How to grow oncidium orchids (warm growing)

What are ‘warm growing’ oncidiums?

These are tropical and sub-tropical orchids, popularly grown as houseplants. They are magnificent in flower, with one plant producing multiple branching stems of small, dainty blooms. The effect is like a cloud of exotic butterflies, and if you look closely, each individual flower resembles a dancing lady. The flowers are often yellow, but can also be pink, reddish-purple or white, and they can be flecked or striped with maroon or white. Narrow evergreen leaves, up to 60cm (2ft) long, sprout from large pseudobulbs (swellings) at the base of the plant.

In addition to these ‘warm growing’ oncidiums, there are also ‘cool growing’ oncidiums (formerly called Odontoglossum) – for advice on growing these, see our guide to cool growing oncidiums.

Choosing warm growing oncidiums

There are many species and hybrids to choose from, with flowers in various colours and often speckled or streaked.

For inspiration, visit the Glasshouse at RHS Garden Wisley, which is home to over 3,000 orchids, including many oncidiums. Also visit RHS Shows, where you’ll find spectacular displays by leading orchid nurseries.

Buying oncidiums

For the widest choice, try specialist orchid nurseries and tropical plant suppliers. Oncidiums may also be available from larger houseplant retailers, especially online.

Buying: garden centre plants

Buying: mail order plants

How to choose healthy plants

These oncidiums can usually stay in their original pot for the first couple of years. They prefer their roots to be quite tightly packed and don’t like root disturbance, so only repot when necessary – once the bark

Bark is the outermost layer of woody plants (trees, shrubs and woody climbers). It is several cells thick and provides protection against physical damage, disease and environmental stresses. Bark comes in a wide variety of colours and patterns, and these can help gardeners when identifying plants. The fissures and crevices of bark on older plants also creates valuable habitat for many garden creatures as well as lichens and small plants.

-based compost

Can refer to either home-made garden compost or seed/potting compost:

• Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

has deteriorated or there is no room in the pot for new growth. This will be about every two years. Repot after flowering, either in February to April or September to October.

Oncidiums produce new pseudobulbs (swellings at the base) beneath the existing ones. When repotting, allow sufficient room for the plant to increase its diameter by two or three new pseudobulbs and root down into the compost. New pseudobulbs shouldn’t overhang the sides of the pot. Oncidium roots are very delicate, so handle carefully when repotting. Cut off any dead or unhealthy roots, which may be brown, soft or papery. Healthy roots should be white and firm.

Replant in the new container using very free-draining, bark-based orchid compost. Oncidiums like plenty of air around their roots, so keep the compost loose. Position the plant at the same level it was previously growing.

Also see our guides to repotting houseplants:

How to repot a plant

Location

These oncidiums like a warm, bright location indoors. They are more forgiving of bright or even direct light than other popular orchids. They can cope with direct sun in the morning or evening, and prefer bright or even very bright conditions. East- and west-facing windowsills are perfect, and south-facing windows are usually fine if plants are given a little shading from intense summer sun. Just take care when in flower, as strong sun can damage the blooms. These oncidiums like as much ventilation

The introduction of outdoor air into a space, for example, a greenhouse. This airflow can reduce temperature, replenish the gases absorbed by plants and keep down fungal diseases like grey moulds.

or air movement as possible, but not cold draughts.

Temperature

Warm growing oncidiums, as the name suggests, like warmth – they generally need about 18–25°C (65–77°F) in the day and 13–17°C (55–63°F) at night. A drop in temperature at night helps to encourage flowering. Windowsills are ideal locations, as they are usually cooler at night.

Watering

• From spring to autumn – water every few days, but allow the compost
  

  Can refer to either home-made garden compost or seed/potting compost:

  
  • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

  to become a little dry between waterings
• In winter – reduce watering to once a month or less. They can withstand considerable drought because their large pseudobulbs (swellings at the base) act as water stores. Wrinkled pseudobulbs generally mean they aren’t getting enough water.

Use rainwater or filtered water whenever possible, especially in hard water areas. Water from above, making sure any excess can drain away. Don’t leave plants sitting in water, as this can lead to rotting.

Be careful not to overwater oncidiums, as their large fleshy pseudobulbs and roots are prone to rot if the compost is too damp. If you see a pseudobulb beginning to rot, cut it out with sterile secateurs then reduce watering.

Humidity

These plants love humid air, especially in warm temperatures. The ideal humidity

The concentration of water vapour in the atmosphere. This affects the rate at which plants transpire (lose water from their leaf pores), which in turn affects how much water their roots absorb from the soil. If air humidity is low, plants transpire quickly and become at risk of wilting if water is lost from the leaves faster than it can be absorbed by the roots. Different plants require different levels of humidity to thrive. Many tropical houseplants need high humidity and are best grown in a steamy bathroom, misted regularly or placed on a saucer of damp pebbles. In a greenhouse, humidity can be raised in hot weather by damping down (wetting) the floor or overhead misting.

level is 55–75 per cent, and it should never be below 40 per cent. To increase humidity:

• Place the pot on an upturned plant saucer in a tray of damp gravel
• Mist the leaves regularly

For more tips, see our quick video guide:

Feeding

Warm growing oncidiums need regular but light feeding throughout the growing season

The period of time when an individual plant is in active growth. This will depend on the local climate and light levels, and can vary between different plants, although it is broadly from spring to autumn.

. Apply a weak orchid fertiliser, diluted to quarter-strength, every second or third watering from spring to autumn.

Encourage flowering

These oncidiums usually flower in autumn to winter, sending up several branching flower stems that carry lots of small yellow blooms. A large plant can produce six or seven sprays of blooms.

A drop in temperature of about 5°C (10°F) at night may help to stimulate flowering. A windowsill is an ideal location for them, as the temperature will usually fall naturally at night.

RHS/Louise Bargus

No pruning or training is necessary, apart from cutting off the old flower stems at the base after all the blooms have faded.

Many warm growing oncidiums form large clumps over time. These can be divided into several smaller new plants when repotting, which is best done after flowering in February to April or September to October. It’s a simple process:

• Remove the plant from its pot
• Cut the clump into several sections, each with at least three pseudobulbs (swellings at the base)
• Pot up each new plant into its own small container, using bark
  

  Bark is the outermost layer of woody plants (trees, shrubs and woody climbers). It is several cells thick and provides protection against physical damage, disease and environmental stresses. Bark comes in a wide variety of colours and patterns, and these can help gardeners when identifying plants. The fissures and crevices of bark on older plants also creates valuable habitat for many garden creatures as well as lichens and small plants.

  -based orchid compost
  

  Can refer to either home-made garden compost or seed/potting compost:

  
  • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

  . Depending on their size, the new plants may take a few years to start flowering

Warm growing oncidiums are generally healthy and disease free. But if their growing conditions aren’t correct, they may develop the following symptoms:

• Wrinkled pseudobulb – usually due to a lack of water
• Black, rotten roots – a sign of overwatering
• Reddish-green leaves – too much light
• Dark green leaves – too little light

Also look out too for sap

Sap is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

-sucking insects, including:

• aphids
• mealybugs
• red spider mites