How big does poison ivy grow?

Poison sumac [Toxicodendron vernix] may reach a mature height of 7 meters [20 feet]. The plant contains the same urushiol oil that's the hallmark of poison ivy [Toxicodendron radicans] and poison oak [Toxicodendron diversilobum, and Toxicodendron pubescens]. But the allergic reaction that it may cause in humans is the most extreme of the three plants. In fact, many botanists rank poison sumac among the most toxic of plants in the United States of America.