by Whit Gibbons

May 16, 2010

Q: I think I have come in contact with poison ivy or maybe poison oak. How can I find out what these plants look like? How much of a problem are they? Do you have to touch the leaves to get the blisters on your skin?

A: Spring is the most likely time for people to encounter an age-old outdoor pest of the plant variety-poison ivy. And poison oak. Most botanical field guides distinguish between poison ivy and poison oak. But the differences are subtle and do not really matter in identifying the plants. The two most definitive traits are the presence of three leaves on each stem and a red coloration at the apex where the three leaves connect. Both plants produce the oils that make you itch. Some biologists claim that poison oak is more virulent than poison ivy. I do not think that has been firmly documented. And I suspect someone who is itching like crazy from exposure to either plant doesn't care which one is more virulent.

Poison ivy can climb trees as a vine (usually attaching close to the trunk), look like a shrub, or be a single, simple plant. All parts of a poison ivy plant--leaves, stem, fruits, and roots--produce oils that can cause skin irritation in some people. Any bodily contact with the oils can cause a problem, whether from patting a dog that has just walked through poison ivy or touching clothes that have come in contact with the plant. You can even get a case of poison ivy internally by inhaling oil droplets that become airborne in smoke when the plants are burned. You can also get poison ivy in the winter simply by touching the stem, even though the leaves are gone.

I work at an ecology lab with people who spend time in swamps, woods, and streams where poison ivy is as common as a household word. Yet few of us ever get a serious case. Perhaps ecologists and others who are in the woods a lot avoid the plant without being aware they are doing so. Also, most people have no reaction when they casually brush against poison ivy, and a third or more are not sensitive at all.

The facts, myths, and disagreements about the properties of poison ivy are legion. You do not spread poison ivy by scratching where it itches, despite what some people say. New blisters and irritated areas can appear more than a week after exposure to the oils, but these merely represent the normal lag time that can occur after initial contact. You cannot give poison ivy to others, except by bringing them into contact with the oils that are on your own body or your clothes after encountering the plant.

Many forms of wildlife can eat poison ivy without being adversely affected. Dozens of kinds of birds including bobwhite quail eat the fruits, which are clusters of smooth, white berries that appear in late summer. I know of a serious case of poison ivy being contracted by a student who sorted through the stomach contents of a recently killed deer. Among the data he recorded for his research on the diet of deer was that they sometimes eat a lot of poison ivy leaves.

According to my favorite dermatologist, the symptoms of poison ivy are a consequence of the oils causing a contact dermatitis (inflammation of the skin). But she says that superficially similar dermatitis can be caused by numerous other plants and even commercial products we might come in contact with. Among the plants known to cause dermatitis are black walnut trees, red cedar, and fresh okra. Fortunately, most people are not affected by those plants.

For photographs and other information about poison ivy go to Click on Educational Materials, then on Ecology Fact Sheets. Under General Ecology, click on Poison Ivy. And when you are out in the woods this spring and summer, remember to watch out for three leaves with a touch of red.

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