There are several hundred recognized Oncidium species, plus a large number of hybrids. For those preferring cool conditions see our page on Oncidium orchids (cool section).
The most easily available oncidiums, however, grow well under normal indoor conditions in the home as well warm conditions in the greenhouse. They have large pseudobulbs and a mass of thin white roots. The large leaves (up to 60cm/2ft) emerge from the pseudobulbs.
Oncidiums are more forgiving of bright, even direct, light than other popular orchids. They can cope with direct light in the morning and prefer bright or even very bright conditions. East, south or west facing windows are all suitable positions.
The leaves should be bright green as opposed to dark green or reddish green. Reddish green indicates too much light and dark green too little light.
- During the growing season, water every few days, but allow the compost to become a little dry between waterings
- In winter, reduce the watering to once a month or less. They can withstand considerable drought because their large pseudobulbs act as stores. Wrinkled pseudobulbs generally indicate a lack of water
Be careful not to overwater. Oncidiums have large, fleshy pseubobulbs and masses of roots; they are prone to rot. If you see a psuedobulb beginning to rot, cut it out with sterile secateurs and reduce the amount of water.
During the growing season, feed with a weak, diluted to 25 percent recommended strength, orchid fertilizer every second or third watering.
Generally, the most popular oncidiums, with small yellow flowers, large pseudobulbs and strappy leaves, are those classed as intermediate to warm orchids. These require a minimum temperature ranging from 13°C (55°F) to 25°C (77°F).
Warm-growing Oncidium can be found in many habitats, from semiarid subtropical lowlands to high mountain region cloud forests.
Oncidiums enjoy moist air, requiring a minimum of 40-50% humidity in the immediate vicinity of the plant. Humidity should be increased with higher temperatures. The ideal humidity is 55-75%. To increase humidity, place the pot on an upturned plant saucer in a tray filled with gravel and topped up with water. In addition, mist using a hand sprayer. In a greenhouse, the floor can be damped once or twice a day.
Give as much ventilation or air movement as possible, but avoid cold draughts.
Oncidiums are magnificent in flower. A large, well-grown plant may send out six or seven branched sprays of yellow flowers. The effect is very much like a cloud of butterflies each flower resembling dancing ladies.
Oncidiums do not like root disturbance and should only be repotted when either the compost becomes broken down or the pot becomes too small for the new growth. They like to be slightly underpotted in a very free-draining bark-based potting media. As a general rule, repot when necessary. Every two years is ideal when the new growth is 5-7.5cm (2-3in) long.
Oncidiums produce new pseudobulbs from the bases of the existing ones. When repotting allow sufficient room for the plant to increase its diameter by two or three new pseudobulbs and root down into the compost. New pseudobulbs should not overhang the sides of the pot.