Missouri's Most Irritating Plant

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Published on: Mar. 2, 2005

Last revision: Nov. 17, 2010

are not symmetrical.

Each outside leaflet often has a distinct notch on its lower half, while its upper half is relatively smooth, with few or no notches. Many times, the outside leaflets resemble pointed mittens. They have a short shaft connecting them to the main leaf stem or petiole, while the middle leaflet appears to have a longer stem.

Poison ivy exhibits some degree of variation, so take the time to look carefully. For example, poison ivy sometimes, but not always, has a red stem. Although green all summer, poison ivy leaflets are among the first to turn color in the fall, usually becoming bright red or orange before falling.

Because you can get a rash from poison ivy in the fall and winter, it's helpful to be able to recognize the plant when it has no leaves. Poison ivy vines are easy to spot. They cling tightly to their host with dark brown, hair-like tendrils. Tendrils are aerial roots.


Poison ivy, as well as poison oak and poison sumac, produce an oleoresin called urushiol. The name is derived from the Japanese word for lacquer. This clear and sticky oil contains chemical transmitters and resins that bind to the surface of skin cells. The oil can trigger immunologic responses that can usually lead to a rash or "Rhus" dermatitis.

Urushiol is highly potent. It's estimated that the amount needed to make 500 people itch would cover the head of a pin. The resin is also stable and long-lasting. It can stay active for up to five years on a dead plant. In fact, centuries old specimens of urushiol have caused dermatitis in people highly sensitive to it.

Urushiol is found in every part of the poison ivy plant throughout the year. This includes the leaf, the stem, the stalk and the roots. The oil can remain active on dead and dried plants for 2-5 years. Unwashed clothing can still deliver active urushiol a year or two later. It is truly a plant or all seasons!

People vary in their reactions to urushiol. An encounter with the same plant may cause a mild rash on one person and severe blisters on another. A third person might not experience any effect. Predicting reactions to urushiol becomes even more confusing because people's reactions to it often changes. You may not have any reaction to poison ivy as a child, but then have severe reactions later in life.

Urushiol only becomes an

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