mystery plant: “Poison ivy," Toxicodendron radicans

EDITOR’S NOTE: The Georgetown Times features John Nelson’s Mystery Plant series each Friday. You can find the answer to today’s mystery plant in next Friday’s paper.

The answer to last week’s Mystery Plant is: ”Bull thistle,” Cirsium horridulum

This is the vine which the Girl Scouts tell us to recognize by its compound, trifoliolate leaves (that is, with three leaflets), and that we are obliged to leave alone, because it is poisonous. Depending on one’s susceptibility to it, it can indeed cause severe contact dermatitis. Because its foliage is rather unremarkable, it tends to blend in to the background, and sometimes becomes “invisible.” You’ll frequently hear a poor victim of this species, slathered in calamine lotion, insisting that he or she never actually saw the vines that caused the itch. But they must have been there, somewhere!

This plant is in fact a perfectly good native resident of eastern North America. It is at home in a variety of woodland ecosystems, and does well in shade. It is common on low ground, abounding in coastal swamps, but may also be found widespread in the mountains. It is a vine, generally, and if given the chance, will climb high into the canopy. Large vines are commonly seen attached to tree trunks, frequently exhibiting plenty of aerial roots, clinging to the tree’s bark. As a vine, its branches frequently emerge at right angles, more or less, to the main stem. It is not always found climbing on trees, and may otherwise, if not climbing, form bushes, or scramble and run on the ground. This species is fully capable of growing in less-than woodsy settings, and does quite well as a component of people’s back yards. Potentially serious rashes may result from contact with any part of the plant, and that includes the vines in the winter (when no leaves are showing).

Now, it turns out that not everybody will end up with a rash after fooling around with this plant. I am one of them; I am constantly reminded after spending all this time in the woods and forests that I’ve never been affected by the stuff, and it is a certainty that I’ve come in contact with it. Some say that such immunity can wear off: I don’t know if that’s always true, but I’ll keep my fingers crossed.

So here I am, going to bat for this plant. It’s certainly not at all “bad.” The foliage is actually quite pretty, and a vigorously growing vine in the early summer can be quite impressive. The plants do produce small, yellow flowers, these eventually forming hard, whitish berries that many birds love to eat. Maybe the horticulturists will come up with a non-toxic variety suitable for growing in gardens. Who knows? Perhaps one of these days there could be garden society dedicated to growing this as a native ornamental.

In the meantime, whether you know you are susceptible to it or not, everyone should take some time to learn how to recognize this plant, and how to distinguish it from the various other non-toxic species which resemble it; there are several. The best way to avoid problems with it is to avoid all contact, whether you are in the woods, or puttering around under the red-tips.

John Nelson is the curator of the A. C. Moore Herbarium at the University of South Carolina, in the Department of Biological Sciences. As a public service, the Herbarium offers free plant identifications. For more information, visit or call 803-777-8196, or email