THE POISON GARDEN website      Arum maculatum berries on a Cannabis leaf 

Search thepoisongarden.co.uk:

This free script provided by JavaScript Kit

This site uses botanical names. Click here for an A to Z common name to botanical name converter.

Rhododendron spp.

Summary

An invasive non-native in the UK that, though poisonous, does most of its harm by destroying habitat for native wildlife.

Blog Entries

Read more about Rhododendron in these blog entries (most recent first);
Is it taste or texture that deters grazing animals?
Plants and trees in a public park
Why and how this page was created

Family

Ericaceae

Meaning of the Name

Rhododendron
From the ancient Greek for 'rose' and 'tree'.

There are over 1,000 species. Three of the best known are;

ponticum
Coming from the south shore of the Black Sea.

catawbiense
From the Appalachian mountains, home of the Catawba tribe.

arboreum
Growing in tree-like form.
(That makes Rhododendron arboreum the rose tree tree.)

Common Names and Synonyms

rhododendron, azalea.

How Poisonous, How Harmful?

All parts of the plant contain toxins variously called andromedotoxin, grayanotoxin, rhodotoxin and acetylandromedol. Unusually, the nectar is believed to have the highest concentration.

The symptoms seen in animals, where poisoning is most likely, include projectile vomiting, abdominal pain, diarrhoea and constipation, slow heart rate, loss of coordination, falling and exhaustion. Most animals will exhibit all these symptoms but vomiting is unusual in cattle.

If sufficient foliage has been consumed death from respiratory failure occurs within hours.

It does not, however, appear on the Horticultural Trades Association list of potentially harmful plants so the chance of human poisoning is extremely low.

Incidents

There are no known incidents of fatal poisoning in humans in recent times, though the reported poisoning in Turkey around 400BC due to toxic honey may have involved fatalities.

Animal poisoning does occur but is rare both because animals tend not to be grazed where Rhododendron is grown and because they prefer other grazing if it is available. There are, however, a number of cases in the literature of poisoning of farm animals and there are anecdotes about zoo animals being poisoned because visitors feed them leaves taken from Rhododendrons growing in the zoo grounds.

Folklore and Facts

Rhododendron is a genus where species will inter-breed freely creating hybrids and new species. In particular, the catawbiense species, brought to the UK in 1809, interbred with the ponticum. There are now around a thousand recognised species of Rhododendron and many thousands more varieties and hybrids. Only the most highly specialised botanists can accurately distinguish one from another. 

Rhododendron

The principle concern with Rhododendron is the potential for toxic honey and the difficulty of distinguishing exactly which species or variety of Rhododendron is what leads to confusion between bee-keepers when they find that bees are dying from visiting Rhododendron bushes ‘exactly the same’ as other bushes that have produced no ill effects.

Of course, it is the question of whether toxic honey gets consumed by humans that is of most interest to the majority. It seems unlikely. First of all, as above, bee-keepers know that their bees are visiting Rhododendron when significant numbers die. Obviously, these dead bees don’t make it back to the hive to unload their toxic cargo. In areas where there can be high concentrations of flowering Rhododendrons, bee-keepers are reported to keep their hives closed until the danger passes.

Then there is the question of how bee-keeping works. Nectar collected early in the year, and Rhododendron are often the first bushes in flower, is often retained to feed the hive and it is not until later in the summer that honey is removed.

Then there are those who say that the toxin decays with time in honey so that by the time honey is consumed the level of toxicity is negligible. And honey with a high level of toxins from Rhododendron is said to be unpleasant to the taste though not, perhaps, as unpleasant as honey made exclusively from Jacobaea vulgaris, common ragwort, as there have been reported cases of honey poisoning.

Perhaps because it has only been known in the UK and northern Europe for a relatively short time, there is little folklore associated with Rhododendron. In southern Europe it is said to have been worn as a generalised protection against all forms of evil and nastiness.

It is said that the Catawba people used the Rhododendron medicinally but it is not mentioned by Charles F. Millspaugh in 'American Medicinal Plants'. But since he makes no mention at all of this tribe he may not be a reliable source on this occasion.

It is said that, at its peak, the Catawba fought three bloody battles on Roan Mountain in the Appalachians and the area is, today, covered with Rhododendron that flower bright red each year to commemorate the slaughter. 

Brownsea Island in Dorset was so overrun by Rhododendron that one of the last red squirrel colonies in England was threatened with extinction. Over a total of fifty years, volunteers have removed the plant and it was expected to have been permanently removed by 2012. The red squirrel population has revived and there are now thought to be 200 of these native animals on the island where they are not in danger from the non-native grey squirrel.