Toxicodendron rydbergii

Poison Ivy

Toxicodendron rydbergii, Rydberg's Poison Ivy

Flora, fauna, earth, and sky...
The natural history of the northwoods



  • Toxicodendron, from the Latin toxicum, "poison", and the Greek dendron, "tree"; hence "poison tree"
  • rydbergii, from the Latin, "Rydberg's"; named after Per Axel Rydberg (1850-1931), an expert on Western flora.
  • Common Name, from
  • Other common names include: Western Poison Ivy, Ryberg's Poison Ivy, Non-Climbing Poison Ivy


  • Kingdom Plantae, the Plants
    • Division Magnoliophyta, the Angiosperms (flowering plants)
      • Class Magnoliopsida, the Dicotyledons
      • Subclass Rosidae, the Roses
        • Order Sapindales
          • Family Anacardiaceae, with cashews, mangos, and pistachios
            • Genus Toxicodendron, the poison ivies, oaks, and sumacs
  • Taxonomic Serial Number: 28822

  • Also known as Rhus radicans, Rhus radicans var. rydbergii, Rhus radicans var. vulgaris, Rhus rydbergii, Rhus toxicodendron var. rydbergii, Rhus toxicodendron var. vulgaris, Toxicodendron desertorum, Toxicodendron radicans var. rydbergii
  • Considered a sub-species of Toxicodendron (Rhus) radicans until the turn of the century. To add to the taxonomic confusion, it hybridizes with that species.
  • Over two hundred years ago, plant taxonomists, starting with Linnaeus himself, lumped the poisonous ivies, oaks, and sumac in with the other less-poisonous varieties of the Rhus genus. This misclassification has caused confusion everywhere. About thirty years ago, taxonomists decided to move the most poisonous varieties into their own genus, the Toxicodendrons.


  • Distinguished from closely related Rhus genus by having cream colored berries where those of Rhus are red.
  • "Leaves of three, let it be."


  • A native, rhizomatous, low shrub. Throughout much of its range, assumes a subshrub growth form, typically less than 3' tall. However, under favorable site conditions, where plants have remained relatively undisturbed for several decades, individuals sometimes reach heights of 10' or more. Exhibits a nonclimbing habit, differentiating it from Eastern Poison Ivy (Toxicodendron radicans), which produces arerial roots.
  • Stems somewhat woody, simple or sparsely branched, arising from much branched rhizomes.
  • Leaves long-stalked, borne alternately near the summit of the stem and divided into three coarse-toothed leaftlets.


  • The most northerly of the Toxicodendron complex, ranging across southern Canada from British Columbia to Nova Scotia, and throughout the northern United States from central Washington and Oregon to New England, extending south to Illinois, Iowa, Nebraska, Kansas, Texas, Arizona, and New Mexico.


  • Considered a ubiquitous weed; easily invades disturbed areas such as roadsides, lakeshores, floodplains, fencerows, logging units, sand dunes, and railroad rights-of-way. Plants rarely dominate large areas but may become locally abundant in mesic sites with moderate amounts of sunlight such as rills, ravines, edges of waterfalls, creekbanks, streambottoms, river terraces, and floodplains.
  • It occurs on a variety of soils.
  • In the East typically occupies mesic sites but is also commonly found on rocky fields, pastures, talus slopes, precipices, gypsum cliffs, and slatey ledges. A successional species in a variety of plant communities throughout its range.
  • Woodlands with poison ivy present in the understory include Bur Oak (Quercus macrocarpa), Green Ash (Fraxinus pennsylvanica), Green Ash/American Elm (Ulmus americana), Bur Oak/Green Ash, and Quaking Aspen (Populus tremuloides).


  • Well adapted to disturbance by fire.
  • Plants resprout via an extensive rhizome system following cool fires.
  • Postfire regeneration may also involve seedling establishment from off-site seeds which are dispersed by birds and mammals.



  • The first European to describe this plant was Captain John Smith in 1609. It was he who coined the name "Poison Ivy." Ever since then, there has been confusion regarding this plant and its cousins.


  • Although contact often causes a debilitating rash in humans, wildlife and livestock can browse without any ill effects. Plants, however, are only sparsely browsed by either livestock or wildlife.
  • A milky oil in the phloem of Poison Ivy may cause blistering of human skin within a few hours of contact. Plants must somehow be damaged in order for the oil to be emitted. Skin rashes can result from contact with either the liquid oil or its dried, blackened residue. Secondary objects such as hand tools can also transmit the poison.


  • Reproduces by seed and vegetatively by rhizomes
  • Sexual reproduction: Numerous, monecious flowers produced in small, dense axillary clusters. Birds and mammals disperse the fruits. Unconsumed fruits are retained on the plant through winter and are deposited beneath parent plant in spring. Bare mineral soil appears to be conducive to germination and establishment.
  • Vegetative reproduction: Vegetative expansion by rhizome is a major mode of reproduction in established plants. Leafy shoots are produced at basal stem nodesalong much branched rhizomes; on some sites, rhizomes may extend up to 7' beyond the parent plant. As a result of this extensive network of rhizomes, Poison Ivy frequently forms thickets under favorable site conditions. These thickets may represent a single clone or several individuals.


  • Like you'd want to!!


  • Noxious native presenting threat to humans.


Boreal border
Last updated on 9 August 1999