Poison Ivy

Toxicodendron radicans, commonly known as poison ivy (older synonyms are Rhus toxicodendron and Rhus radicans),[1] is a poisonous North American and Asian flowering plant that is well known for causing an itching, irritating, and sometimes painfulrash in most people who touch it, caused by urushiol, a clear liquid compound in the sap of the plant.[2] Urushiol, however, is not a defensive measure; it helps the plant to retain water.[citation needed] It is variable in its appearance and habit, and despite its common name it is not a true ivy (Hedera). T. radicans is commonly eaten by many animals, and the seeds are consumed by birds,[3] but poison ivy is most often thought of as an unwelcome weed.

There are numerous subspecies and/or varieties of T. radicans ,[4] which can be found growing in any of the following forms, all have woody stems:

• • as a trailing vine that is 10–25 centimetres (3.9–9.8 in) tall

  • as a shrub up to 1.2 metres (3 ft 11 in) tall

  • as a climbing vine that grows on trees or some other support

The deciduous leaves of T. radicans are trifoliate with three almond-shaped leaflets.[1] 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.[5] 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.

The urushiol compound in poison ivy is not meant as a defensive measure; rather, it helps the plant to retain water. It is frequently eaten by animals such as deer and bears.[6]

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 (3.1 in) above the leaves. The berry-like fruit, adrupe, mature by August to November with a grayish-white colour.[1] 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

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:[8]

• 1. "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, but other, non-harmful plants have similar leaves.[9]

  2. "Hairy vine, no friend of mine. "[10]

  3. "Berries white, run in fright" and "Berries white, danger in sight. "[10]