Generally known as the Dancing Lady Orchid, the varicosum or Strap-Leaf Oncidiums are all derived from species that have similarly shaped flowers, inflorescences and leaves. The plants have small to large bulbs that carry one or more, large to medium sized thin, flexible wide leaves which are strap like in shape. The inflorescence or flower spike arises from the axil of the leaves on new-season bulbs. The flower spikes are long, branched and very flexible, and sway easily in the slightest breeze. The flowers can have large, bright yellow labellum or lip which people likened to the skirt of a ballerina. As the flower stems sway in the breeze the flowers dance, giving the impression of dancing ladies.
Most varicosum Oncidium hybrids have been derived from four species and one natural hybrid:
This spring flowering species originates in Brazil. Its flowers are 5-7cm (2-3") in diameter, dark brown in colour, and have wide petals and sepals. The labellum is somewhat small. This species has been used to improve the width of the petals in hybrids.
The flowers, which appear in autumn, are slightly smaller than those of Oncidium crispum but have the same colouring and general shape.
This species flowers in autumn and prefers much more humidity than the other species. The flowers are similar in shape to those of Oncidium crispum but the segments have yellow or golden flecked borders. This species has been used to impart spotting to the hybrids and improve the size of petals.
This is the original Dancing Lady with its large, bright yellow lip, which dominates its hybrids. The petals are small, dull yellow, and barred in red-brown. The barring of the petals often adds contrast and interest in its hybrids. It flowers in autumn.
This is now considered to be a natural hybrid between Oncidium diasryle and Oncidium forbesii. It is similar in shape and colouring to its two parents but has inherited the yellow marking on its petals from Oncidium forbesii. It is usually winter flowering.
Growth habit and Ecology
The key to successful culture of any orchid is an understanding of its habit of growth and its natural habitat (ecology). The varicosum group of Oncidium, are medium-sized epiphytic plants that have small to large pseudobulbs joined by a creeping rhizome. The flexible, long, wide leaves are carried on top of the pseudobulb in most species. The roots range far and widely over the limbs and trunks of the tree hosts. The plants grow on the inner branches or trunks of the trees where it is shadier and more humid.
Some species (Oncidium forbesii) prefer to dry out during its resting period in winter, while others (Oncidium varicosum) appear to need continuous humidity and moist conditions and have little or no resting period.
Most of the species come from the coastal mountains of southeast Brazil, where the minimum temperatures may fall to 5-6 deg C during the relatively dry winter. Summers are humid and moist with temperatures in the high twenties and low thirties.
Pots or Slabs
Some people have been able to grow varicosum Oncidiums in coarse fir bark composts in plastic or terracotta pots. However the rambling nature of the plants soon takes them over the edge of the pot. It is easier to mount the plants on natural or compressed cork, hardwood palings, or slabs of treefern. The plants can be tied to their mounts with fishing line, plastic coated wire, pantyhose or knitting nylon. Make sure the plants are firmly, but not too tightly tied to their mounts, and that the new, emerging shoot is closest to the mount and pointed towards the top of the mount. The roots like to roam freely and a large mount should be chosen to provide plenty of space for root growth.
Air movement has three important roles to play in the successful culture of varicosum Oncidiums. Firstly it provides the carbon dioxide needed for transformation into sugars. Secondly it cools the leaves thereby preventing overheating. Finally, air movement dries the leaves rapidly and so prevents fungal spores from germinating and infecting plants.
In their native environment, the varicosum Oncidiums grow on the inner branches or trunk of their host trees where they get every passing breeze but high humidity. If you can provide a similar buoyant atmosphere in the shade-house, they will grow well for you. However, if the breezes of the shade-house are too strong or too dry, the atmosphere will lack humidity and the plants loose too much water. The bulbs shrink and become very shrivelled. To provide the right amount of air movement and humidity, it may be necessary to grow the plants low down on the walls of the shade-house where a fan keeps the air moving continuously.
Varicosum Oncidiums prefer "cool" to "intermediate" temperatures. In their natural habitat the temperatures would be similar to those experienced in the coastal ranges of southeast Queensland (or the Manly-Warringah peninsular area). Heavier shading may be needed to keep the temperatures down to about 25-30 deg C during summer, but no heating would be needed at any time of the year. If your area experiences harsh winter temperatures, provide sheltered conditions sufficient to maintain temperatures above 5 deg C.
When and how much to give are always problems with any orchid. Remember that varicosum Oncidiums come from an area that has a wet, humid summer and a relatively dry winter. Some species (Oncidium varicosum) like wetter, or more humid conditions, while most others prefer to dry out before rewatering. During spring and summer, water every day or second day. In autumn, water once a week, and during winter, mist just enough to prevent the plants dehydrating and the bulbs becoming shrivelled.
Light and Shelter
In nature, most of the species prefer mottled bright light, slightly more subdued than that experienced by the single leaf Cattleya species. None will tolerate the bright, hot light preferred by Dendrobiums and vandaceous orchids. Bush or shade-house covered with cloth giving 60-70% shade are sufficient in most areas, but in regions where significant rain falls during winter, it may be advisable to provide weather proof area under fibreglass or plastic sheeting.
Remember that Varicosum Oncidiums and medium-sized plants need only moderate amounts of fertilisers to grow. Also if you use cork mounts the plants will receive no food from their mounts and so will rely on you to provide all their needs. My plants get no special feeding; the get whatever is going along with the other plants in the collection. Remember that they should be fed during their season of active growth; so feed during spring and summer.