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3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915

6/19/00

Contacts:
Jeff Iles, Department of Horticulture, (515) 294-0029, iles@iastate.edu
Elaine Edwards, Extension Communication Systems, (515) 294-5168, eedwards@iastate.edu

Yard and Garden Column for the Week Beginning June 23

Leaflets of Three...Beware of Me!

By Jeff Iles
Extension Horticulturist
Iowa State University Extension

Each year, an estimated two of three Americans develop an allergic rash after contact with either poison ivy, poison sumac or poison oak. And because we spend a considerable amount of time in our gardens and natural areas during the summer months, I'd hazard a guess that most of us have had a close encounter with one or several of these devilish weeds. Unfortunately, there is considerable confusion regarding the identification of these plants and the ways they are able to inflict their pain and misery on our skin.

More than 200 years ago, Carl Linnaeus, the father of plant taxonomy, lumped the various species of poison ivy, oak and sumac into the Rhus genus. Recently, however, taxonomists have moved these troublemakers into their own genus, the Toxicodendrons (Toxico meaning poison and dendron referring to plant or tree). A good way to differentiate between the two genera is to remember the Toxicodendrons have whitish or cream-colored fruit while members of the Rhus genus have red fruit.

Today, poison ivy, oak and sumac are divided into five distinct species. Poison sumac (Toxicodendron vernix) is the species we are least likely to encounter because it is a water-loving swamp plant of the Northeast, Midwest and Southeast. It has pinnately-compound leaves with 5 to 13 smooth leaflets, can grow 6 to 20 feet in height and looks somewhat similar to smooth or staghorn sumac, but remember, only poison sumac has cream-colored fruit.

Eastern poison oak (Toxicodendron toxicarium), found growing in sandy soils from southern New Jersey to Florida and west into Texas, Oklahoma and Kansas, has multi-lobed leaflets (three leaflets per leaf) and looks much more like oak than the Western poison oak (Toxicodendron diversilobum) whose leaflets appear wrinkled with a scalloped leaf margin. Firefighters in California and other Western states are especially wary of Western poison oak because smoke from burning plants is extremely toxic, capable of causing lung infections and a burning, itching rash all over the skin of anyone unlucky enough to be downwind.

Climbing poison ivy (Toxicodendron radicans) is extremely common in the Eastern, Midwest and Southern United States. Vines grow practically straight up, reaching heights of 10 to 20 feet. Around the Great Lakes and in the far Northern and Western United States and Canada, poison ivy grows as a shrub and is classified as Toxicodendron rydbergii. In either case, the leaves are glossy, bright green, usually trifoliate (having three leaflets), but on occasion 5 or 7 leaflets per leaf can be found. Virginia creeper (Parthenocissus quinquefolia) often is confused with poison ivy, but the harmless creeper has five leaflets and bluish-black fruit.

The rash we get from our association with these plants is an allergic rash (dermatitis) caused by contact with an oil called urushiol (oo-ROO-she-ol). All species of poison ivy, oak and sumac have urushiol in their roots, stems, leaves and fruit. The oil or sap is released when plants are bruised. For this reason Toxicodendron-dermatitis is more common in the spring and early summer when leaves and stems are tender. The sap may be deposited on the skin by direct contact with the plant, through contact with contaminated objects such as shoes, clothing, tools and animals, or as airborne urushiol particles from burning plants.

It is interesting to note that an allergic reaction seldom occurs on the first exposure to poison ivy or its relatives. A second encounter, however, will usually result in a severe reaction in about 85 percent of the population. Approximately 15 percent of the population is thought to be resistant, never developing an allergic reaction to urushiol.

Once urushiol touches the skin it begins to penetrate in minutes. In fact, usually within 10 to 15 minutes of contact, urushiol binds to skin proteins. If the sap can be washed off before that time (rubbing alcohol followed by plenty of cold water), a reaction may be prevented. Approximately 24 to 36 hours after a sensitized person is exposed to urushiol, a blistery, itching rash develops. If you can keep from scratching (this will lessen the risk of secondary infection) your skin will heal in about two to four weeks.

Besides keeping a watchful eye out for these unpleasant plants, is there any way to prevent exposure to poison ivy, oak and sumac? Barrier skin creams such as a lotion containing bentoquatum (IvyBlock [registered]) may provide some protection, but its effectiveness in preventing an allergic reaction may vary from person to person. Recently, immunization has become possible thanks to a prescription pill made from the active extract of poison ivy. Unfortunately, the immunization procedure can take four months to achieve a reasonable degree of "hyposensitization" and may be accompanied by uncomfortable side effects.

The characteristic rash and itch caused by urushiol exposure can be relieved by taking cool showers and applying over-the-counter preparations like calamine lotion or Burrow's solution. Soaking in a lukewarm bath with an oatmeal or baking soda solution may help ease the itching and dry oozing blisters. An antihistamine like Benadryl also may help relieve the itching. Of course, if the reaction seems unusually bad, get to a doctor.

Finally, let's dispel a few myths about poison ivy and its relatives:

Myth #1 - Scratching poison ivy blisters will spread the rash. Not true. Fluid leaking from blisters will not spread the rash. Well before the blisters form, however, you may spread the urushiol on your hands to other parts of your body.

Myth #2 - Poison ivy is contagious. Not true. The rash is simply a reaction to urushiol. The rash cannot pass from person to person; only urushiol can be spread by direct contact.

Myth #3 - You can "catch" poison ivy simply by being near the plants. Not true. Direct contact or contact with smoke from burning plants is needed to introduce urushiol onto the victim.

Myth #4 - Once allergic, always allergic to poison ivy. Not true. A person's sensitivity changes over time, even from season to season. People who were sensitive to urushiol as children may not be allergic as adults.

Myth #5 - Don't worry about dead plants. Not true. Urushiol remains active on any surface, including dead plants, for up to 5 years.

Myth #6 - One way to protect yourself from poison ivy is to keep yourself covered outdoors. Partly true. Urushiol can stick to your clothes which you can touch and spread to your skin later.

The problem with an article like this is that you'll begin to see, or think you see poison ivy everywhere. And of course, that is rarely the case. Still, before you grab the vine on that tree you're about to prune, remember, "leaflets of three...beware of me."

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ml: isugarden


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