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Leaves of Three, Let It Be

Poison ivy, Rhus radicans or Toxicodendron radicans, is an undeniable plant menace in home landscapes. I’m allergic to its plant oils, as you may be. It’s the plant we all love to hate.

Purdue experts Rosie Lerner (retired) and Travis Legleiter co-wrote a Purdue Extension publication entitled, “Poison Ivy,” which explains several key aspects of managing poison ivy.

Poison ivy leaves are what we call compound leaves. Each leaf has 3 leaflets, hence the adage, “Leaves of three, let it be.” Another key identifying characteristic of poison ivy is that one side of a leaflet may have an irregularly toothed margin, while the opposite margin remains smooth or without serration.

To me, it closely resembles a boxelder tree’s compound leaf. Boxelder, also known as ash-leaved maple, has compound leaves with 3-7 leaflets, while poison ivy always has 3. If you have trouble telling the difference between a new boxelder seedling and a new poison ivy plant, another identifying difference is the leaf arrangement. Leaves of poison ivy are arranged alternately on a stem, while leaves of boxelder are arranged oppositely (right across from each other) on the stem.

The Purdue experts said that people often use the names “poison ivy” and “poison oak” interchangeably; this is incorrect. Poison ivy is the only species found throughout Indiana. Poison oak, Rhus toxicodendron, is a low-growing, non-climbing shrub, that is not known to occur in Indiana.

For those who love to walk trails in the woods, or wander off the beaten path a bit, poison ivy may be lurking among other plants. Keep an eye out for those leaves of three, or you may be dealing with an itchy rash in a few days.

Poison ivy can be a low-growing shrub or a vine in the home landscape. “It has aerial rootlets that enable the vine to attach itself to whatever it may be growing on,” they said. It can climb to the top of the tallest tree.

Poison ivy flowers are rather inconspicuous and usually not noticed by gardeners. They produce greenish-white, smooth berries that appear waxy. “The berries grow in clusters about the size of currants,” they said. “Birds and other wildlife eat the berries and then spread the seeds in their droppings.”

The Purdue experts said that all parts of the poison ivy plant, including the stem and roots, contain and secrete a nonvolatile oil (oleo resin) that affects the skin. “This oil is insoluble in water,” they said. “That means if you simply wash with water alone after coming into contact with poison ivy, you may spread the oil to other areas and increase the discomfort.” Of course, seek medical attention as needed.

“However, washing with a strong alkali soap, such as yellow laundry or naptha, will relieve the discomfort,” they said. “Alcohol will dissolve and remove the oily substance from the skin, and if you apply it soon enough, may prevent irritation.”

The experts said that once established, the woodiness of the poison ivy plant makes it difficult to control. Repeatedly cutting the plant back to the ground may eventually starve the plant. However, each time you cut it you expose yourself to the oil. You can dig up and discard small plants, but if you leave behind any portion of the root system the plant will likely re-sprout.

One important caution: never burn poison ivy! “The smoke from burning the plant contains particles that can seriously injure your eyes, skin, and respiratory system,” they said.

Several herbicides are available for poison ivy control. “However, keep in mind that any herbicide that will kill poison ivy will also kill any desirable plants,” they said. “So if the poison ivy is growing among shrubs and trees, you must apply chemical controls directly to the poison ivy plant and not to any of the other plants.” They said that if the poison ivy growth is severe enough, it may be worth sacrificing some desirable plants to eliminate the poison ivy. Always read and follow the label directions before using any pesticide product.

For more information, including poison ivy look-alikes and specific control recommendations, search for Purdue Extension Bulletin HO-218-W, “Poison Ivy,” at Purdue Extension’s Education Store,

poison ivy

Poison ivy; Photo by Rosie Lerner, Purdue University

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