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GALLERIES > PLANTS AND TREES > POISON IVY [Toxicodendron radicans]

Poison Ivy Photo @
Location: Sycamore Canyon, AZ
GPS: 31.4N, -111.2W, elev=3,940' MAP
Date: August 1, 2009
ID : ? [3888 x 2592]

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Toxicodendron radicans (Poison ivy; older synonyms Rhus toxicodendron, Rhus radicans) 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.

Habitat and range

Poison Ivy grows throughout much of North America, including all Canadian provinces except Newfoundland (and the Territories) and all U.S. states except Alaska, Hawai"?i, and California (which instead houses Poison-Oak, a very similar plant), as well as in the mountainous areas of Mexico up to around 1500 m (5,000 ft) (see caquistle or caxuistle"?the 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. 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 1500 m (5,000 ft), although the altitude limit varies in different locations. The plants can grow as a shrub up to about 1.2 metres (4 ft) tall, as a groundcover 10"?25 cm (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.

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.

Characteristic appearance This article may be confusing or unclear to readers. Please help clarify the article; suggestions may be found on the talk page. (March 2009) Poison ivy vine with typical reddish "hairs"

The leaves are ternate with three almond-shaped leaflets. The fruit, a drupe, is grayish-white color and is a favorite winter food of some birds.

Various mnemonic rhymes describe the characteristic appearance of poison ivy:

  1. "Leaves of three, let it be."
  2. "One, two, three? Don't touch me."
  3. "Hairy vine, no friend of mine."
  4. "Berries white, run in fright" and "Berries white, danger in sight."
  5. "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.
  6. "Longer middle stem, stay away from them." This refers to the middle leaflet having a notably longer stem than the two side leaflets and is a key to differentiating it from the similar-looking Rhus aromatica - Fragrant sumac.
  7. "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 tougher to distinguish from other plants, while in autumn they can be reddish-orange.)
  8. "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.)
  9. "If butterflies land there, don't put your hand there." This refers to the fact that some butterflies land on poison ivy, since they are not affected, which provides them protection as their predators avoid eating the plant.

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 to 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 sometimes believed sufficient to positively identify the plant: (a) clusters of three leaflets, (b) alternate, and (c) lack of thorns (although there are a great number of other plants which also fit that simplified description). 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 remain viable after passing through the digestive tract of birds.

Effects on the body Main article: Urushiol-induced contact dermatitis

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 people will become sensitized with repeated or more concentrated exposure to urushiol. Reactions can progress to anaphylaxis.

Poison ivy on a roadside

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's solution compresses or baths to relieve discomfort, though recent studies have shown some of these traditional medicines to be ineffective. Antihistamines, bentoquatam and other antipruritics are now recommended by dermatologists as more effective in the treatment of poison ivy and prevention of its effects. In severe cases, clear fluids ooze from open blistered sores and corticosteroids are the necessary treatment.

Blisters from contact with poison ivy

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. A poison ivy rash 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.

Similar reactions have been reported occasionally from contact with the related Fragrant Sumac (Rhus aromatica) or Japanese lacquer tree.

Similar-looking plants
  • 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 only grows 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. The chief difference between blackberries and raspberries, on the one hand, and poison ivy, on the other, is that blackberries and raspberries almost always have prickles on the stems, whereas poison ivy is 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 color, 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.

See also
  • Poison oak
  • Poison sumac
  • Urushiol

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