Poison Ivy Season

Don’t find out the hard way. Poison ivy (Toxicodendron radicans) is actually easy to identify by visual inspection. Because of the fear that it inflicts upon the unsuspecting and unknowing home gardener, it is often confused with other plants – plants that are not actually toxic at all. I think of snakes as analogous to this plant – there are plenty of non-venomous species in Missouri’s ecology, but only a small number of them are actually poisonous. But, it is those few that stand out in most people’s minds that pack a punch of fear. Let’s dispel a few myths concerning poison ivy and compare this plant to several other species that have been on the blunt end of mistaken identity.

First, poison ivy’s key physical feature is its leaves. The leaflets are always in a group of three, called trifoliate leaves. The two lateral leaflets often develop a lobe along their margins, which gives them a "mitt" shape. The petioles, or small stems, that support the lateral leaflets are shorter than the petiole of the terminal leaflet. Poison ivy lacks hairs. Stems and leaf veins of young plants often have a reddish color; in the fall, the leaves often take on a bright red color. The appearance of this native perennial can have several forms – it may be a low-growing shrub or a vine that will climb to the top of tall objects with the aid of its aerial rootlets. The fruit is a greenish-white berry that contains a single seed. Because birds and other wildlife will feed on them, poison ivy can be easily spread by their droppings. Photographs of poison ivy and other weeds may be viewed on-line at http://www.p su.missouri.edu/fishel/poison_ivy.htm.

One plant that is often confused with poison ivy is Virginia creeper (Parthenocissus quinquefolia). It is also a high climbing vine with the aid of its tendrils and has a perennial life cycle, but it does not take on a shrub-like appearance. We see Virginia creeper throughout Missouri, often completely engulfing telephone poles and fences. But, just because it has a similar growth habit doesn’t make it poison ivy. Instead of each leaf containing three leaflets as poison ivy does, Virginia creeper has leaves made up of five leaflets. This plant has no toxic principles that cause skin rash, although the berry, which is dark blue, is toxic if ingested.

Another plant that has been confused with poison ivy, but on a smaller scale, is fragrant sumac (Rhus aromatica). The leaves also occur as trifoliates, but there are a few differences between sumac and poison ivy. Its leaves emit a fragrant, aromatic odor, especially when crushed, and the center terminal leaflet does not have a long petiole. Also, the leaf surface of fragrant sumac is hairy, whereas poison ivy lacks hairs on its leaves. The berries of fragrant sumac are red. Although there is a poisonous species of sumac, it is not known to occur in Missouri.

I’ve often heard people use the term "poison oak" when referring to poison ivy, although it is a different species. Poison oak (Toxicodendron pubescens) causes the same dermatitis problems as poison ivy, but its distribution is limited in the state, primarily to the extreme southern counties. Like poison ivy, its close cousin, it has trifoliate leaves, but grows in a shrub-like fashion, not a vine. The leaf tips of poison oak are blunter than those of poison ivy.

The toxic principles contained in poison ivy are known as urushiols. These compounds are major contributors to allergenic dermatitis in the United States. All parts of the plant, not solely the leaves, contain urushiols. Several myths can get one in trouble with this plant: 1) a person will not develop a skin rash by contacting dead poison ivy tissue. Not true. Urushiols remain active within plant tissue for over a year following death. 2) A person will not develop a skin rash by contacting smoke from burning the plant. Not true again. Don’t burn poison ivy in attempting to eradicate it because the smoke contains the urushiols. What’s worse is that inhaling the smoke and getting the smoke in the eyes can cause severe consequences. 3) A person will not develop a skin rash by contacting tools, pets or clothing that have had direct contact with poison ivy. Again, not true. Remember, urushiols can remain active for over a year.

Control of poison ivy can be achieved by several means, mainly dependent on the growth stage of the plant. Small, immature poison ivy plants may be mowed or removed by hand. Larger, mature plants will take more of an effort with the assistance of herbicides and patience. For home gardeners, products containing the active ingredient, glyphosate, may be readily purchased from the local lawn and garden center. Glyphosate products may be applied directly to the foliage or to the cut stems, which is a useful technique for controlling vine plants. Remember that glyphosate is nonselective and will kill or at least cause harm to any vegetation that it contacts. Other products that will list poison ivy on their labels include those that contain some combination of 2,4-D, 2,4-DP, dicamba and/or triclopyr. These may be selectively applied to turfgrass and other grassy areas, but desirable broadleaf vegetation will sustain death or injury. With larger, mature poison ivy, be prepared to make at least one repeat application. Regardless of product chosen, be sure to read and follow all herbicide label directions.

Don’t learn the hard way. Poison ivy can be easy to identify. MU Extension has recently published a guide covering vine weed control. It is in full color and also provides written descriptions of poison ivy and other common vine weeds found in Missouri. It may be ordered through your local University of Missouri Extension Center or by calling the MU Extension Publication Office at 1-800-292-0969. If you are not sure, let the MU Extension Plant Diagnostic Clinic assist you. You can submit samples of poison ivy or any other plants for identification through your local University of Missouri Extension Center.

Fred Fishel, Extension Pest Management/Pesticide Training Coordinator, UMC (573) 884-6361