Poison ivy is one of those plants most everyone has heard about, if not suffered miserably from. It is the only woody perennial you will learn about in this course. As a young boy constantly running through the woods I got blisters just about every summer. I still get blisters if I get near it. Some people can rub it and not get any iritation. This is a good example of genotypic-phenotypic variability among human beings. My daughter ate some once and her head become very swollen, she was in absolute misery for 2 weeks. I was very worried for her, but she survived. Learn this weed for survival, if not for this course.
The best way to ID poison ivy is by the 3-leaflet leaf. The outside leaflets have a little notch, like a thumb, on one side (below).
Usually we think poison ivy is only a little plant in the woods or along the edge of fields (below, left). Actually, it is a vine that gets very large and long. Often you will see it growing very long and twining around larger trees in its struggle to get light (below, right). All forms of the plant have the active ingredient that causes the blisters. The blistering oil gets on your skin and quickly degrades or rubs off. The blisters you get are a result of this often brief encounter. There is nothing you can do to avoid this process except wash immediately after contact and hope you are one of those lucky, resistant, genotypes.