Oncidiums are very popular because they are easy to grow and have long lasting sprays of small, distinct flowers which often have unique color combinations and a pleasant fragrance. They typically bloom once a year and can be grown in the home as well as in a sheltered area in the garden. Oncidiums have been combined with Odontoglossum and other related orchid types through hybridizing to offer a seemingly endless array of striking color patterns.
Most Oncidiums and their hybrids do best under the same temperature and light conditions in which Cattleyas thrive: intermediate temperature ranges: 60° F at night; 70° to 85° F day and bright light — 1500 to 2000 ft. candles. Leaves should be medium green to slightly yellow or red-tinged, never dark, glossy green. Reddish green indicates too much light; dark green indicates not enough light.
Water, Fertilizer, and Humidity
Oncidiums should become moderately dry between waterings. Generally, they require more water while the new shoot is growing and less once the bulb has formed. Take care to keep water out of the new growth at the base of the plant. Never allow the bottom of the pot to stand in water. Never use artificially softened water. Plants not actively growing should be watered less; many species have winter rest periods.
We recommend the use of Grow More Premium Orchid Food at half strength dilution at every watering, year-round.
The ideal humidity for Oncidiums is between 55-75%, but 30-40% is workable. If leaves are growing crinkled in an accordion fashion, it is lacking humidity. Try giving more water and placing on a humidity tray or bed of gravel. Misting the plant may help but make sure the water is at least room temperature.
Repot Oncidiums at least every two years. As a general rule, repot them when the new shoot is two to three inches tall or when new roots appear. All the old mix should be removed from the roots and any dead roots should be removed. If dividing, keep the divisions in clumps of three to five mature bulbs. Replant in Kiwi premium grade pine bark from New Zealand which has been pre-moistened, placing the base of the new growth about 1/2″ (no deeper) into the bark.
Water thoroughly with SUPERthrive (add one drop to watering can), then keep bark barely damp until you see the new roots penetrating the bark; resume normal watering.
Our source for much of this information is the American Orchid Society Guide: Growing Orchids.