Poison Ivy

Poison ivy is a type of allergenic plant in the genus Toxicodendron native to Asia and North America. Formerly considered a single species, Toxicodendron radicans, poison ivies are now generally treated as a complex of three separate species: Toxicodendron radicans, Toxicodendron rydbergii, and Toxicodendron orientale. They are well known for causing urushiol-induced contact dermatitis, an itchy, irritating, and sometimes painful rash, in most people who touch it. The rash is caused by urushiol, a clear liquid compound in the plant's sap. They are variable in appearance and habit, and despite its common name, it is not a "true" ivy (Hedera), but rather a member of the cashew and pistachio family (Anacardiaceae). T. radicans is commonly eaten by many animals, and the seeds are consumed by birds, but poison ivy is most often thought of as an unwelcome weed.

Kingdom:
Plantae
Clade:
Tracheophytes
Clade:
Angiosperms
Clade:
Eudicots
Clade:
Rosids
Order:
Sapindales
Family:
Anacardiaceae
Genus:
Toxicodendron
Species:
T. radicans

Numerous subspecies and/or varieties of T. radicans are known.[4] They can be found growing in any of the following forms, all having woody stems:

as a climbing vine that grows on trees or some other support
as a shrub up to 1.2 metres (4 feet) tall
as a trailing vine that is 10–25 centimetres (4–10 inches) tall
The deciduous leaves of T. radicans are trifoliate with three almond-shaped leaflets.[5] 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+1⁄4–4+3⁄4 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.[6] 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.

T. radicans spreads either vegetatively or sexually. It 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 in) above the leaves. The berry-like fruit, a drupe, mature by August to November with a grayish-white colour.[5]

These 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 middle stem is longer.[7] The appearance of poison ivy can vary greatly among environments, and even within a large 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:[8]

"Leaves of three, let it be" is the best known and most useful cautionary rhyme. It applies to poison oak, as well as to poison ivy. However, some other innocuous plants have similar leaves.[9]
"Hairy vine, no friend of mine"[10]
"Berries white, run in fright" and "Berries white, danger in sight"[10]

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