FILE - A poison ivy plant appears at Lancaster County Park, in Lancaster, Pa., on July 22, 2010. Botanically known as Toxicodendron radicans, poison ivy contains oily chemical compounds called urushiols in its leaves, stems and roots. (Marty Heisey/LNP/LancasterOnline via AP, File)
FILE - A poison ivy plant appears at Lancaster County Park, in Lancaster, Pa., on July 22, 2010. Botanically known as Toxicodendron radicans, poison ivy contains oily chemical compounds called urushiols in its leaves, stems and roots. (Marty Heisey/LNP/LancasterOnline via AP, File)
This undated image provided by Bugwood.org shows a dense mat of poison ivy growing in Ohio. (Ohio State Weed Lab/The Ohio State University/Bugwood.org via AP)
This undated image provided by Bugwood.org shows poison ivy growing in Bulloch County, Ga. (David J. Moorhead/University of Georgia/Bugwood.org via AP)
This undated image provided by Bugwood.org shows the hairy, woody vines of poison ivy climbing a tree in Ohio. (Ohio State Weed Lab/The Ohio State University/Bugwood.org via AP)
This undated image provided by Bugwood.org shows a poison ivy plant. (Richard Gardner/ Bugwood.org via AP)
This undated image provided by Bugwood.org shows poison ivy growing in Hubbard County, Minnesota. (Steven Katovich/Bugwood.org via AP)
This undated image provided by Bugwood.org shows the young, red foliage of a poison ivy vine growing in Sault Ste. Marie, Ontario, Canada. (Rob Routledge/Sault College/Bugwood.org via AP)