Poison ivy is a toxic vine that climbs to 60 feet high, trailing or climbing by aerial roots. Sometimes it appears as a low, upright shrub.
Leaves are alternate, compound, with 3 leaflets (“leaves of 3, let it be”) that are variable in size and shape; the end (center) leaflet has a stalk ½–1¾ inches long, which is longer than the stalks on the other 2 leaflets; side leaflets have unequal sides. Leaflets take on different textures and hues as the season progresses; they turn red, orange, or yellow in fall.
Stems are light brown, hairy, with raised pores, climbing by aerial rootlets. Stems trail until they find support; lacking support, they assume an erect, shrublike posture with single stems.
Flowers May–June, with clusters 1–4 inches long on new growth of stems. Flowers are small, greenish white, and fragrant.
Fruit ripens August–November, berries in grapelike clusters, persistent, about ¼ inch across, creamy white, waxy, globe-shaped, usually smooth.
Eastern poison oak (T. pubescens) is a closely related species. It occurs in low-nutrient, sandy or rocky soils, such as in glades, openings of dry upland forests, sand prairies, sand savannas, and roadsides, but it is uncommon and limited to the southern part of the Missouri Ozarks and the northern part of the Bootheel Lowlands (extreme southern and southeastern Missouri). Poison oak does not climb and does not produce aerial roots, and its leaflets usually have 3–7 deep, relatively rounded lobes, so the leaflets resemble the leaves of white or blackjack oak. The berry-like fruits have inconspicuous hairs.
There is a plant called “poison sumac,” but although some people have used that name for Missouri species, it technically belongs to a plant that does not occur in Missouri. True poison sumac (T. vernix) has feather-compound leaves with 7–13 leaflets whose margins are entire (lack teeth or lobes); it occurs in swamps and bogs to the east and north of our state.
Three other common Missouri plants are often mistaken for poison ivy:
A trailing or climbing vine that can reach 60 feet high, often growing on trees or other objects.
Occurs in floodplain and upland forests, alluvial soil along streams, thickets, along fence rows, roadsides, and railroads. Birds often distribute the seeds via their droppings. To control poison ivy, spraying with glyphosate such as Roundup is recommended over burning. (Burning poison ivy causes its oil to vaporize and become airborne, which can cause severe rashes; people breathing the smoke may have to be hospitalized.)
Poison ivy is in the same family as sumacs, and this family also has several economically important fruits and nuts, including pistachio, mango, and cashew. To some degree and in at least certain parts of the plants, all contain urushiols — the same chemicals that cause dermatitis from poison ivy. One reason why cashews are so expensive is that the process of shelling them releases a caustic resin that can cause skin to blister.
Poison ivy’s toxin is an oil that on many people causes an itchy rash with clear blisters. If you touch poison ivy, change clothing and immediately wash the affected area with soap and cold water. The oil can remain on fabric until it is washed off.
In fall, this plant’s leaves often turn attractive red, or sometimes orange or yellow. As leaves fall from trees, their bright red, poison-ivy-clad branches can be rather pretty . . . from a distance!
The white, waxy berries are a popular, and important, food for songbirds during fall migration and in winter when other foods are scarce. Many birds like the berries as well as the insects hiding in the tangled vines. Small mammals and deer browse on poison ivy foliage, twigs, and berries.
There are no sharp dividing lines between trees, shrubs, and woody vines, or even between woody and nonwoody plants. “Wood” is a type of tissue made of cellulose and lignin that many plants develop as they mature — whether they are “woody” or not. Trees are woody plants over 13 feet tall with a single trunk. Shrubs are less than 13 feet tall, with multiple stems. Vines require support or else sprawl over the ground.