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Rhus radicans, poison ivy


One of the most frequently mentioned 'poison' plants, especially in North America, but some would call it allergenic rather than poisonous.

Blog Entries

Read more about Rhus radicans, poison ivy, in these blog entries (most recent first);
Confusion between allergens and poisons could cause problems



Meaning of the Name

Often given a circular definition; 'Rhus' means 'sumac', 'sumac' means 'Rhus'. Some suggestion that the word comes from the Greek 'reo, meaning 'to flow' and indicates the spreading nature of the plants. It has also been suggested that 'sumac' means 'red' and 'Rhus' is intended to convey redness.

From the Latin for radiating, thus a plant with radiating stems which will form additional roots.

Common Names and Synonyms

Poison ivy. Also known as Rhus toxicodendron or Toxicodendron radicans. There are those who claim Toxicodendron as the proper modern genus but Dr Liz Dauncey's new book goes for Rhus radicans and Google produces many more results for that name than for Toxicodendron radicans. The American Association of Poison Control Centers, however, favours Toxicodendron radicans.

This may be a rare example where the common name is more useful that the botanical names.  

How Poisonous, How Harmful?

Though ingestion is known to cause gastrointestinal upset this is very rare and only mild.

Contact with the allergenic oil, urushiol, is the much greater problem. Initial contact may take some time to produce the symptoms of dermatitis; itching, blistering and pigmentation changes. Like all allergens, the effects tend to become more serious with repeated exposure and very severe dermatitis can occur for people who become sensitised.

Urushiol is not destroyed when the plant dies so dead leaves can be problematic.

It is believed, however, that up to 30% of people suffer no ill effects adding to the view that this is an allergen rather than a true poison.


In the USA, in particular, contact with poison ivy (and the related poison oak and poison sumac) is a regular risk for many gardeners. The effects are so well known that it is almost certain that most cases do not result in contact with healthcare facilities or poison control centers. Typically, poison ivy accounts for between 5-10% of calls to PCCs where a plant is specifically identified. This amounts to less that 2,000 calls per annum.

Folklore and Facts

Poison ivy is, thankfully, a plant whose harmful potential is so obvious that it was never brought to the UK in any quantity. It is known that John Parkinson, author of the 1640 Theatrum botanicum, had, at that date, poison ivy in his own garden. 

But, in 1668, Richard Stafford sent some from Bermuda to England with a note reminding the recipient of its dangers and claiming ‘I have seen a Man, who was so poyson’d with it, that the skin peel’d off his face, and yet the Man never touch’d it, onely look’d on it as he pass’d by’. This was, probably, an exaggeration of the effects but it served to reinforce the notion that this was not a plant which should be widely grown.

The question remains as to whether Rhus radicans should be defined as a poisonous plant. Its effects are those of an allergic reaction; some people are known to be completely unaffected and, as stated above, repeated exposure can result in sensitisation leading to far worse symptoms. These effects are, however, attributable to an identifiable substance, urushiol, and, in general, the symptoms produced are related to the concentration of urushiol present.

But, whether it is called an allergen or a poison, there is no doubting the extent of the pain and discomfort caused each year in the spring and early summer when gardeners clear unwanted plants from the garden and either fail it to see or fail to identify its presence. But it is not just gardeners who are at risk. There are plenty of reports of hikers failing to recognize poison ivy and suffering the consequences. Indeed, you would think, from reading local papers in the USA or gardening magazines and blogs, that Rhus radicans was about the only 'poison' plant in America.