What is a Dendrobium orchid?
As explained in my key to the terrestrial orchid genera of Thailand, the orchid family (Orchidaceae) is defined by the column, a flower structure composed of fused stamens and pistils. Orchid flowers have three petals of which one is usually a lip, and then there are three sepals surrounding the petals. About 22 500 species in the world belong to the orchid family, so further divisions are needed into genera.
One genus is Dendrobium, common as ornamentals and also in some protected Thai forests. There are about 1250 species in the world, confined to Asia and Australia. In Thailand we have 161 wild species and zillions of man-made hybrids and cultivars. If you can identify this genus, you have significantly increased your chances to identify your orchid to species level. If you know the name, then you can find out how to grow it successfully. We therefore need to make an understandable definition of the genus Dendrobium. For terms like column, spur, viscidium, pollinia and lip, see pictures on the article key to the terrestrial orchid genera of Thailand.
1. Dendrobiums are mostly growing in trees (epiphytic), or if you wish, on wood, baskets or nets, preferably not in closed pots. Some Dendrobium orchids may sometimes grow on steep sandy slopes or rocks but rarely in the flat soil. Some grow in swamps.
2. Dendrobium stems (rhizomes) are branched (sympodial), in contrast to for example Vanda and Phalaenopsis orchids which only grow at the top shoot (monopodial). Sometimes the rhizomes are lacking.
Sympodial growth in Coelogyne flavida.
Monopodial growth in Vanda liouvillei.
3. Most Dendrobiums have pseudobulbs. These are nutrient storing organs, sometimes shaped like real bulbs resembling onions. They can also be 50-200 cm long and no thicker than a pencil. Epiphytic Cattleya orchids have pseudobulbs too, while Papilionanthe orchids do not.
Flowers of Dendrobium primulinum are formed directly on elongated pseudobulbs.
4. The pseudobulbs in Dendrobium are often seemingly segmented, having a series of nodes. This is never the case in two other large orchid genera, Bulbophyllum and Coelogyne. In these genera the pseudobulbs seem to be made of one unit. In such a case the pseudobulbs are described as having ‘a single internode’.
Pseudobulbs of Dendrobium chrysotoxum are segmented into several internodes.
Coelogyne flaccida has pseudobulbs with one internode (no segments).
5. Many orchids have a spur, a longer or shorter protruding element behind the flower, often an extension of the lip containing nectar. Technically, Dendrobium orchid lips do not have spurs, but instead the flower may have a spur-like mentum, i.e. a sac-like structure formed by the lateral sepals and the column-foot. A true spur is formed by one organ only, usually the lip.
In Dendrobium lindleyi (left) and D. jenkinsii (right) you can see that the mentum is formed by the side (lateral) sepals.
In Vanda liouvillei you can see that the white spur is a part of the violet-brown lip, separate from the lateral sepals which I hold between my fingers.
6. If you grow an epiphytic orchid with segmented pseudobulbs, and see spur-less lips, it is likely you have a Dendrobium. To make sure, you take a tooth-pick or needle and free the pollinia from the apex of the column. These pollinia lack a viscidium, the sticky tape-like structure found in many other orchids. With a magnifying glass you will see that the ‘pair’ of pollinia are in fact four, but densely packed in pairs.
To further facilitate the identifications, the Dendrobiums have been separated into 40 sections, but that is another story.
Dendrobium capituliflorum is not showy like most other Dendrobiums. Using the definitions given above, it is still a member of this aristocratic genus. The flowers emanate from the elongated pseudobulb. The species is native to New Guinea and the Solomon islands. It is currently in blossom at Dokmai Garden.
The scientific name Dendrobium, meaning ‘tree of life’ or ‘life on tree’, was coined by the Swedish botanist Olof Peter Swartz (1760-1818), a disciple of the great Carl Linnaeus from Uppsala University. Swartz was the first orchid taxonomist ever, and a friend of Joseph Banks.
Orchids have a reputation of being useless to man, spare ornamentals and vanilla (Vanilla planifolia, soon in blossom at Dokmai Garden). That belief is usually due to lack of examples, which in turn is due to loss of rural knowledge and biodiversity decline, such as when we claim lions are solely African. Dendrobium moniliforme was used to scent clothes in Japan, other Dendrobium were used as medicine (D. crumenatum) or for making baskets, some species produced gum to fix body painting in northern Australia.
Knowledge about orchids are often confined to expensive books or technical research papers. Simplified and free texts are therefore crucial: If basic knowledge about orchids becomes widespread, then the orchids’ chances for survival in the forests will increase. Knowledgeable people are concerned, and many knowledgeable people will report findings to universities and take proper action for orchid preservation, such as never buying roadside orchids stolen from the forest. To reach a civilized stage where knowledge about orchids is widespread and not considered ‘snobbish’, I am most happy for any suggestions improving my text(s).