Toxicodendron is a genus of woody trees, shrubs and vines in the Anacardiaceae or Sumac Family. All members of the genus produce the skin-irritating oil urushiol, which can cause a severe allergic reaction; hence the scientific name which means "poison tree".
Members of this genus are sometimes included in the genus Rhus, although recent molecular evidence points to keeping Toxicodendron as a separate monophyletic genus.
They have pinnately compound, alternate leaves and whitish or grayish drupes. The best known members of the genus in North America are poison ivy, practically ubiquitous throughout most of eastern North America, and poison oak, similarly ubiquitous throughout much of the western part of the continent.
The plants are quite variable in appearance. The leaves may have smooth, toothed or lobed edges, and all three types of leaf edge may be present in a single plant. The plants grow as creeping vines, climbing vines, shrubs, or, in the case of Lacquer Tree and Poison Sumac, as trees. While leaves of Poison ivy and poison oaks usually have three leaflets, sometimes there are five or, occasionally, even seven leaflets. Leaves of Poison Sumac have 7-13 leaflets, and of Lacquer Tree, 7-19 leaflets.
The common names come from similar appearances to other species that are not closely related and to the allergic response to the urushiol. Poison oak is not an oak (Quercus, family Fagaceae), but this common name comes from the leaves' resemblance to white oak (Quercus alba) leaves, while Poison ivy is not an ivy (Hedera, family Araliaceae), but has a superficially similar growth form. Both Poison oak and Poison ivy are members of the sumac family, Anacardiaceae. Technically, the plants do not contain a poison; they contain a potent allergen.
The resins of certain species native to Japan, China and other Asian countries, such as T. vernicifluum (Lacquer Tree) and T. succedaneum (Wax Tree), are used to make lacquer, and, as a byproduct of lacquer manufacture, their berries are used to make japan wax.
Western Poison oak (Toxicodendron diversilobum or Rhus diversiloba) is found throughout much of western North America, ranging from the Pacific coast into the Sierra Nevada and Cascade mountain ranges between southern British Columbia and into Baja California. It is extremely common in that region, where it is the predominant species of the genus. Indeed, it is California's most prevalent woody shrub. Extremely variable, it grows as a dense shrub in open sunlight, or as a climbing vine in shaded areas. Like poison ivy, it reproduces by creeping rootstocks or by seeds. The leaves are divided into three leaflets, 35-100 mm long, with scalloped, toothed, or lobed edges. Californians learn to recognize it by the rhyme "leaves of three, let it be". The leaves may be red, yellow, green, or some combination of those colors, depending on various factors, such as the time of year.
Asian Poison ivy (Toxicodendron orientale or Rhus orientale) is very similar to the American Poison ivy, and replaces it throughout east Asia (so similar that some texts treat it as just a variety of the American species).
Potanin's Lacquer Tree or Chinese Varnish Tree (Toxicodendron potaninii or Rhus potaninii) from central China, is similar to T. vernicifluum but with (usually) fewer leaflets per leaf. Growing up to 20 m tall, like T. vernicifluum it is used for lacquer production. The leaves have 7-9 leaflets.
Poison ivy (Toxicodendron radicans or Rhus radicans) is extremely common in some areas of North America. In the United States it grows in all states except Alaska, Hawaii, and California, but it is much less common than Poison oak in western North America. It also grows in Central America. Appearing as a creeping vine, a climbing vine, or a shrub, it reproduces both by creeping rootstocks and by seeds. The appearance varies. Leaves, arranged in an alternate pattern, usually in groups of three, are from 20 to 50 mm long, pointed at the tip, and may be toothed, smooth, or lobed, but never serrated. Leaves may be shiny or dull, and the color varies with the season. Vines grow almost straight up rather than wrapping around their support, and can grow to 8-10 m in height. In some cases, Poison ivy may entirely engulf the supporting structure, and vines may extend outward like limbs, so that it appears to be a Poison ivy "tree".
Western Poison ivy (Toxicodendron rydbergii or Rhus rydbergii) is found in northern parts of the eastern United States. It also exists in the western United States and Canada, but is much less common than Poison oak. It may grow as a vine or a shrub. It was once considered a subspecies of Poison ivy. It does sometimes hybridize with the climbing species. Western Poison ivy is found in much of western and central United States and Canada, although not on the West Coast. In the eastern United States it is rarely found south of New England.
Wax Tree (Toxicodendron succedaneum or Rhus succedanea), a native of Asia, although it has been planted elsewhere, most notably Australia and New Zealand. It is a large shrub or tree, up to 8 m tall, somewhat similar to a sumac tree. Because of its beautiful autumn foliage, it has been planted outside of Asia as an ornamental plant, often by gardeners who were apparently unaware of the dangers of allergic reactions. It is now officially classified as a noxious weed in Australia and New Zealand. The fatty-acid methyl ester of the kernel oil meets all of the major biodiesel requirements in the USA (ASTM D 6751-02, ASTM PS 121-99), Germany (DIN V 51606) and European Union (EN 14214). 
Atlantic Poison oak (Toxicodendron pubescens or Rhus toxicarium) grows mostly in sandy soils in eastern parts of the United States. Growing as a shrub, its leaves are in groups of three. Leaves are typically rounded or lobed, and are densely haired. Although it is often confused with the more common poison ivy, even in the scientific literature, Atlantic Poison oak has small clumps of hair on the veins on the underside of the leaves, while Poison ivy does not.
Lacquer Tree or Varnish Tree (Toxicodendron vernicifluum or Rhus verniciflua) grows in Asia, especially China and Japan. Growing up to 20 m tall, its sap produces an extremely durable lacquer. The leaves have 7-19 leaflets (most often 11-13). The sap contains the allergenic oil, urushiol. Urushiol gets its name from this species which in Japanese is called Urushi. Other names for this species include Japanese lacquer tree, Japanese Varnish Tree and Japanese Sumac (Note: the term "varnish tree" is also occasionally applied to the Candlenut, Aleurites moluccana, a southeast Asian tree unrelated to Toxicodendron).
Poison Sumac (Toxicodendron vernix or Rhus vernix) is a tall shrub or a small tree, from 2-7 m tall. It is found in swampy, open areas and reproduces by seeds. The leaves have between 7-13 untoothed leaflets, in a feather-compound arrangement. In terms of its potential to cause urushiol-induced contact dermatitis, poison sumac is far more virulent than other Toxicodendron species, even more virulent than poison ivy and poison oak. According to some botanists, poison sumac (Toxicodendron vernix) is the most toxic plant species in the United States (Frankel, 1991).
^ Pell, Susan Katherine (2004-02-18). Molecular Systematics of the Cashew Family (Anacardiaceae) (PhD dissertation at Louisiana State University)., page 89
^ Brooks, Bill (1999-03-04). [http://nac.tamu.edu/x075bb/caddo/frameidx.html The Toxicodendrons:
Poison Ivy, Poison Oak and Poison Sumac].