Toxicodendrons of the United States

Abstract

What is a Toxicodendron and why is poison ivy now termed Toxicodendron radicans rather than Rhus radicans? Many scientific articles today continue to use Rhus to describe poison oak and poison ivy, because Linnaeus, the father of taxonomic botany, classified poison ivy, poison oak, and poison sumac in the genus Rhus. The name Toxicodendron originated with Tournefort,1 who preceded Linnaeus, so it is not new. The reasons for its separation from Rhus are cogent,1,2 but until classic treatises by Barkley3 on the Anacardiaceae and Gillis1 on Toxicodendron, there was little tendency to change from the old ways. Brizicky,4 emphasizing exceptions, argued against separation of Toxicodendron, and Barkley5 wrote a rebuttal. Fortunately, a major shift toward use of Toxicodendron now has occurred in scientific publications.Excluding Toxicodendron, (true) Rhus rarely causes contact dermatitis, so these plants should be considered benign. There also are allergenic Anacardiaceae other than Toxicodendron that contain alk(en)yl catechols, so the separation of these genera is based not only on allergenicity, but on a multiplicity of reliable botanic features, many of which can be used in field recognition. Accurate identification of poison ivy throughout all four seasons and in various stages of growth requires practice as well as knowledge of other plants in that locality and terrain. Unfortunately, many persons who can “reliably” recognize these plants often use leaf morphology as their primary criterion, even though leaves are not present much of the year.The material in this chapter is intended as a basis for developing a successful identification system; however, to become proficient, one should identify several plants in a location where they can be observed throughout the year. Before long, the differences between the benign and toxic plants will become so obvious that field identification of typical plants will be almost automatic. Extremely young plants, some winter specimens, and some clones of (eastern) poison oak still can be a problem for almost anyone.