Dendrobium tetragonum is found in Section Dendrocoryne. The Dendrobium tetragonum complex consists of four closely related species, which, for many years, have been considered varieties of Den. tetragonum. These are:
- Dendrobium tetragonum
- Dendrobium cacatua Clements et. D. Jones (usually known incorrectly as Dendrobium tetragonum var. hayesianum)
- Dendrobium capitisyork Clements et. D. Jones (syn. Dendrobium tetragonum var. giganteum)
- Dendrobium melaleucaphilum Clements et. D. Jones (syn. Dendrobium tetragonum ‘North Coast Form’)
These four species are very similar vegetatively. They all possess pendulous pseudobulbs that are very thin toward a basal swelling and thicken to a square shape toward the apex. However, each species has distinctive floral characteristics. The range and habitat of each species is distinct and in only a few areas do any of these overlap.
Dendrobium tetragonum occurs from the Illawarra region of New South Wales to southeast Queensland. Plants are found from the coastal lowlands to above 2,666 feet, on trunks and the larger branches of trees along stream banks in subtropical or temperate rain-forest. These areas are heavily shaded and most plants grow near creeks. Those that occur at higher altitudes away from permanent water sources do so in locations where frequent fogs and mists keep the atmosphere moist. Seedlings occur almost to ground level.
Dendrobium tetragonum has the shortest pseudobulbs of the complex, rarely more than 12 to 16 inches long. Most plants consist of approximately 10 canes, but those grown under good conditions and given a chance to mature can become quite large, with up to 30 or more pseudobulbs. The flowers are the smallest of this complex, usually only 2 to 2.5 inches, but what they lack in size they make up with their strong, sweet, vanilla-like perfume. There are between two and five flowers per raceme, and several racemes per pseudobulb. The flowers are pale green or cream-colored to yellow with vivid red margins and a wide labellum with lateral lobes similar in width to the midlobe. The labellum is usually white with red markings although plants with an unmarked midlobe are common. Flowering is in the spring .
There is a form of Den. tetragonum with longer pseudobulbs that can be almost as long as those of Den. melaleucaphilum. The flowers are small, like Den. tetragonum, but may have more red diffused throughout the sepals. The plant has the flattened appearance characteristic of Den. tetragonum. These plants usually flower prolifically from nodes all along the pseudobulbs. A green-flowered form found within populations of Den. tetragonum is the true Den. tetragonum var. hayesianum. This is a color variant of Den. tetragonum.
Dendrobium melaleucaphilum, is found in isolated colonies from the Blue Mountains (west of Sydney) to central Queensland at altitudes less than 1,333 feet. The larger colonies, in northern New South Wales, occur in swampy, heavily-grassed coastal areas just above high-tide level. The most common host of this species is Melaleuca stylphelioides, a native “paperbark” tree, with bark that grows in thin papery layers into which the roots of Den. melaleucaphilum intertwine. Although the trees can form dense stands, Den. melaleucaphilum in these areas receives high light levels. Other orchids in this habitat include the epiphytes Cymbidium madidum, Peristeranthus hillii, Dendrobium linguiforme, Dendrobium teretifolium and several terrestrials. In its natural habitat, Den. melaleucaphilum is quite vulnerable. Colonies have been devastated by illegal collecting and others are threatened by coastal development. One site we saw several years ago had been stripped of the larger specimens, and the seedlings were left to rot on trees that had been felled specifically to collect the one or two large plants.
Dendrobium melaleucaphilum can grow quite large. A rhizome may produce up to 100 pseudobulbs 32 inches or more long. The pseudobulbs of Den. melaleucaphilum tend to remain more upright than those of Den. tetragonum. The 4 inch to 6 inch flowers, paler than those of Den. tetragonum, are typically a pale cream colored to a greenish cream colored or even bronzish with red or red-brown margins of varying intensity and thickness. The broadly flared labellum is superficially similar to that of Den. tetragonum, white with red markings, but tends to roll under in most clones. There are usually four or five flowers per raceme but some plants have eight. The somewhat unpleasant scent is different from Den. tetragonum. Flowering is in early spring.
High light levels are necessary for this species to flower prolifically in cultivation. A regular feeding program and ample water are recommended.
Dendrobium capitisyork is endemic to Cape York (Iron Range to Eungella Range, Queensland) and is distributed across a range of altitudes of 333 feet to 2,666 feet along the coast. This resident of the dense tropical rainforest is found in heavily shaded areas clinging to the trunks and branches of trees, usually along streams. It also occurs in the more open and dry deciduous scrub close to the ground.
Mature plants are similar in size and appearance to Den. melaleucaphilum, with penduous pseudobulbs up to 20 inches. The flowers are entirely different: yellow to gold in base color with varying amounts of red spotting on the segments and occasionally a red margin. Flowers up to 6.5 inches tall are not uncommon. Racemes 1/2 to 3/4 inches long bear one to three flowers. The labellum, white with red markings, is different from the other species. The midlobe is thin and pointed, and the lateral lobes are very much larger than the midlobe. Unlike the other species, flowering occurs throughout the year; the peak flowering period is October to May in the U.S. Common faults of this species include hooked sepals or lateral sepals that cross. Due to its flowering habit and ability to interbreed with other sections of Dendrobium, Den. capitisyork has been widely employed in hybridizing.
This species produces the strongest growths on those specimens raised in the greenhouse . Being a lowland tropical species, Den. capitisyork does not tolerate prolonged periods of cold as well as the other species. Flowering is more frequent in the heated house. This species is still relatively common in its natural habitat.
Dendrobium cacatua is the lesser-known species of this group. Many consider it to be a color form of Den. capitisyork, but the flowers have a distinctive shape. There are pure populations that do not occupy the same habitat as Den. capitisyork. This species is endemic to northeast Queensland (south to the Eungella Range). It is a high-altitude rainforest species found in cool, moist conditions with Dendrobium adae, Dendrobium fleckeri and Dendrobium agrosto-phyllum. Most plants grow low on tree trunks and are well-shaded. Pseudobulbs 20 inches or more in length are green-brown.
The flowers of Den. cacatua are pale green to greenish yellow or white, sometimes even bronzish in color. There may be pale red markings on the white labellum. Two to five flowers are borne on 3/4 inch long racemes. The flowers are more starry and the segments are thinner than those of the other species of this group.
These species are pendulous epiphytes. New pseudobulbs grow upward during their development, then become pendulous under their own weight as they mature. Leaves set at an angle to the pseudobulbs maximize the light-gathering surface. This is important in cultivation because plants mounted upside down will possibly die. These species are best grown mounted although, as seedlings, they do well started in pots. Suitable mounts include cork, jacaranda logs and milled timber or any normal orchid mount. Divisions attach quickly and readily to many mounts; wiry, multibranched roots are soon sent across and through the support. The main requirement is that the mount be durable and large enough to support a mature plant; re-mounting can cause a major setback. Being epiphytic, and often growing attached to the trunks of their hosts with little collected leaf litter, these species require very good drainage. The presence of roots in furrows of the bark of the host (or under the bark in the case of Den. melaleucaphilum) suggests the mount must also retain moisture after watering.
Despite the wide distribution of these species they will grow side by side in the greenhouse in temperate areas. They withstand light frost with minimal protection. The temperature range for these species is quite wide, from frost to in excess of 100 degrees (for short periods). The main requirements are high humidity and regular air movement.
Feed when in active growth after flowering to induce specimens to develop club-like pseudobulbs that flower prolifically. They are somewhat sensitive to water quality and quantity, particularly Den. melaleucaphilum. It will flower poorly when kept too dry during the winter.
Although the ranges of some species overlap, natural hybrids are unknown. Artificial hybrids between Den. melaleucaphilum and Den. capitisyork show intermediate characteristics. Crosses between selected plants within these species are known, and seedlings are usually readily available. Den. tetragonum has suffered somewhat in popularity due to being compared directly with the large and showy Den. melaleucaphilum. However, its intense color and incredible scent ensure that many plants are grown.
The star-shaped flowers of these species share many character-istics, although size and coloration vary considerably. The sepals are quite large in comparison to the thin, small petals. The dorsal sepal usually has a slight twist or kink, a feature often seen in hybrids (where it is considered detrimental). The labellum of each species is quite distinct, but they do share a tendency for the midlobe to roll under. This trait is particularly apparent in Den. melaleucaphilum. In some specimens, the lateral sepals may twist the “hips.” This undesirable trait can be quite pronounced in poorly grown specimens.
Flowering in all species occurs from nodes along the length of the pseudobulbs. This occurs more frequently as pseudobulbs age and the apical nodes are depleted. The flowers are scented, but this varies among the species.
In recent years, a number of Den. capitisyork have appeared with a heavily marked labellum that can be almost solid red in color. These have been used to produce more of their kind; the effect of these ‘red throats’ on hybridizing has been quite strong.
References: Clements, M. A., and D. J. Jones. 1990. Recently named Australian Orchid Taxa – I Dendrobium. Lindleyana 5(4): 235-243.
Mark and Allison Webb raise orchids at their home in New South Wales, Australia. Mark’s recently published book, The Essentials of Orchid Growing, with Gordon C. Morrison, is a practical introduction to the Orchidaceae. It is appropriate for growers of all levels of expertise who want to understand how orchids grow.
Used with permission of The American Orchid Society (AOS Bulletin, June 1992)