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ORCHID SPECIES CULTURE Charles and Margaret Baker Oncidium wentworthianum Bateman ex Lindley. AKA: Oncidium tenue Lindley. ORIGIN/HABITAT: Mexico and Guatemala. In Mexico, plants are found in the States of Vera Cruz and Chiapas. In Guatemala, they are found in the mountains of Santa Rosa. Plants grow as epiphytes in humid forests at elevations up to 4900 ft. (1500 m). CLIMATE: Station #76781, Minatitlan, Mexico, Lat. 18.0N, Long. 94.5W, at 90 ft. (27 m). Temperatures are calculated for an elevation of 4500 ft. (1370 m), resulting in probable extremes of 91F (33C) and 35F (2C). N/HEMISPHERE JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC F AVG MAX 61 65 68 71 74 72 71 71 70 68 64 61 F AVG MIN 51 52 54 55 58 58 59 58 58 56 53 52 DIURNAL RANGE 10 13 14 16 16 14 12 13 12 12 11 9 RAIN/INCHES 3.4 2.5 1.5 1.9 3.7 9.4 10.7 12.1 20.0 14.9 17.6 4.1 HUMIDITY/% 90 88 85 82 82 86 87 87 88 87 88 90 BLOOM SEASON * * ** *** ** * * * DAYS CLR N/A RAIN/MM 86 64 38 48 94 239 272 307 508 378 447 104 C AVG MAX 16.1 18.3 20.0 21.7 23.3 22.4 21.7 21.7 21.1 20.0 17.8 16.1 C AVG MIN 10.6 11.4 12.5 13.0 14.7 14.7 15.2 14.7 14.7 13.6 11.9 11.4 DIURNAL RANGE 5.5 6.9 7.5 8.7 8.6 7.7 6.5 7.0 6.4 6.4 5.9 4.7 S/HEMISPHERE JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN Cultural Recommendations: LIGHT: 2500-3500 fc. High light may be necessary to initiate blooms, but plants should not be exposed to direct midday sun. Strong air movement should be provided at all times. TEMPERATURES: Summer days average 71-72F (22C), and nights average 58-59F (15C), with a diurnal range of 12-14F (7-8C). These temperatures represent about the coolest conditions under which this species should be grown. Because of the range in habitat elevation, plants should adapt to conditions somewhat warmer than indicated. HUMIDITY: 85-90% most of the year, dropping to near 80% in spring. WATER: Rainfall is moderate to heavy almost all year, but conditions are slightly drier for 2-3 months in late winter and early spring. Cultivated plants should be watered heavily while actively growing; but their roots should be able to dry rapidly after watering, and the medium should never be allowed to become stale or soggy. FERTILIZER: 1/4-1/2 recommended strength, applied weekly while plants are actively growing. A high-nitrogen fertilizer is beneficial from spring to midsummer, but a fertilizer high in phosphates should be used in late summer and autumn. REST PERIOD: Winter days average 61-65F (16-18C), and nights average 51-52F (11C), with a diurnal range of 9-13F (5-7C). O. wentworthianum should adapt to conditions somewhat warmer than indicated. Winter rainfall remains rather high in the habitat, but water should be reduced somewhat for cultivated plants, especially those grown in the dark, short-day conditions common in temperate latitudes. Plants should not be allowed to dry out completely, however. Tomlinson (see references) reported that these plants do not go through the same dormant period required by many oncidiums and, as a consequence, need to be watered all year, although water should be reduced in the colder winter months. Fertilizer should be reduced or eliminated until new growth starts and heavier watering is resumed in spring. GROWING MEDIA: Plants may be mounted on tree-fern or cork slabs if humidity is high and plants are watered at least once daily in summer. Several waterings a day may be necessary for mounted plants during very hot, dry periods. Because most growers find it difficult to keep mounted plants moist enough, they are usually grown in pots using a very open, fast draining medium such as medium sized fir bark. Materials such as perlite that help hold the medium open and also retain some moisture are often added to the fir bark. Charcoal is often added to help keep the medium open and prevent it from becoming sour. Plants should be repotted when the medium starts to break down or when the plant has outgrown its pot. Repotting should be done just as new root growth is starting to enable the plant to become re-established in the shortest possible time before the added stress of hot, dry, summer weather. This is usually after flowering in late winter or early spring. MISCELLANEOUS NOTES: The bloom season shown in the climate table is based on cultivation records. Plant and Flower Information: PLANT SIZE AND TYPE: A large sympodial epiphyte to 17 in. (43 cm) tall. PSEUDOBULB: Usually 3-4 in. (7.5-10.0 cm) long, but pseudobulbs may be as large as 5 in. (12.7 cm). They are clustered, robust, ovoid-oblong, somewhat compressed, and become furrowed with age. They are dark green, but they are typically transversely barred with bands of tiny brown spots. They have thin, dry, fibrous sheaths at the base. LEAVES: 5-14 in. (13-35 cm) long by 0.6-1.1 in. (1.5-2.8 cm) wide. There are usually 2 rather leathery, sharply pointed leaves at the apex of each pseudobulb. INFLORESCENCE: Usually up to 39 in. (100 cm) long, but the arching, wavy, branching inflorescence may be as long as 84 in. (213 cm). The slender branches of the panicle may be short or rather elongated and are spaced well apart. The longer ones may then be branched again. The peduncle emerges from the base of the pseudobulb and is subtended by a leaflike sheath. FLOWERS: 3-10 relatively large, showy blossoms are carried on each branch of the panicle. The flowers are usually about 1 in. (2.5 cm) across. The spreading-reflexed sepals and petals are deep yellow, irregularly blotched with red-brown, and have undulate margins. The lip has pale yellow, rounded sidelobes and a yellow midlobe with some red-brown spots around the crest. HYBRIDIZING NOTES: Chromosome count is 2n = 56. REFERENCES: Ames, O. and D. Correll. (1952-1965) 1985. Orchids of Guatemala and Belize. Dover Publications, New York. Hamilton, R. 1988. When does it flower? 2nd ed. Robert M. Hamilton, 9211 Beckwith Road, Richmond, B.C., Canada V6X 1V7. Hawkes, A.  1987. Encyclopaedia of cultivated orchids. Faber and Faber, London. Tanaka, R., and H. Kamemoto. 1984. Chromosomes in orchids: counting and numbers. Appendix in Orchid biology: reviews and perspectives, vol. III. J. Arditti, ed. Comstock Publishing, Cornell University Press, Ithaca, N.Y. Tomlinson, P. Not dated (1986?). Oncidiums --a cultural guide. Wellington Orchid Society, Inc., 5 Norwich Street, Wellington, New Zealand. Williams, L.  1986. The Orchidaceae of Mexico. CEIBA 2(1-4):1-256. PHOTOS/DRAWINGS: . Copyright 1997, Charles O. Baker and Margaret L. Baker Sheet version 546395 ......................................................................... Please remember that this sheet is for your use only, and though it was provided free of charge, it may not be reproduced or retransmitted in any way without permission. ......................................................................... __________________________________________________________________________ "Orchid Species Culture" Charles & Margaret Baker, Portland, Oregon USA Orchid Culture & Pollination site http://www.orchidculture.com email <email@example.com> __________________________________________________________________________ "Orchid Species Culture Vol. 1 - Pescatorea, Phaius, Phalaenopsis, Pholidota, Phragmipedium, Pleione" 250 pages of culture information. 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