|Toxicodendron diversilobum (western poison oak)|
Toxicodendron pubscens (poison oak)
Toxicodendron radicans (poison ivy)
Toxicodendron rydbergii (western poison ivy)
Toxicodendron succedaneum (Rhus tree)
Toxicodendron vernicifluum (lacquer tree)
Toxicodendron vernix (poison sumac)
Members of this genus have pinnately- compound, alternate leaves and whitish or grayish drupes. The best known member is poison ivy, Toxicodendron radicans, which is practically ubiquitous throughout eastern North America.
The plants are quite variable in appearance. Leaves may have smooth, toothed or lobed edges, and all three types of leaves may be present in a single plant. Plants grow as creeping vines, climbing vines, or shrubs or, in the case of poison sumac, as trees. While leaves of poison ivy and poison oak usually come in sets of three, sometimes there are groups of five or, occasionally, even seven. Leaves of poison sumac come in groups of seven to thirteen.
While the triple leaf pattern is the best known method of identification of the genus, poison sumac is an exception, and there are other plants with a similar triple leaf pattern.
The common names are somewhat misleading. Technically, the plants don't contain a poison; they contain an allergen. Poison oak is not an oak at all, although leaves in some plants bear a striking resemblance to oak leaves.
There are several species of Toxicodendron:
- Climbing poison ivy (Toxicodendron radicans) is extremely common in much of North America. In the United States it grows in all states except Alaska and Hawaii. It also grows in Central America and parts of Asia, Africa, Europe and Australia.
- Non-Climbing poison ivy (Toxicodendron rydbergii) is found in western United States and Canada, and in northern parts of eastern United States.
- Western poison oak (Toxicodendron diversilobum) is found only on the Pacific Coast of the United States and Canada. It is extremely common in that region, where it is the predominant species of the genus.
- Eastern poison oak (Toxicodendron toxicarium) grows mostly in sandy soils in eastern parts of the United States.
- Poison sumac (Toxicodendron vernix) grows exclusively in very wet or flooded soils, usually in swamps and peat bogs in eastern United States and Canada.
- The varnish tree or lacquer tree (Toxicodendron vernicifluum, formerly Rhus Verniciflua) grows in Asia, especially China and Japan.
- The Rhus tree or wax (Toxicodendron succedaneum, formerly Rhus Succedanea) is a native of Asia, although it is found elsewhere, most notably Australia and New Zealand.
Non-climbing poison ivy (Toxicodendron rydbergii) may grow as a vine or a shrub. It was once considered a sub-species of climbing poison ivy. It does sometimes hybridize with the climbing species. Non-climbing 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.
Western poison oak (Toxicodendron diversilobum) is 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 3 leaflets, 3½ to 10 centimeters long, with scalloped, toothed, or lobed edges. Western Poison Oak is found only on the Pacific Coast, where it is common, and ranges from Southern Canada to Baha, California. It is California's most prevalent woody shrub.
Eastern poison oak (Toxicodendron toxicarium) grows as a shrub. Its leaves are in groups of three. Leaves are typically rounded or lobed, and are densly haired. Poison ivy shrubs are sometimes mistaken for, or simply called, poison oak. (Poison oak has small clumps of hair on the veins on the underside of the leaves, while poison ivy does not.)
Poison sumac (Toxicodendron vernix) is a tall shrub or a small tree, from 5 to 25 feet tall. It grows in wet soils such as in bogs, swamps, and flooded areas. It reproduces by seeds. Between 7 and 13 leaves on alternate sides of the vine form clusters that, in many cases, resemble a feather. The tree is found in parts of eastern North America, especially in the coastal plains and the Great Lakes region.
The varnish tree or lacquer tree (Toxicodendron vernicifluum) is a small Asiatic tree, the sap of which produces an extremely durable lacquer. Leaves are in groups of 9 to 15 leaflets (usually 11). The sap contains the allergenic oil, urushiol. Urushiol gets its name from this species which, in Japan, has been called the Urushi tree. Other names for this species include Japanese lacquer tree, Chinese lacquer tree, Chinese varnish tree, Japanese varnish tree and Japanese sumac. (Note: the term varnish tree is also applied to the candlenut tree, Aleurites moluccana, a southeast Asian tree that is not a Toxicodendron)
The Rhus tree (Toxicodendron succedaneum) is a large shrub or tree up to 8 m tall, somewhat similar to a sumac tree. It is native to Asia. Because of it's beautiful autumn foliage, the Rhus tree has been planted outside of Asia as an ornamental, by gardeners who were apparently unaware of the dangers of allergic reactions. Particularly in Australia and New Zealand, where it was once an ornamental, it is now officially classified as a noxious weed.