poison ivy Weed Description: A woody vine that may occur as a weed of landscapes, woods, fencerows, pastures, and hay fields. Poison ivy is the major cause of allergenic dermatitis in the eastern United States, which causes inflammation, blistering, and itching of the skin. The plant sap contains a chemical called urushiol, which is found within ducts in the leaves, flowers, stems, and roots of this weed. When poison ivy plants are bruised or damaged, this chemical is emitted onto the leaf and stem surfaces where humans and animals may come into contact with it. Poison ivy is found throughout the southern United States east of the Mississippi River. It also occurs more sporadically in the midwestern and northern United States.