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Rhododendron ponticum

Heather Family [Ericaceae]  

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28th May 2008, Rowsley, Derbyshire. Photo: © RWD

25th June 2004, Grindleford, Derbyshire. Photo: © RWD

25th June 2004, Grindleford, Derbyshire. Photo: © RWD

23rd Aug 2008, Butterley Reservoir, Marsden, Yorkshire Photo: © RWD

1st July 2002, Tixal Hill, Whaley Bridge. Photo: © RWD
Colonising the whole of south-facing slopes.

2nd Nov 2007, Stanley Gill Force, Eskdale Valley, Cumbria. Photo: © RWD
Saplings springing up in a clearing.

2nd Nov 2007, Stanley Gill Force, Eskdale Valley, Cumbria. Photo: © RWD
Ready to smother the whole fells again...

Uniquely identifiable characteristics : there is no other shrub like this.

The word Rhododendron is derived from the Greek language, meaning Purple Tree.

Rhododendrons are slowly taking over the whole of many valleys in North Wales, and to a lesser extent in Cumbria. They are everywhere, especially acid lowland soils. They colonise especially south facing slopes to the detriment of all other plants, even bracken, for it is so dark under the shade of these bushes that nothing else can germinate or grow. Slowly they take over the landscape. Controlling their spread is very time consuming.

Rhododendrons can reach a height of 5m. They love woods, open heaths, and lowish moorland. Their elliptical leaves are glossy, large and untoothed, in whorls up the branches. The purple/mauve flowers are large and completely envelop a flowering bush in a very showy spectacle. For this reason they were once planted by landed gentry in Victorian gardens, but their un-controlled spread is putting a stop to that practice.

It is thought that Rhododendrons were here before the last ice-age, about 20,000 years ago, but were eliminated by the ice-age, but brought back into the UK about 2 centuries ago as an ornamental garden plant. This has proved to be a bad mistake.

More importantly, rhododendrons now harbour a disease caused by the fungus Phytophthora ramorum which was first found in the UK late 1900's and which is also deadly to some species of trees. Uprooting these rhododendrons is becoming much more imperative. Ramorum disease hosted in Rhododendron pontica is now spreading to other trees such as Sweet Chestnuts, Turkey Oak, Beech, Birch, Douglas Fir, Western Hemlock amongst about 220 other vulnerable trees. This includes Japanese Larch, which is extensive in forestry plantations. The disease was first spotted in Larch trees in 2009 in South West England. Since then it has spread rapidly and is now found, mainly in the West, in Devon, West Somerset, Wales, and with pockets in Lancashire, Cumbria and Western Scotland. All infected trees are being felled as a matter of some urgency, which is having a devastating effect on the plantations; the wood cannot be used for fear of spreading the fungus further afield. Infected Japanese Larch trees are especially prone to release high levels of the fungal spores in spring and summer. In moist air (such as is likely in the West) the spores can spread significant distances to infect other trees, even in gardens and parks. Infected Larch trees shed their needles early, well before autumn. The shoots visibly wither and the needles go black, branches die back and the upper trunk can bleed resin. The fungus infects the tree just beneath the bark; infected trees show an identifying wine-red stain if the bark is peeled away.


Both Rhododendrons and American Laurels (Kalmia), especially the leaves, contain glycosides called Grayanotoxins, which are the toxic compounds present also in Bog Rosemary and other members of the Heather Family.

When ingested, it lowers the blood pressure which can lead to dizziness, vomiting, diarrhoea, tremors, heart disturbances and may cause breathing problems. Convulsions coma and death may result if sufficient quantities are consumed. Cattle and sheep are more at risk, for they are more likely to eat Rhododendrons or Azaleas.

Honey made from nectar where bees have visited Rhododendrons or Azaleas cannot be sold, it is toxic, and must not be consumed.

Honey gathered by bees from these plants is also toxic, so bee colonies should not be placed near Rhododendrons or Azaleas (such as Yellow Azalea). In fact, the nectar from these plants is so toxic to bees themselves that many beekeepers tend to keep their hives shut during the flowering season if there are Rhododendron or Azaleas around within a mile or two. Oleanders also have toxins so poisonous to bees that a whole colony can be wiped out.

Grayanotoxins were previously known by the names Rhodotoxin, Andromedatoxin and Acetylandromedol.

 Azaleatin is an anthocyanid based upon the flavonol Quercetin and is also found in the same flowers as Azalein below. It must derive its name from Azaleas, for they are both yellow (well, Yellow Azalea is) and in the same family as Rhododendron.

 Azalein is an anthocyanidin (the glycoside of the anthocyanin Azaleatin) found in both Rhododendrons and in flowers of the Thrift Family Plumbaginaceae. The glycoside involved is L-Rhamnoside. It is a pigment imparting a pinkish-red hue to the flowers.

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Rhododendron ponticum

Heather Family [Ericaceae]  

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