This is an extraordinarily large and diverse New World genus with an equally
diverse number of habitats. Oncidiums may originate anywhere from sea level
in the tropics to the high elevations of the Andes. This obviously makes
cultural generalizations difficult. More specific instructions may be available
from the grower. Some genera included are Aspasia, Brassia, warm-growing
miltonias (often called the Brazilian type) and many of their hybrids.
Light needs can vary from bright to nearly full direct sun depending
on the species. Most will thrive with one to several hours of sun a day.
Generally, thicker-leaved plants, such as "mule-ear" and "equitant"
oncidiums, can stand 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.
Temperatures for this group are generally considered intermediate
to warm: 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.
Water requirements vary with the type of plant. Generally, plants
with large fleshy roots or leaves need less-frequent watering than thin-leaved
or 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 10 days depending on 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.
Humidity should be between 30 and 60 percent. Many oncidiums require
less humidity than other orchids. Most green-houses have adequate humidity.
In the home, placing the plants above moist pebbles in trays is ideal.
Fertilize 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 formulation should be used on plants in other
media or on slabs. If skies are cloudy, applications once a month are
Potting 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 large-rooted plants; the
standard size is 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. Firm
the medium around the roots. Keep humidity high and the potting medium
dry until new roots form. Equitant and mule-ear oncidiums, as well as
other fleshy-leaved or large-rooted plants, can be grown on slabs of cork
bark or tree fern or in pots filled with a coarse, well-drained medium
such as charcoal. This allows the drying between waterings that these