The pentadecyl catechols 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. Over 350,000 people are affected by urushiol annually in the United States. Typically, the rash from the urushiol oil lasts about five to twelve days, but in extreme cases, it can last a month or more. Around 15 to 25 percent of people have no allergic reaction to urushiol, but most people have a greater reaction with repeated or more concentrated exposure. In extreme cases, a reaction can progress to anaphylaxis. Urushiol-induced contact dermatitis is the allergic reaction caused by poison ivy. Main article: Urushiol-induced contact dermatitis Poison ivy's growth and potency has already doubled since the 1960s, and it could double again once carbon dioxide levels reach 560 ppm. Higher carbon dioxide levels increase the rate of plant growth, and cause them to produce more unsaturated urushiol, which causes stronger reactions in humans. Poison ivy is particularly sensitive to carbon dioxide levels, greatly benefiting from higher concentrations in the atmosphere. radicans is also found in parts of China. It is listed as a noxious weed in the US states of Minnesota and Michigan and in the Canadian province of Ontario. 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 more common now than when Europeans first arrived in North America. It can grow in areas subject to seasonal flooding or brackish water.
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). Older vines on substantial supports send out lateral branches that may be mistaken for tree limbs at first glance. The plants can grow as a shrub up to about 1.2 m (3.9 ft) tall, as a groundcover 10–25 cm (3.9–9.8 in) high, or as a climbing vine on various supports. radicans rarely grows at altitudes above 1,500 m (4,900 ft), although the altitude limit varies in different locations. orientale in Taiwan, Japan, Korea and Sakhalin. rydbergii (western poison ivy) are found in western North America, and T. The plant is extremely common in suburban and exurban areas of New England, the Mid-Atlantic, and the Southeastern United States. It may grow as a forest understory plant, although it is only somewhat shade-tolerant. It also grows in exposed rocky areas, open fields, and disturbed areas. It is normally found in wooded areas, especially along edge areas where the tree line breaks and allows sunshine to filter through. Caquistle or caxuistle is the Nahuatl term for the species. states east of the Rocky Mountains, as well as in the mountainous areas of Mexico up to around 1,500 m (4,900 ft). radicans grows throughout much of North America, including the Canadian Maritime provinces, Quebec, Ontario, Manitoba, and all U.S.
Leaves may be smooth or notched on the same plant. Seeds are spread mainly by animals and remain viable after passing through the digestive tract.
Fruits are a favorite winter food of some birds and other animals. The berry-like fruit, a drupe, mature by August to November with a grayish-white colour. The yellowish- or greenish-white flowers are typically inconspicuous and are located in clusters up to 8 cm (3 in) above the leaves. It is dioecious flowering occurs from May to July. radicans spreads either vegetatively or sexually. The milky sap of poison ivy darkens after exposure to the air. The vines develop adventitious roots, or the plant can spread from rhizomes or root crowns. Vines growing on the trunk of a tree become firmly attached through numerous aerial rootlets. Leaflet clusters are alternate on the vine, and the plant has no thorns. Each leaflet has a few or no teeth along its edge, and the leaf surface is smooth. The leaflets of mature leaves are somewhat shiny. 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. radicans are trifoliate with three almond-shaped leaflets.