Toxicodendron radicans, commonly known as poison ivy (older synonyms are Rhus toxicodendron and Rhus radicans), is a poisonous North American and Asian plant that is well known for its production of urushiol, a clear liquid compound in the sap of the plant that causes an itching, irritation and sometimes painful rash in most people who touch it. The plant is not a true ivy (Hedera).
Poison ivy can be found growing in any of the following forms:
as a trailing vine that is 10–25 centimetres (3.9–9.8 in) tall
Poison ivy grows throughout much of North America, including the Canadian Maritime provinces, Quebec, Ontario, Manitoba, and all U.S. states east of the Rocky Mountains, as well as in the mountainous areas of Mexico up to around 1,500 m (4,900 ft) (caquistle or caxuistle is the Nahuatl term[clarification needed] ). It is normally found in wooded areas, especially along edge areas where the tree line breaks and allows sunshine to filter through. It also grows in exposed rocky areas, open fields and disturbed areas.
It may grow as a forestunderstory plant, although it is only somewhat shade tolerant. The plant is extremely common in suburban and exurban areas of New England, the Mid-Atlantic, and southeastern United States. Similar species, poison oak, and Toxicodendron rydbergii are found in western North America. Poison ivy rarely grows at altitudes above 1,500 m (4,900 ft), although the altitude limit varies in different locations. The plants can grow as a shrub up to about 1.2 metres (3.9 ft) tall, as a groundcover 10–25 cm (3.9–9.8 in) high, or as a climbing vine on various supports. Older vines on substantial supports send out lateral branches that may be mistaken for tree limbs at first glance.
It grows in a wide variety of soil types, and soil pH from 6.0 (acidic) to 7.9 (moderately alkaline). It is not particularly sensitive to soil moisture, although it does not grow in desert or arid conditions. It can grow in areas subject to seasonal flooding or brackish water. 
It is more common now than when Europeans first arrived in North America. The development of real estate adjacent to wild, undeveloped land has engendered "edge effects," enabling poison ivy to form vast, lush colonies in these areas. It is listed as a noxious weed in the U.S. states of Minnesota and Michigan, and the Canadian province of Ontario.
Outside North America, poison ivy is also found in the temperate parts of Asia, in Japan, Taiwan, the Russian islands of Sakhalin and the Kuriles, and in parts of China.
A study by researchers at the University of Georgia found that poison ivy is particularly sensitive to CO2 levels, greatly benefiting from higher CO2 in the atmosphere. Poison ivy's growth and potency has already doubled since the 1960s, and it could double again once CO2 levels reach 560 ppm.
Poison ivy vine with typical reddish "hairs" (like leaves, vines are extremely poisonous to humans)
The deciduousleaves of poison ivy are trifoliate with three almond-shaped leaflets. Leaf 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 reddish when expanding, 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 (1.2–4.7 in) long, rarely up to 30 cm (12 in). 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. Vines growing on the trunk of a tree become firmly attached through numerous aerial rootlets. The vines develop adventitious roots, or the plant can spread from rhizomes or root crowns. The milky sap of poison ivy darkens after exposure to the air.
Poison ivy spreads either vegetatively or sexually. Poison ivy is dioecious; flowering occurs from May to July. The yellowish- or greenish-white flowers are typically inconspicuous and are located in clusters up to 8 cm (3.1 in) above the leaves. The berry-like fruit, a drupe, mature by August to November with a grayish-white colour. Fruits are a favorite winter food of some birds and other animals. Seeds are spread mainly by animals and remain viable after passing through the digestive tract.
Aids to identification
Poison ivy leaf and berries
The following four characteristics are sufficient to identify poison ivy in most situations: (a) clusters of three leaflets, (b) alternate leaf arrangement, (c) lack of thorns, and (d) each group of three leaflets grows on its own stem, which connects to the main vine.
The appearance of poison ivy can vary greatly between environments, and even within a single area. Identification by experienced people is often made difficult by leaf damage, the plant's leafless condition during winter, and unusual growth forms due to environmental or genetic factors.
Various mnemonic rhymes describe the characteristic appearance of poison ivy:
"Leaflets three; let it be" is the best known and most useful cautionary rhyme. It applies to poison oak, as well as to poison ivy.
"Longer middle stem, stay away from them." This refers to the middle leaflet having a visibly longer stem than the two side leaflets and is a key to differentiating it from the similar-looking Rhus aromatica (fragrant sumac).
"Raggy rope, don't be a dope!" Poison ivy vines on trees have a furry "raggy" appearance. This rhyme warns tree climbers to be wary. Old, mature vines on tree trunks can be quite large and long, with the recognizable leaves obscured among the higher foliage of the tree.
"One, two, three? Don't touch me."
"Berries white, run in fright" and "Berries white, danger in sight."
"Red leaflets in the spring, it's a dangerous thing." This refers to the red appearance that new leaflets sometimes have in the spring. (Note that later, in the summer, the leaflets are green, making them more difficult to distinguish from other plants, while in autumn they can be reddish-orange.)
"Side leaflets like mittens, will itch like the dickens." This refers to the appearance of some, but not all, poison ivy leaves, where each of the two side leaflets has a small notch that makes the leaflet look like a mitten with a "thumb." (Note that this rhyme should not be misinterpreted to mean that only the side leaflets will cause itching, since actually all parts of the plant can cause itching.)
"If it's got hair, it won't be fair." This refers to the hair that can be on the stem and leaves of poison ivy.
Over 350,000 people are affected by poison ivy annually in the US.
The pentadecylcatechols of the oleoresin within the sap of poison ivy and related plants causes the allergic reaction; the plants produce a mixture of pentadecylcatechols, which collectively is called urushiol. After injury, the sap leaks to the surface of the plant where the urushiol becomes a blackish lacquer after contact with oxygen.
Poison ivy on a roadside
Urushiol binds to the skin on contact, where it causes severe itching that develops into reddish coloured inflammation or non-coloured bumps, and then blistering. These lesions may be treated with Calamine lotion, Burow's solution compresses or baths to relieve discomfort, though recent studies have shown some traditional medicines to be ineffective. Over-the-counter products to ease itching—or simply oatmeal baths and baking soda—are now recommended by dermatologists for the treatment of poison ivy. A plant based remedy cited to counter urushiol induced contact dermatitis is jewelweed. "The results of a clinical study, in which a 1:4 jewelweed preparation was compared for its effectiveness with other standard poison ivy dermatitis treatments was published in 1958 (Annals of Allergy 1958;16:526-527). Of 115 patients treated with jewelweed, 108 responded ‘most dramatically to the topical application of this medication and were entirely relieved of their symptoms within 2 or 3 days after the institution of treatment.' It was concluded that jewelweed is an excellent substitute for ACTH and the corticosteroids in the treatment of poison ivy dermatitis. However, other sources have since concluded that jewelweed is ineffective or of questionable effectiveness. 
The oozing fluids released by scratching blisters do not spread the poison. The fluid in the blisters is produced by the body and it is not urushiol itself. 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 mucus lining of the mouth and digestive tract can be damaged. A poison ivy rash usually develops within a week of exposure and can last anywhere from one to four weeks, depending on severity and treatment. In rare cases, poison ivy reactions may require hospitalization.
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. Clothing, tools, and other objects that have been exposed to the oil should be washed to prevent further transmission.
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. A related allergenic compound is present in the raw shells of cashews. Similar reactions have been reported occasionally from contact with the related Fragrant Sumac (Rhus aromatica) and Japanese lacquer tree. These other plants are also in the Anacardiaceae family.
Immediate washing with soap and water or rubbing alcohol may help prevent a reaction. During a reaction, Calamine lotion or diphenhydramine may help mitigate symptoms. Corticosteroids, either applied to the skin or taken by mouth, may be appropriate in extreme cases.
Virgin's Bower (Clematis virginiana) (also known as Devil's Darning Needles, Devil's Hair, Love Vine, Traveller's Joy, Virginia Virgin's Bower, Wild Hops, and Woodbine; syn. Clematis virginiana L. var. missouriensis (Rydb.) Palmer & Steyermark ) is a vine of the Ranunculaceae family native to the United States. This plant is a vine that can climb up to 10–20 ft tall. It grows on the edges of the woods, moist slopes, and fence rows, and in thickets and streambanks. It produces white, fragrant flowers about an inch in diameter between July and September.
Box-elder (Acer negundo) saplings have leaves that can look very similar to those of poison ivy, although the symmetry of the plant itself is very different. While box-elders often have five or seven leaflets, three leaflets are also common, especially on smaller saplings. The two can be differentiated by observing the placement of the leaves where the leaf stalk meets the main branch (where the three leaflets are attached). Poison ivy has alternate leaves, which means the three-leaflet leaves alternate along the main branch. The maple (which the box-elder is a type of) has opposite leaves; another leaf stalk directly on the opposite side is characteristic of box-elder.
Virginia creeper (Parthenocissus quinquefolia) vines can look like poison ivy. The younger leaves can consist of three leaflets but have a few more serrations along the leaf edge, and the leaf surface is somewhat wrinkled. However, most Virginia creeper leaves have five leaflets. Virginia creeper and poison ivy very often grow together, even on the same tree. Be aware that even those who do not get an allergic reaction to poison ivy may be allergic to the oxalate crystals in Virginia creeper sap.
Western Poison-oak (Toxicodendron diversilobum) leaflets also come in threes on the end of a stem, but each leaflet is shaped somewhat like an oak leaf. Western Poison-oak grows only in the western United States and Canada, although many people will refer to poison ivy as poison-oak. This is because poison ivy will grow in either the ivy-like form or the brushy oak-like form depending on the moisture and brightness of its environment. The ivy form likes shady areas with only a little sun, tends to climb the trunks of trees, and can spread rapidly along the ground.
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.
Blackberries and raspberries (Rubus spp.) can resemble poison ivy, with which they may share territory; however, blackberries and raspberries almost always have thorns on their stems, whereas poison ivy stems are smooth. Also, the three-leaflet pattern of some blackberry and raspberry leaves changes as the plant grows: Leaves produced later in the season have five leaflets rather than three. Blackberries and raspberries have many fine teeth along the leaf edge, the top surface of their leaves is very wrinkled where the veins are, and the bottom of the leaves is light minty-greenish white. Poison ivy is all green. The stem of poison ivy is brown and cylindrical, while blackberry and raspberry stems can be green, can be squared in cross-section, and can have prickles. Raspberries and blackberries are never truly vines; that is, they do not attach to trees to support their stems.
The thick vines of Riverbank Grape (Vitis riparia), with no rootlets visible, differ from the vines of poison ivy, which have so many rootlets that the stem going up a tree looks furry. Riverbank grape vines are purplish in colour, tend to hang away from their support trees, and have shreddy bark; poison ivy vines are brown, attached to their support trees, and do not have shreddy bark.
Fragrant Sumac (Rhus aromatica) has a very similar appearance to poison ivy. While both species have three leaflets, the center leaflet of poison ivy is on a long stalk, while the center leaflet of fragrant sumac does not have an obvious stalk. When crushed, fragrant sumac leaves have a fragrance similar to citrus while poison ivy has little or no distinct fragrance. Fragrant sumac produces flowers before the leaves in the spring, while poison ivy produces flowers after the leaves emerge. Flowers and fruits of fragrant sumac are at the end of the stem, but occur along the middle of the stem of poison ivy. Fragrant sumac fruit ripens to a deep reddish color and is covered with tiny hairs while poison ivy fruit is smooth and ripens to a whitish color.
Hoptree (Ptelea trifoliata) has leaves that are remarkably similar. It is, however, a much larger plant so confusion is unlikely for any but the smallest specimens. The flowers and seeds are also easily distinguished from those of poison ivy.
^Tyler’s Herbs of Choice: The Therapeutic Use of Phytomedicinals. Varro Tyler, PhD. ISBN 978-0789028099
^D. Long, N. H. Ballentine, J. G. Marks. Treatment of poison ivy/oak allergic contact dermatitis with an extract of jewelweed. Am. J. Contact. Dermat. 8(3):150-3 1997 PMID 9249283
^Gibson, MR; Maher, FT (1950). "Activity of jewelweed and its enzymes in the treatment of Rhus dermatitis.". Journal of the American Pharmaceutical Association. American Pharmaceutical Association39 (5): 294–6. PMID15421925.