Rhododendron viriosum - picture by Brian Walters

Rhododendron viriosum – picture by Brian Walters

Just about now many of the heritage gardens of Britain are coming alive with Rhododendron colour. Rhododendron is named from the ancient Greek words for  “rose” (rhódon)  and “tree” (déndron). It is a genus of over 1000 species of woody plants in the heath family, and are either evergreen or deciduous. Most species have showy flowers. Rhododendrons are extensively hybridized in cultivation, and natural hybrids often occur in areas where species ranges overlap.They were introduced to the UK in the late 18th century from the Himalayas and China.

There are over 28,000 cultivars of Rhododendron in the International Rhododendron Registry held by the Royal Horticultural Society. Most have been bred for their flowers, but a few are of garden interest because of ornamental leaves and some have ornamental bark or stems. Recent genetic investigations have caused an ongoing realignment of species and groups within the genus. Horticulturally, rhododendrons may be divided into the following groups:

  • Evergreen rhododendrons: the main category

  • Vireya (Malesian) rhododendrons: these are tender shrubs

  • Azaleas (a section of generally small-sized, small-leaved and small-flowered shrubs), further divided into deciduous and evergreen hybrids. They are distinguished from “true” rhododendrons by having only five anthers per flower.

  • Azaleodendrons – semi-evergreen hybrids between deciduous azaleas and rhododendrons

Rhododendron luteum

Rhododendron luteum

Species names include:

R. arborescens = tree like

R. augustini = after Dr. Augustine Henry, famous 19th /20th century irish plantsman.,

R. balsamiaeflora = balsam-flowered, the double flowered florist’s balsam.

R. campanulatum = bell – shaped flowers

R. campylocarpum = bearing bent fruits

R. cinnarbarinum = cinnabar red

R. decorum = shapely or becoming

R. fastigiatum = fastigiate or erect branches taperign to a point

R. flavum = yellow (also known as R. luteum)

R. impeditum = twiggy branches

R. laponicum = of lapland

R. molle = soft or velvety, refering to the leaves

R. myrtilloides = myrtle – like

R. nudiflorum = naked flowered, i.e coming before the leaves

R. ponticum = Pontic, a  region of the Black Sea

R. rhodora = old generic name signifying rosy-red

R. russatum = reddened- the foliage

R. sutchense = from Szechuan

R.vaseyi = discovered by Mr. G.R. Vasey, 19th century botanist

R. viscosum = sticky or viscous

R. yunnanense = of Yunnan, southern China

Rhododendron ponticum

Rhododendron ponticum

The rhododendron is the national flower of Nepal, where the flowers are considered edible and enjoyed for their sour taste. The pickled flower can last for months and the flower juice is also marketed. The flower, fresh or dried, is added to fish curry in the belief that it will soften the bones!

Some species of rhododendron are poisonous to grazing animals because of a toxin in their pollen and nectar. Rhododendron is extremely toxic to horses, with some animals dying within a few hours of ingesting the plant.

People have been known to become ill from eating honey made by bees feeding on rhododendron and azalea flowers. Xenophon described the odd behavior of Greek soldiers after having consumed honey in a village surrounded by Rhododendron ponticum during the ‘March of the Ten Thousand’ in 401 BC.Pompey’s soldiers reportedly suffered lethal casualties following the consumption of honey made from Rhododendron deliberately left behind by Pontic forces in 67 BC. Later, it was recognized that honey resulting from these plants has a slightly hallucinogenic and laxative effect., the suspect rhododendrons being R. ponticum and R. luteum (also known as R. flavum). Eleven similar cases have been documented in Turkey during the 1980s. The effects of R. ponticum was mentioned in the 2009 film Sherlock Holmes as a proposed way to arrange a fake execution.

Rhododendron 'Lemon Dream'

Rhododendron ‘Lemon Dream’

Rhododendrons are grown for their spectacular flowers, usually borne in spring. Some also have young leaves and stems covered in a striking dense woolly covering (indumentum) and some – the deciduous rhododendrons or azaleas – have good autumn colour. Some species (e.g. Rhododendron ponticum in Ireland and the United Kingdom) are invasive plants, spreading in woodland areas replacing the natural understory. R. ponticum is difficult to eradicate, as its roots can make new shoots.

Sources and further information:


BBC video of Rhododendrons in the Himalayas

RHS- growing Rhododendrons

Royal Botanic Garden, Edinburgh – Rhododendron collection

Quizzicals: answers to the two clues given in Plantax 10…

  • Evader of women – Ladies Slipper
  • Oriental busybody – Japanese Medlar

..and 2 more cryptic clues to the names of plants, fruit or veg…

  • Where policemen spend their holidays
  • Feline relative

(thanks to Les Palmer, answers in the next Plantax!)

Old School Gardener

If you’ve enjoyed reading this post and others on this blog, why not comment and join others by signing up for automatic updates via email (see side bar, above right ) or through an RSS feed (see top of page)?