Missouri's Most Irritating Plant

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Published on: Mar. 2, 2005

Last revision: Nov. 17, 2010

Although a fortunate few are immune to poison ivy's rashes and blisters, between 50 and 70 percent of people experience physical reactions to contact with the plant. The unpleasant results of a "brush" with poison ivy may last for days, weeks or months. Some people are so sensitive to the plant that they suffer after petting a dog that has been in poison ivy, inhaling smoke from burning poison ivy or handling the clothes of someone who has walked through poison ivy.

Poison ivy has been irritating people for quite some time. In 1609, Captain John Smith was the first to call it poison ivy. He said it resembled the English ivy or Boston ivy, but he noted that the plant "caused itchynge, and lastly blisters."


Poison ivy is a member of the Cashew Family (Anacardiaceae). Most Missourians have probably heard of at least three "poison" members of this family: poison ivy (Toxicodendron radicans), poison oak (Toxicodendron toxocarium) and poison sumac (Toxicodendron vernix). However, only poison ivy is common in Missouri. Poison oak is rare, and poison sumac has never been recorded here.

Poison ivy is the most widespread of the three plants. Found from the East Coast to the West Coast and from southern Canada to Mexico, it has been found in every county in Missouri and in every type of terrestrial habitat, including prairies, swamps, forests, fields, and glades. Poison ivy can grow in full sun and in nearly full shade. You might find it in your flower garden or lawn, or along your driveway. Its most preferred habitats are forest edges and recently disturbed open fields.

"Leaves of Three"

Poison ivy can be a woody shrub or a vine. As a shrub, it can grow about 6 feet high. As a vine, it can climb 40 feet up a tree.

The best way to distinguish poison ivy from other plants is to look at its leaves and tendrils.

The old saying "Leaves of three, leave it be" is good advice. Poison ivy has a compound leaf with three leaflets. However, many useful plants, including aromatic sumac, strawberries, and even green beans, also have three leaflets.

The leaflets of poison ivy are arranged alternately, rather than opposite one another, on the stem.

Poison ivy leaves are sometimes -- but not always -- waxy or shiny. The three leaflets are pointed. The middle or upper leaf is symmetrical. The two sides are mirror images of one another, but they

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