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February 2001 This Month ONT will focus on: "The Genus Thunia" Written By Mr. F. A.
Marais. According to literature there are approximately six species within the genus, which differ only slightly from each other. This may be the reason why some taxonomists regard certain species, within the genus, to be con-specific (a term used to describe individuals belonging to the same species). This article will only deal with Thu. marshalliana because it is the only species with a fair representation in literature and it is readily available in the orchid trade.
Thunia marshalliana (Close up of flowers) Owner of the plant: Mrs. H. Ströh Photographer: Frikkie Marais HISTORY Thunia marshalliana was discovered at Moulmein in Burma; described by H.G. Reichenbach in 1887, and named after Count Thun of Hohenstein, who had an important collection of orchids about the middle of the nineteenth century. PLANT DESCRIPTION Thunia marshalliana is a herbaceous, sympodial, epiphytic/terrestrial orchid, which form clumps of erect, biennial leafy canes/stems terminating in a raceme of white showy Coelogyne-like flowers with yellow and orange-marked lips. Plants can grow to a height of between 50cm and 100 cm. The leaves on the new growths are grey-green and fall off soon after flowering. The older canes/stems are usually bare and silver-grey in colour and stay on the plant for another year. During the first year the cane/stem is in active growth and flowers at the end of the growing season. The second year it loses its leaves and serves as a food reservoir for the new growth. After the new growth is mature it has used up all the energy from the older cane, which then shrivels and dies. NATURAL HABITAT Thunia marshalliana grows either epiphytically or terrestrially up to an elevation of 2000 metres in the tropical and sub-tropics. It has a very wide distribution occuring from Burma, Thailand, South China and India (most of these places are subject to humid monsoon conditions during their summer and cool dry conditions during winter). This wide distribution is also an indication of its ease of culture and willingness to adapt to various horticultural conditions. CULTIVATION The culture of Thunia marshalliana is very simple, especially since it can be cultivated as a terrestrial in South Africa. When the plants begin to grow, at the start of spring, the potting medium should be topped-up with a little mature compost or cow manure. For the first four to six weeks of the growing season, water should be given sparingly until the roots have made good growth. Because Thunias are very rapid growing "hungry" orchids, they should be planted in a moisture-retaining but well drained and aerated compost/potting soil. Most of the plants in the Pretoria region flower between December and January after which the plants should still be watered and fertilised regularly with a balanced fertiliser until the end of the growing season, which is during April/May. They should be grown in a fairly bright situation with adequate fresh air movement to ensure healthy plant growth. The ideal temperature during the growing season should be between 18ºC and 28ºC, however, during winter (the dormant season) the temperatures can drop to 10ºC or even lower without damaging the plants, provided that they are kept fairly dry. My plants grow outside in Pretoria under ±30% shade where the winter temperatures regularly drop to 1ºC for a few days. During winter the plants should be kept in a cool rather dry atmosphere and be given only enough water to prevent the canes/stems from shrivelling (maybe once every two weeks). Most of the roots also seem to die during this period, resulting in a "floppy" plant, and it might be wise to support the canes/stems until the new roots can support the plant again. Although Thunia marshalliana grows rapidly into large plants it is important to know that it should always be potted in rather small pots. Plants potted in large pots are slower growing and tend to rot easily.
Specimen plant in flower during December Owner of the plant: Mrs. H. Ströh Photographer: Frikkie Marais
PROPAGATION
This is one of the few orchids which can be profitably propagated by cutting the old mature canes/stems into lengths of about 15cm and rooting them in sand or sphagnum moss (similar to the soft-cane Dendrobiums). When rooted, the small plants may be planted and grown as described above. Allow approximately three years for the plant to reach flowering size.
Alternatively large plants can be devided into smaller divisions, each retaining at least two mature canes/stems. Plants divided in this manner will flower the same year, after completion of the new growth. Plants regularly also produce keikies from the terminal nodes on the canes/stems of the previous season, which can be removed and planted up as individual plants.
PESTS AND DISEASES
Although Thunia marshalliana is not generally troubled by many diseases except the occasional root rot, it is, however severely attacked by red spider mite when in active growth. Root rot can be controlled preventatively by applying VIRIKOP as a drench to the potting medium, whereas red spider mite should be controlled curatively with an insecticide like RED SPIDERCIDE, as a full cover application, at the recommended dose and frequently.
Diagram 1 - plant growth habit
SPECIES AND HYBRIDS IN CULTIVATION
The only species in general cultivation are Thu. alba, Thu. bensoniae, Thu. majorensis, Thu. marshalliana, Thu. venosa and Thu. winniana of which Thu. alba and Thu. marshalliana seems to be the most popular among the orchid growers.
Up to 1998 only three hybrids were registered within the genus namely:
REFERENCES: American Orchid Society Bulletin. Vol. 54/1200, 37/421, 40/687, 41/439, 60/1096, 37/421.
Bailey, L.H. 1930. Thunia. In: The Standard Cyclopedia of Horticulture, Vol.3. New York: The Macmillan Company: 3339-3340.
Bechtel, Cribb, and Launert. 1992. The manual of cultivated orchid species. Third edition. U.K. Blandford Press. 535.
Leroy-Terquem, G. and Parisot, J. 1991. Orchids: Care and cultivation. Durban (RSA): Bok Books International. 86-87.
Liberty Hyde Baily Hortorium. 1978. Thunia. In: Hortus Third, A concise dictionary of cultivated plants in the United States and Canada. Third printing. Macmillan Publishing Co. Inc. 1110.
Pradhan, U.C. and Pradhan, S.C. 1997. 100 Beautiful Himalayan orchids and how to grow them. Darjeeling (Kalimpong) India: Primulaceae Books: 120-121.
National Department of Agriculture Directorate Genetic Resources National Gene Bank Private Bag X973 Pretoria 0001
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