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Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Sapindales
Family: Anacardiaceae
Genus: Toxicodendron
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)
Toxicodendron is a small genus of woody shrubs and vines, all of which produce a skin-irritating oil, urushiol, which can cause a severe allergic reaction; hence the scientific name which means "poison tree". Members of this family were formerly included in the genus Rhus.

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) grows throughout the United States 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 2 to 5 centimeters 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 20 feet in height. In some cases, climbing 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.

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, 3 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.

See also poison ivy, poison ivy rash

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