Oncidiums may originate anywhere from sea level in the tropics, to the high elevations of the Andes. This obviously makes cultural generalizations difficult since they are such an extraodinarily large and diverse New World genus with an equally diverse number of habiats.
Light needs can vary depending on the species. Most will do best on one to several hours of sun a day. Usually thicker leaved plants can withstand more light. In a greenhouse, 20 to 60 percent shade is required, or about 2,000 to 6,000 foot candles, depending on the plants. In the home, east, south or west windows are ideal. Many types of oncidiums will grow under artificial light: Four fluorescent tubes supplemented with incandescent bulbs and placed 6 to 12 inches over the plants are necessary for proper growth. Metal-halide and sodium-vapor bulbs also provide sufficient light without needing to be so close to the plants.
For this group of orchids, temperatures considered intermediate to warm are best. 55 to 60 F at night, and 80 to 85 F during the day. Temperatures up to 95 to 100 F are tolerated if humidity and air movement are increased as the temperatures rise, a good general rule in any case.
Requirements vary with the type of plant. Generally, plants with large fleshy roots or leaves need less frequent watering than thinner leaved ot thin-rooted plants. Watering should be thorough, and the medium should dry at least halfway through the pot before watering again. This may be every two to ten days, depending on the weather, pot size and material, type of orchid and type of potting medium. Plants not actively growing should be watered less; many species have winter rest periods.
Should be between 30 and 60 percent. Many Oncidiums require less humidity than other orchids. Most greenhouses have adequate humidity. In the home, placing the plants above moist pebbles in a tray is ideal.
Should be done regularly while plants are actively growing. Applications of 30-10-10 formulations twice a month are ideal for plants in a bark based potting medium. A 20-20-20 formultion should be used on plants in other media or on slabs. If skies are cloudy, applications once a month are sufficient.
Should be done when new growth is about one-half mature (which is usually in the spring). Fine-grade potting media are usually used with fine-rooted plants and coarser mixes with larger rooted plants; the standard size is of a medium grade. The plant should be positioned in the pot so that the newest growth is farthest away from the edge of the pot, allowing the maximum number of new growths before crowding the pot. Spread the roots over a cone of potting medium and fill in around the roots. Keep humidity high and teh potting medium dry until new roots form.