Toxicodendron radicans (syn. Rhus toxicodendron, Rhus radicans; Poison ivy) is a plant in the family Anacardiaceae. The name is sometimes spelled "Poison-ivy" in an attempt to indicate that the plant is not a true Ivy (Hedera). It 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.
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Poison Ivy grows throughout much of North America, including all Canadian provinces except Newfoundland (and the Territories) and all U.S. states except Alaska, Hawaii, and California (which instead houses Poison-Oak, a very similar plant), as well as in the mountainous areas of Mexico up to around 5,000 ft (see caquistle or caxuistlethe Nahua term), and is normally found in wooded areas, especially along edge areas. It also grows in exposed rocky areas and in open fields and disturbed areas. It also grows as a forest understory plant, although it is only somewhat shade tolerant[1]. The plant is extremely common in suburban and exurban areas of New England, the Mid-Atlantic and Southeastern United States. It rarely grows at altitudes above 1,500 meters (5,000 ft), although the altitude limit varies in different locations.[1] The plants can grow as a shrub up to about 1.2 meters (4 ft) tall, as a groundcover 1025 centimeters (410 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.

It is not particularly sensitive to soil moisture, although it does not grow in desert or arid conditions. It grows in a wide variety of soil types, and soil pH from 6.0 (acidic) to 7.9 (moderately alkaline). It can grow in areas subject to seasonal flooding or brackish water.

It is more common now than when Europeans first entered North America. Real estate development adjacent to wild, undeveloped land has engendered "edge effects," enabling poison ivy to form vast, lush colonies in such places. It is listed as a noxious weed in the U.S. states of Minnesota and Michigan and the Canadian province of Ontario.Poison ivy flowers Box Elder on the left, poison ivy on the right
Poison ivy vine with typical reddish "hairs"

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. This is the basis of mnemonics such as "Leaves of three, let it be; leaves of four, eat some more"; variants include "... hairy vine, no friend of mine", "... berries white, run in fright" and "... berries white, danger in sight".

The color ranges from light green (usually the younger leaves) to dark green (mature leaves), turning bright red in fall; though other sources say leaves are red when young, turn green through maturity, then back to red, orange, or yellow in the 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 are sufficient to positively identify the plant:  clusters of three leaflets,alternate, and lack of thorns. If it is growing up the trunk of a tree, the presence of copious root-hairs will identify it, leading to the "hairy vine, no friend of mine" warning.

Poison ivy spreads both vegetatively and sexually. The vines put down adventitious roots, or the plant can spread from rhizomes or root crowns. The plant flowers in May to July and produces mature fruits by August to November. Seeds are spread mainly by animals, and are viable after passing through the digestive tract of birds.

The reaction caused by poison ivy, urushiol-induced contact dermatitis, is an allergic reaction. Around 15%[5] to 30% of people have no allergic response, but most people will become sensitized with repeated or more concentrated exposure to urushiol. Reactions can progress to anaphylaxis.

Urushiol binds to the skin on contact, where it causes severe itching that develops into reddish colored inflammation or non-colored bumps, and then blistering. These lesions may be treated with calamine lotion, Burow solution compresses, or Aveeno baths to relieve discomfort.In severe cases, clear fluids ooze from open blistered sores and corticosteroids are the necessary treatment.

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 or that contamination is still occurring from contact with objects to which the original poison was spread. 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. An untreated rash can last up to four 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) 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. Mangoes are in the same family (Anacardiaceae) as poison ivy; the sap of the mango tree and skin of mangoes has a chemical compound similar to urushiol.

Similar reactions have been reported occasional

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-- Edited by Ranade on Wednesday 17th of June 2009 11:36:31 AM