Oncidiums are popular indoor and florist orchids for a very good reason: their large sprays of flowers often sag with dozens of blooms. There are actually several hundred recognized oncidium species, but the naming isn't stable, so there is considerable flux as experts reclassify plants. Additionally, they have been freely hybridized. The most common oncidium grow well under normal indoor conditions. They have large pseudobulbs that arise from a mass of thin white roots. The large leaves (up to 2 ft.) emerge from the pseubobulbs. They flower in fall.
I find that oncidiums are much more forgiving of bright, even direct, light than other popular orchids, especially the phalaenopsis. Oncidiums can handle direct morning light and prefer bright to very bright conditions. They tend to enjoy the same light I give dendrobiums.
During the growing season, water daily or every other day. Be careful, though, because drainage is an absolute priority, and the potting media must be perfectly free draining. The plants can also be grown on slabs or in baskets. Because oncidiums have large, fleshy pseubobulbs and masses of roots, they are very prone to rot. If you see a psuedobulb beginning to rot, cut it out with sterile snippers and reduce the amount of water. In the winter, reduce watering to bimonthly or less. They can withstand considerable drought because of their large pseudobulbs. Wrinkled pseudobulbs generally indicate lack of water.
During the growing season, feed with a weak orchid fertilizer bimonthly or scatter slow-release pellets in the growing media at the beginning of the season. Although there are many species, in general, the larger the plant, the more heavily it will feed.
Oncidiums can be found in many habitats, from semiarid subtropical lowlands to cool and misty cloud forests. Generally, the most popular oncidiums, which feature small yellow flowers, large pseudobulbs and strappy leaves, are intermediate to warm orchids. Do not expose them to cold drafts or tempatures below about 50. Even temps in the mid-50s will cause the plant to slow its growth if they last too long.
Oncidiums are magnificent in bloom. A large, well-grown plant might send out six or seven branched sprays of yellow flowers. The effect is very much like a cloud of buttery butterflies. The most popular species include O. leucochilum, O. longipes, O. sarcodes, O. pulchellum, as well as many hybrids. Although oncidiums are known for their yellow flowers, other varieties are available. The O. Sharry Baby is sometimes called the chocolate orchid for its sprays of brownish flowers with a rich cocoa scent.
Potting and Repotting:
Oncidiums like to be slightly underpotted in a very free-draining bark-based potting media. Many of the oncidiums will form large clumps of pseudobulbs and develop into rather large plants. They can be easily divided into clumps when repotting. Just make sure you have at least three pseudobulbs in each division. In general, only repot when necessary.
Like many orchids, once an oncidium has adjusted to its conditions, they're not difficult plants to grow. My greatest success with these plants has come almost by accident: I inherited a very large one and didn't really have room for it, so I stuck it in the corner of my collection and essentially forgot about it. That fall, I was rewarded with a profusion of blooms. Since then, I've enthusiastically grown oncidium. Watch your water to avoid rot, make sure the big plants are fed well, give it lots of light, and your oncidium will produce.