Growing Oncidium indoors and in greenhouses
Elegant and floriferous, Oncidium orchids form a large and diverse group ranging from cool-growing mountain species to warm-growing lowland ones. Many are popularly known as “dancing lady” orchids, producing arching sprays of long-lasting flowers that move freely in the slightest air current. Historically, several closely related genera – including Odontoglossum – were grouped separately, and although modern taxonomy has merged them into Oncidium, their cultural requirements remain distinctly different. Understanding whether a plant is a cool-growing Odontoglossum-type or a warm-growing Oncidium is essential for long-term success.
OVERVIEW
Oncidiums, in a broad sense, are distributed across Central and South America, from Mexico and the Caribbean to Brazil and Bolivia. ‘True' oncidiums typically originate from lower elevations with brighter light and warmer, more seasonal conditions. By contrast, Odontoglossum-type oncidiums are native to cool Andean cloud forests, where temperatures are moderate year-round and humidity is consistently high.
All oncidiums are sympodial orchids, with flattened pseudobulbs that store water and nutrients. New growths arise from the base of mature pseudobulbs and flower once mature, while older pseudobulbs continue to support the plant and should not be removed prematurely. Growth tends to be seasonal, with flowering linked to the maturation of new growths.
Oncidium Swizzle
Oncidium Prince Ahmad
Oncidium alexandrae
CARE ESSENTIALS
■ Light
Cool-growing Odontoglossum types
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Bright but diffuse light – a hand held 30cm above the leaves at noon should cast a semi-distinct shadow.
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Best in north or east-facing windows.
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Avoid strong direct sun, which quickly causes leaf scorch and overheating.
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Leaves should be medium green; yellowing or red tints indicate excess light.
Warm-growing Oncidium
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Brighter light is required – a hand held 30cm above the leaves at noon should cast a more distinct shadow.
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East- or west-facing windows are ideal; south-facing windows are suitable with light shading in summer.
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Tolerate some direct sun, especially in the morning or late afternoon.
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Dark green leaves usually indicate insufficient light and plants are unlikely to flower.
■ Temperature
Cool-growing Odontoglossum types
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Daytime range: 15-20°C; night-time 10-14°C.
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The drop from day to night-time noted above of c.5°C is essential for vigour and flowering.
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Prolonged temperatures above 22°C can result in stress, leaf loss or failure to flower; plants will stop growing until optimum temperatures are restored.
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Best suited to cool rooms or temperature-controlled greenhouses.
Warm-growing Oncidium
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Daytime range: 18-25°C; night-time 13-18°C.
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The modest temperature drop noted above from day to night-time of c.4-6°C is beneficial but not critical.
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Tolerate short periods of higher temperatures if humidity and airflow are adequate.
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Well-suited to typical UK household conditions.
■ Watering and humidity
Cool-growing Odontoglossum types
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Keep the potting medium evenly moist but never waterlogged.
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Roots are sensitive to drying and to stale, wet conditions.
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Daytime humidity of 60-80% is ideal, dropping at night to prevent fungal issues; constant but gentle air movement should be maintained.
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Use rainwater, RO or deionised water wherever possible as these plants are particularly salt-sensitive.
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Reduce watering only slightly in winter; do not allow prolonged drying.
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Ideally stand plants on capillary matting or a tray of damp gravel over the summer months to create a moist microclimate around the plant, particularly as summers are getting warmer.
Warm-growing Oncidium
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Water thoroughly when the medium approaches dryness.
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Daytime humidity of 50-70%, dropping at night to prevent fungal issues, is sufficient for most hybrids.
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Slightly drier conditions are tolerated in winter when growth slows.
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Pseudobulbs should remain plump; excessive shriveling indicates under-watering.
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Ideally stand plants on capillary matting or a tray of damp gravel over the summer months to create a moist microclimate around the plant, particularly as summers are getting warmer.
■ Fertilising
Both groups
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Feed lightly, “weakly, weekly” during active growth using a balanced fertiliser at a maximum of 0.5 microsiemens.
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Reduce feeding during winter or when growth slows.
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Flush pots often with plain water to prevent salt accumulation, leaf tip burn and root damage.
■ Potting and repotting
Both groups
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Potting medium: medium-grade bark with added perlite or charcoal; cool growers often benefit from the addition of a small amount of sphagnum to improve moisture retention.
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Repot every 1-2 years, or when the medium begins to break down. Be gentle with the fine roots where possible during repotting as they resent root disturbance.
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Best time to repot is when new growths begin producing fresh roots.
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Plants prefer to be slightly under-potted.
QUICK ID GUIDE
Is your oncidium a cool-growing Odontoglossum type or a warm-growing Oncidium?
■ Cool-growing Odontoglossum types
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Flowers often large, starry or flat-faced, in a range of colours, frequently with bold markings or spotting.
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Inflorescences usually upright to gracefully arching rather than freely branching.
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Leaves tend to be softer, thinner and more prone to stress marks if conditions are unsuitable.
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Plants often struggle or decline in warm, centrally-heated rooms.
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Traders may still refer to them as despite modern reclassification.
■ Warm-growing Oncidium
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Flowers typically smaller but produced in greater number; the lip is more prominent than the petals and sepals, and many are predominantly yellow (“dancing lady” types).
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Inflorescences often highly branched and freely arching.
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Leaves generally firmer and more tolerant of household conditions.
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Plants grow and flower well in typical indoor temperatures.
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Available in garden centres and supermarkets.
If unsure:
When in doubt, treat the plant as a cool grower initially and observe its response. Heat stress develops more quickly and irreversibly than mild cool stress.
COMMON ISSUES
■ Cool-growing Odontoglossum types
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Leaves yellowing or collapsing rapidly: usually caused by excessive heat, particularly combined with low humidity. Move plants to a cooler position and improve airflow.
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Failure to flower despite healthy growth: often due to insufficient night-time temperature drop or light that is too low.
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Root loss despite regular watering: typically linked to stale, waterlogged compost or salt build-up from hard water. Improve drainage and flush frequently with pure water.
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Leaf spotting or fungal issues: high humidity without adequate air movement is the most common cause. Increase airflow rather than reducing humidity.
Oncostele Margaret Holm
Oncidium Sweet Suger
■ Warm-growing Oncidium
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Strong vegetative growth but no flowers: almost always caused by insufficient light. Gradually increase light levels, especially in winter.
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Shrivelled pseudobulbs: indicates underwatering or roots that are no longer functioning. Check root health and adjust watering accordingly.
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Leaf tip burn: often caused by fertiliser salts or hard water. Reduce feed strength and flush pots regularly.
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Bud blast: may result from sudden temperature changes, cold draughts, or drying out during bud development.
SUMMARY
Oncidiums encompass orchids suited to a wide range of growing environments, but success depends on recognising whether a plant is from the cool-growing Odontoglossum group or a warm-growing Oncidium. Odontoglossum types reward growers who can provide cool temperatures, high humidity and diffused light, while warm-growing Oncidium are among the most adaptable and floriferous orchids for UK homes and greenhouses. By matching culture to origin, growers can enjoy vigorous plants and spectacular floral displays across this remarkably diverse group.