves all along its length. Epidendrum has similar canes but they are covered with a protective layer of dead leaves called bracts. In the related Cattleya orchid, the storage organs are still significantly longer than their girth, but only one or two leaves appear at their tops. In Bulbophyllum, they lack a bract and are typically very thick, almost spherical. There may also be prominent lengthwise ridges producing a star-shaped cross-section. Catasetum orchids experience a long dry season, and spend much of the year with no leaves to avoid dessication. They have very thick, conspicuous, bract-covered stems to store water and nutrients. When the plant resumes growth during the wet season, it utilizes the stored nutrients to develop its new growth with extreme rapidity.

If you are cultivating an orchid very well, each new growth that develops should be larger than the previous one.

Many orchids with canes (long, thickened stems that have leaves all along their length), are capable of developing keikis at the nodes where leaves emerge, a natural form of vegetative propagation. In some cases, you can take an old cane and cut it between each pair of nodes, setting the pieces on moist sphagnum until they begin to grow.

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