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Rhododendron

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Rhododendron 'Spicy Lights'
Photo: Jardin botanique de Montréal (Gilles Delisle)
Rhododendron 'Spicy Lights'
  • Rhododendron 'Spicy Lights'
  • Rhododendron 'Mary Flemming'
  • Rhododendron 'Ingrid Melquist'
  • Rhododendron 'Christo Rey'

Onglets

Botany

Origin and description

Azaleas, once known as genus Azalea, are now included in genus Rhododendron, although most people usually consider rhododendrons and azaleas to form two separate groups. In this case, the name azalea is usually applied to small rhododendrons with small leaves and small five-stamened flowers that are often grown as indoor plants. The name azalea includes several species and numerous hybrids. They are sold as potted flowering plants and rarely grow to more than 50 cm indoors. The many cultivars have single, semi-double or double blooms in shades of white, pink, red and salmon. The flowers appear from December to May depending on the cultivar (early, mid-season or late).

Species, cultivars and related plants

Azaleas are often sold as Azalea indica or Rhododendron indicum. Rhododendron x obtusum (kurume azaleas) are very compact, bushy hybrids. The stems are covered in fine brown hairs. The Rhododendron simsii (Indian azalea) species has been used to create many large-flowered or double-flowering hybrids. The flowers are grouped at the tips of the stems.

Toxicity

Careful, this plant is toxic. Keep it out of the reach of children and pets.

Common name

Rhododendron / Azalea

Latin name (genus)

Rhododendron cvs.

English common name

Rhododendron / Azalea

French common name

Rhododendron / Azalée

Botanical family

  • Ericaceae
Horticulture

Growing conditions

Azaleas like cool temperatures and bright indirect sunlight, especially during the blooming period. It is best to place the plant near an east- or west-facing window. An azalea that is already in bloom when purchased should be placed in a cool room (about 16ºC) out of direct sunlight. In these conditions, the blooms may last for four to six weeks. Starting in June, after all risk of spring frost is past, you can move the plant outdoors, to a partially shaded spot, but it must never lack water. Bring it indoors before any risk of frost and give it nighttime temperatures ranging from 10 to 15ºC for four to six weeks to encourage it to bloom the next spring. To keep relative humidity high, you can set the pot on a saucer filled with gravel and water, being careful not to let the base of the pot sit in the water.

Easy to grow?

Azaleas are difficult to grow because they do not like high temperatures and dry air. They also need a period with cool temperatures to stimulate the blooming period.

Watering and fertilizer

Avoid using hard water on azaleas. If your tap water is hard, use rainwater or distilled water instead. When the plant is in bloom, the soil should be kept moist but not drenched. Never leave water standing in the saucer. After the plant has finished blooming, let the top few centimetres of soil dry out between waterings, but never let the roots get completely dry. Fertilize with an acidifying fertilizer, like rhododendron and azalea food, every other week from April to October. Moisten the soil before feeding to avoid burning the roots.

Pruning and maintenance

Remove faded blooms. After the plant has finished blooming, pinch back the new shoots to encourage the formation of lateral stems. Prune if necessary to keep the plant’s shape and remove any dried stems. Cut back to just above a bud. The plant should not be pruned after early July, because flower buds are initiated starting in August.

Repotting

Repot the plant every two or three years after it has finished blooming. Use a well-drained, acidic potting mix without horticultural lime – for instance, three parts peat moss to one part perlite. Because azaleas have shallow roots, it is best to use wide, shallow pots (called azalea pots).

Propagation

By woody stem cuttings.

See also

Pests and diseases
  • Thrips

    Thrips

    Over 5,000 species of thrips have been identified around the globe, hundreds of them in North America. Western flower thrips (Frankliniella occidentalis) and onion thrips (Thrips tabaci) are the greenhouse species that cause the most damage in Quebec.

  • Tetranychus urticae

    Two-Spotted Spider Mite

    Two-spotted spider mites are not true insects, but tiny arthropods, just like other mites and ticks. With their four pairs of legs, they look like miniature spiders. They are so small that they are difficult to see, but they do weave tell-tale whitish webbing.

  • Trialeurodes vaporariorum

    Whitefly

    Whiteflies are major greenhouse and houseplant pests.

  • Cyclamen mites
  • Root rot
Physiological disorders
Symptoms Possible causes Solutions

Green leaves that dry out on the plant and drop off and faded blooms

Improper watering (too little or too much water)

Adjust watering
Never allow soil to dry out completely
Never leave water standing in the saucer

Flower buds drop off and leaf edges turn brown

Air too dry

Increase humidity or place plant on a saucer filled with gravel and water

Short blooming period

Lack of water
Temperatures too high

Immerse pot in room-temperature water and then let drain
Place plant in a cool room (about 16ºC) to extend the blooming period

Yellow leaves

Alkaline soil, excessively hard water or build-up of mineral salts in the soil
Too much water

Repot using acidic soil and do not use hard water for watering
Water soil repeatedly to flush mineral salts
Never leave water standing in the saucer

Failure to bloom

Late pruning

Prune after blooming