COLUMBIA CITY, Ind. — An undeniable plant menace in home landscapes is poison ivy, Rhus radicans or Toxicodendron radicans. I’m allergic to its plant oils, as I’m sure many readers are. It’s the plant we all love to hate.
Poison ivy leaves are what we call compound leaves. Each leaf has 3 leaflets. You may have heard the expression, “Leaves of three, let it be,” or “leaves of three, leave it be.” I’m not sure which is the appropriate original expression of folklore, but the message is the same, and it contains a good general warning to heed.
Rosie Lerner, Purdue consumer horticulture specialist, recently wrote that another key identifying characteristic of poison ivy is that one side of a leaflet may have an irregularly toothed margin, while the opposite edge may be smooth or barely toothed.
If you are one who enjoys trees, you may notice that leaves of boxelder trees resemble poison ivy. However, boxelder leaves can have 3-7 leaflets, while poison ivy always has 3. If you have boxelder in your landscape, and cannot tell the difference between a young 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.
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.
Poison ivy can adapt to many situations in the landscape. “Poison ivy is typically a vine that can climb quite high by means of aerial rootlets,” said Lerner. “But older poison ivy plants, especially those that have been cut back repeatedly, can take the form of a shrub.”
Lerner said that poison ivy flowers are rather inconspicuous and usually not noticed by gardeners. “The subsequent fruits are greenish white, smooth berries in clusters about the size of currants,” said Lerner. “Birds and other wildlife eat the berries and then spread the seed in their droppings.”
Lerner said that all parts of the poison ivy plant, including the stem and roots, contain and secrete a nonvolatile oil that affects the skin. “This oil is insoluble in water,” said Lerner. “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.
The compound that causes all the trouble is an oleo resin called urushiol oil (or toxicodendrol).
Lerner 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: don’t burn poison ivy! The smoke from burning the plant contains particles that can seriously injure your eyes, skin, and respiratory system.
Several herbicides are available for poison ivy control. “Keep in mind, however, that any herbicide that will kill poison ivy will also kill any desirable plants,” said Lerner. “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.” She said 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: https://mdc.itap.purdue.edu/.
— John E. Woodmansee, Purdue Extension Educator, Agriculture/Natural Resources
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