Horticulture/Toxicodendron radicans

Toxicodendron radicans (syn. Rhus toxicodendron, Rhus radicans; Poison ivy ) is a woody vine that is well known for its ability to produce urushiol, a skin irritant that causes an itching rash for most people, technically known as urushiol-induced contact dermatitis.

Poison ivy is subject to frequent taxonomic reclassification and confusion; it is currently divided into eastern and western species in the genus ../Toxicodendron/. At least six distinct subspecies of Toxicodendron radicans are recognized. Complicating identification and taxonomy are the fact that the species (even a particular subspecies) can be highly variable in growth habit and leaf appearance.

It is more common now than when Europeans first entered North America. Real estate development adjacent to wild, undeveloped land has created "edge effects", enabling it to form vast, lush colonies in such places. It is listed as a noxious weed in the U.S. states of Minnesota and Michigan.

Description
The plants can grow as a shrub up to about 1.2 meters (4 ft) tall, as a groundcover 10–25 centimeters (4–10 in) high, or as a climbing vine on various supports. Older vines on substantial supports send out lateral branches that may at first be mistaken for tree limbs.

The leaves are ternate with three almond-shaped leaflets. The berries (actually drupes) are a grayish-white color and are a favorite winter food of some birds. Hence the mnemonic,

"'Leaves of three, let it be; berries white, danger in sight.'"

Another version is:

"'Leaves of three let it be; if it's hairy, it's a berry.'"

The color ranges from light green (usually the younger leaves) to dark green (mature leaves), turning bright red in fall. The leaflets of mature leaves are somewhat shiny.The leaflets are 3-12 cm long, rarely up to 30 cm. 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. These three characteristics: (a) clusters of three leaflets, (b) alternate, and (c) lack of thorns, are sufficient to positively identify the plant. If it is growing up the trunk of a tree, the presence of root-hairs will identify it.

The color ranges from light green (usually the younger leaves) to dark green (mature leaves), turning bright red in fall. The leaflets of mature leaves are somewhat shiny.The leaflets are 3-12 cm long, rarely up to 30 cm. 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. These three characteristics: (a) clusters of three leaflets, (b) alternate, and (c) lack of thorns, are sufficient to positively identify the plant. If it is growing up the trunk of a tree, the presence of root-hairs will identify it.

The plants can grow as a shrub up to about 1.2 meters (4 ft) tall, as a groundcover 10–25 centimeters (4–10 in) high, or as a climbing vine on various supports. Older vines on substantial supports send out lateral branches that may at first be mistaken for tree limbs.

The leaves are ternate with three almond-shaped leaflets. The berries (actually drupes) are a grayish-white color and are a favorite winter food of some birds. Hence the mnemonic,

"'Leaves of three, let it be; berries white, danger in sight.'"

Another version is:

"'Leaves of three let it be; if it's hairy, it's a berry.'"

Growing Conditions
It grows throughout much of North America, including all Canadian provinces except Newfoundland (but not the territories) and all U.S. states except Alaska, and is normally found in wooded areas, especially along edge areas. It also grows in exposed rocky areas, in open fields and disturbed areas, and even in areas with wet soils. It rarely grows at altitudes above 1,500 meters (5,000 ft).

Varieties
Aside from the many subspecies, individual plants can vary greatly in the shape of their leaflets.

Confusion with other plants

 * Boxelder (../Acer negundo/) saplings can look almost indistinguishable from poison ivy. While Boxelder Maples often have five or seven leaflets, three leaflets are also common. The two can be differentiated by the fact that Poison-ivy has alternate leaves, while the maple has opposite leaves; in other words, by observing where the leaf stalk (the "branch" the three leaflets are attached to) meets the main branch. Another leaf stalk directly on the opposite side is characteristic of Boxelder Maple. If the three-leaflet leaves alternate along the main branch, it may be Poison-ivy.


 * Virginia creeper (../Parthenocissus quinquefolia/) vines can look like poison ivy. The younger leaves can consist of three leaflets (as opposed to the usual five) but have a few more serrations along the leaf edge, and the leaf surface is somewhat wrinkled. The rootlets of a climbing Virginia creeper are paler and much stouter than the hairy roots of poison ivy, which are dark red. Virginia creeper and poison ivy grown in the same habitat and frequently co-occur, often on the same trees.


 * Poison Sumac (Toxicodendron vernix) has compound leaves with 7–15 leaflets. Poison sumac never has only three leaflets.


 * Kudzu (../Pueraria lobata/) is a non-toxic 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.


 * Blackberry and raspberry vines bear a passing resemblance to poison ivy, with which may share territory. The chief difference between blackberry vines and poison ivy is that blackberry vines have spines on them, whereas poison ivy is smooth. Also, the three-leaflet pattern of blackberry vine leaves changes as the plant grows: the two bottom leaves both split into two leaves, for a total of five in a cluster. They have many teeth along the leaf edge, and the top surface of their leaves is very wrinkled where the veins are, and the bottom of the leaves is light minty - greenish white, while poison ivy is all green. The stem and vine of poison ivy are brown and woody, while blackberry stems are green with thorns.

Toxic effects

 * See ../Urushiol-induced contact dermatitis/ for more details about the rash, including prevention and treatment.

The reaction caused by poison-ivy, urushiol-induced contact dermatitis, is an allergic reaction. Around 15% to 30% of people have no allergic response, but most if not all will become sensitized over time with repeated or more concentrated exposure to urushiol.

For those who are affected by urushiol, it causes a very irritating rash. In extreme cases, corticosteroids are needed to treat rashes and severe itching. The first symptom of contact is a severe itching of the skin that develops into reddish colored inflammation or non-colored bumps, and then blistering of the skin occurs. In severe cases, clear fluids ooze from open blistered sores. The urushiol poison is quickly bound to the skin after contact.

The oozing fluids released by itching blisters do not spread the poison. The appearance of a spreading rash indicates that some areas received more of the poison and reacted sooner than other areas. The blisters and oozing result from blood vessels that develop gaps and leak fluid through the skin; if the skin is cooled, the vessels constrict and leak less. If poison ivy is burned and the smoke then inhaled, this rash will appear on the lining of the lungs, causing extreme pain and possibly fatal respiratory difficulty. If poison ivy is eaten, the digestive tract, airway, kidneys or other organs can be damaged.

Understanding why new lesions may develop for two weeks (studied on forearm) after one exposure was made clear by a University of Miami scientist: Larger amounts have earliest onset and largest reaction, smaller amounts produce a delayed reaction. The overall severity "progresses" with the combined active lesions. Therefore, the last new lesion should occur at two weeks after last exposure, the total rash (untreated) may go on for 3-4 weeks.

Urushiol oil can remain active for several years, so handling dead leaves or vines can cause a reaction. In addition, oil transferred from the plant to other objects, such as pet fur or gardening tools, can cause the rash if it comes into contact with the skin.

People who are sensitive to poison-ivy can also experience a similar rash from mangoes; the skin of mangoes has a chemical compound similar to urushiol. Similar reactions have been reported occasionally from contact with the related aromatic sumac or Japanese lacquer tree.