Toxicodendron radicans

The species is found in North America. Although commonly eaten by animals, with birds consuming the seeds, T. radicans is considered a noxious weed. As a poison ivy, it causes urushiol-induced contact dermatitis in most people who touch it, producing an itchy, irritating, and sometimes painful rash.

Description

Numerous subspecies and/or varieties of T. radicans are known. They can be found growing in any of the following forms, all having woody stems:

• as a climbing vine that grows on trees or some other support
• as a shrub up to 1.2 metres (4 feet) tall
• as a trailing vine that is 10–25 centimetres (4–10 inches) long.

The deciduous leaves of T. radicans are trifoliate with three almond-shaped leaflets. The leaf color ranges from light green (usually when young) to dark green (in maturity), turning red, orange or yellow in fall; some sources say the leaves are also reddish when expanding (before becoming dark green). The leaflets are 3–12 cm (1+1⁄4–4+3⁄4 in) long, rarely up to 30 cm (12 in), and somewhat shiny in maturity. Each leaflet has a few or no teeth along its edge, and the leaf surface is smooth. Leaflet clusters are alternate on the vine, and the plant has no thorns. Vines growing on the trunk of a tree become firmly attached through numerous aerial rootlets. The vines develop adventitious roots, or the plant can spread from rhizomes or root crowns. The milky sap of poison ivy darkens after exposure to the air.

T. radicans spreads either vegetatively or sexually. It is dioecious; flowering occurs from May to July. The yellowish- or greenish-white flowers are typically inconspicuous and are located in clusters up to 8 cm (3 in) above the leaves. The berry-like fruit, a drupe, mature by August to November with a grayish-white colour.

Identification

These four characteristics are sufficient to identify poison ivy in most situations: (a) clusters of three leaflets, (b) alternate leaf arrangement, (c) lack of thorns, and (d) each group of three leaflets grows on its own stem, which connects to the main vine, the middle stem is longer. The appearance of poison ivy can vary greatly among environments, and even within a large area. Identification by experienced people is often made difficult by leaf damage, the plant's leafless condition during winter, and unusual growth forms due to environmental or genetic factors. Various mnemonic rhymes describe the characteristic appearance of poison ivy:

1. "Leaves of three, let it be" is the best known and most useful cautionary rhyme. It applies to poison oak, as well as to poison ivy. However, some other innocuous plants have similar leaves.
2. "Hairy vine, no friend of mine"
3. "Berries white, run in fright" and "Berries white, danger in sight"

Similar-looking plants

• Box elder (Acer negundo) saplings have leaves that can look very similar to those of poison ivy, although the symmetry of the plant itself is very different. While box elders often have five or seven leaflets, three leaflets are also common, especially on smaller saplings. The two can be differentiated by observing the placement of the leaves where the leaf stalk meets the main branch (where the three leaflets are attached). Poison ivy has alternate leaves, which means the three-leaflet leaves alternate along the main branch. Boxelder has opposite leaves; another leaf stalk directly on the opposite side is characteristic of box elder.
• Virgin's bower (Clematis virginiana) is a vine of the buttercup family native to eastern North America. This vine can climb up to 10–20 feet (3.0–6.1 m) tall. It grows on the edges of the woods, moist slopes, and fence rows, and in thickets and streambanks. It produces white, fragrant flowers about 1 inch (2.5 cm) in diameter between July and September.
• Virginia creeper (Parthenocissus quinquefolia) vines can look like poison ivy. The younger leaves can consist of three leaflets but have a few more serrations along the leaf edge, and the leaf surface is somewhat wrinkled. However, most Virginia creeper leaves have five leaflets. Virginia creeper and poison ivy very often grow together, even on the same tree. Even those who do not get an allergic reaction to poison ivy may be allergic to the oxalate crystals in Virginia creeper sap.
• Western poison oak (Toxicodendron diversilobum) leaflets also grow in threes on the end of a stem, but each leaflet is shaped somewhat like an oak leaf. Western poison oak grows only in western North America, although many people refer to poison ivy as poison oak, because poison ivy grows in either the ivy-like form or the brushy oak-like form depending on the moisture and brightness of its environment. The ivy form likes shady areas with only a little sun, tends to climb the trunks of trees, and can spread rapidly along the ground.
• Poison sumac (Toxicodendron vernix) has compound leaves with 7–15 leaflets. Poison sumac never has only three leaflets.
• Kudzu (Pueraria lobata) is a nontoxic edible vine that scrambles extensively over lower vegetation or grows high into trees. Kudzu is an invasive species in the southern United States. Like poison ivy, it has three leaflets, but the leaflets are bigger than those of poison ivy and are pubescent underneath with hairy margins.
• Blackberries and raspberries (Rubus spp.) can resemble poison ivy, with which they may share territory; however, blackberries and raspberries almost always have thorns on their stems, whereas poison ivy stems are smooth. Also, the three-leaflet pattern of some blackberry and raspberry leaves changes as the plant grows: Leaves produced later in the season have five leaflets rather than three. Blackberries and raspberries have many fine teeth along the leaf edge, the top surface of their leaves is very wrinkled where the veins are, and the bottom of the leaves is light minty-greenish white. Poison ivy is all green. The stem of poison ivy is brown and cylindrical, while blackberry and raspberry stems can be green, can be squared in cross-section, and can have prickles. Raspberries and blackberries are never truly vines; they do not attach to trees to support their stems.
• Strawberries (Fragaria), wild or cultivated, have trifoliate leaves and a red vine, and can be found in similar ecosystems. However, strawberry leaflets are serrated and have same-length petiolules. The stems also have small trichomes, which poison ivy lacks.
• The thick vines of riverbank grape (Vitis riparia), with no rootlets visible, differ from the vines of poison ivy, which have so many rootlets that the stem going up a tree looks furry. Riverbank grapevines are purplish in color, tend to hang away from their support trees, and have shreddy bark; poison ivy vines are brown, attached to their support trees, and do not have shreddy bark.
• Fragrant sumac (Rhus aromatica) has a very similar appearance to poison ivy. While both species have three leaflets, the center leaflet of poison ivy is on a long stalk, while the center leaflet of fragrant sumac does not have an obvious stalk. When crushed, fragrant sumac leaves have a fragrance similar to citrus, while poison ivy has little or no distinct fragrance. Fragrant sumac produces flowers before the leaves in spring, while poison ivy produces flowers after the leaves emerge. Flowers and fruits of fragrant sumac are at the end of the stem but occur along the middle of the stem of poison ivy. Fragrant sumac fruit ripens to a deep reddish color and is covered with tiny hairs, while poison ivy fruit is smooth and ripens to a whitish color.
• Hoptree (Ptelea trifoliata) has leaves that are remarkably similar. It is, however, a much larger plant, so confusion is unlikely for any but the smallest specimens. The flowers and seeds are also easily distinguished from those of poison ivy.

Similar allergenic plants

• Toxicodendron rydbergii (western poison ivy)
• Smodingium argutum (African poison ivy)
• Toxicodendron pubescens (poison oak – eastern)
• Toxicodendron diversilobum (poison oak – western)
• Toxicodendron vernix (poison sumac)
• Gluta spp (rengas tree)
• Toxicodendron vernicifluum (Japanese lacquer tree)
• Lithraea molleoides (aruera – South America)