Oncidiinae Culture

Reprinted from the August 2000 issue of Orchids
-- The magazine of the American Orchid Society

Light A good indication of proper light is the color of the leaves: they should be bright green as opposed to dark green (too much shade) or reddish green (too much light). I have often moved a plant that was large and strong, but which had not yet bloomed, into brighter light conditions and watched as it responded almost immediately by sending up spikes. Care must be taken when doing this, however, that the increased light is not too great or injury to the leaves will result.

Temperature If your temperatures are seldom above 100 F or below 45 F, then temperature-tolerant Oncidiinae should perform well.

Humidity Provide a humidity range of 50 to 90 percent. Seedlings benefit from 70 percent or higher humidity. Generally, humidity should be increased as temperature, light intensity and air movement increase. This can be accomplished by misting the plants and periodically wetting down the greenhouse floor with water.

Air Movement Adequate air movement reduces leaf temperature, thereby allowing high light intensity and more vigorous growth. An additional advantage is the reduction of fungal and bacterial infections, which high humidity alone might otherwise cause.

Watering Quantity Oncidiinae intergeneric hybrids should be kept somewhat moist. Never allow them to dry out completely, but be sure to provide for excellent drainage, as soggy or waterlogged conditions are not conducive to healthy growth. Generally, they require more water when making new growths, and less once the pseudobulbs have developed. When watering, be sure to water copiously to ensure a thorough wetting of the potting medium, and to reduce the buildup of toxic minerals.

Fertilizing These plants respond favorably to a balanced formula, such as 20-20-20, at 1/2 strength, every second or third watering. If potted in fir bark, a high-nitrogen fertilizer, such as 30-10-10, will be required. Fertilizing can generally be increased during warmer weather when plants are in active growth, while the reverse is true in cooler months. Potting As a rule, repot every two years. Exceptions are those plants in rock wool or mounted on tree-fern slabs or cork bark; these should be repotted only when the medium starts to break down or the plant has outgrown the slab. Plants potted in sphagnum moss may need to be repotted every 12 to 18 months. While both plastic and clay pots offer advantages, the heavier clay pots are suggested for plants with tall spikes to help prevent plants in flower from tipping over. Repot when new growth is 2 or 3 inches tall or when the new roots first appear. Remove any organic mix and trim off dead roots with a sterilized blade. If it is necessary to divide, keep at least three to five mature pseudobulbs together. Water the newly repotted plants lightly until the new roots have penetrated the medium, then resume normal watering. Pot size will depend on the size of the plant and its root system. Small, shallow pots allow room for one- or two-years’ growth.

Potting Media Any porous mix with good water-retentive qualities can be used. Our preferred medium is rock wool, an inert material made from molten spun rock. We combine two parts Grodan Stone Wool with one part expanded perlite with excellent results. The advantage to rock wool is that it will not break down, and thus can be left on the root system indefinitely if particular attention is paid to flushing the medium routinely with pure water to remove accumulated salts. Other possible media are New Zealand sphagnum moss, tree fern, fir bark, and all sorts of combination mixes. The key is not to allow them to completely dry between waterings. Keep them slightly moist at all times while providing good drainage. Primary hybrids with Brazilian miltonias do well in fingernail-size horticultural charcoal. — Milton O. Carpenter.

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