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Wisconsin Plant of the Week

Toxicodenron radicans (L.) Kuntze

Anacardiaceae (Cashew Family)

Common Name:

poison-ivy

11/08/1999

Poison ivy is a native woody species that grows in 3 forms in Wisconsin -- an erect shrub about 1 m tall, a short shrub that spreads by underground stems or rhizomes, and a climbing woody vine with aerial roots.  The shrub forms are found statewide in sandy or rocky soil of clearings and at the forest edge.  The vine form is more common in wooded floodplains especially in the south and west.

On all forms the leaves are alternate and compound with 3 leaflets.  Each leaflet is stalked with the longest stalk on the terminal leaflet.  Leaflets are 5 to 15 cm long and pointed, with edges either smooth or coarsely toothed. Emerging leaves may be red, then shiny green, later turning dull green.  The closest "look alike" is a box elder seedling, but box elder leaves are opposite and always dull green.

Small green flowers, less than 3 mm, bloom in June in loosely branched axillary clusters.  The fruit are ivory to gray, smooth, about 5 mm across and persist into the winter.

Like poison sumac, all parts of poison-ivy are poisonous due to an oil that causes contact dermatitis in many, sometimes after a few hours and sometimes not for days.  The oil is not volatile, and despite many claims, one can't be affected merely by being near the plant. The fluid in the blisters of an infection also does not spread the rash.

The plant must be bruised to release the oil and only a minute amount of oil is required to cause the rash in sensitive people. The oil is released in droplets in smoke from burning plants and is also easily transferred from bruised plants to shoes, clothing or an animal, where it may remain for a long time before being transferred to sensitive skin.

  • Origin of the Name: Toxicodendron, L., toxicum, poison; Gr., dendron, a tree; radicans:  L., radix, a root; refers to the rooting stems
  • Range:  s. N.S. to FL, w. to MI, se MN, OK, TX, also China & Japan
  • WI Range:  Statewide
  • Common associates:  Oaks, silver maple, bluegrass
  • Wetland Indicator Status: FAC+
  • Coefficient of Conservatism: C = 2 (S&W), C = 2 (MI))
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