Toxicodendron rydbergii  (Small ex Rydb.) Greene

by Marie Snyder, Native Plants of Arizona 2009
Common names: Western Poison Ivy, Poison Ivy (1).
Family: Andardiaceae (Sumac Family) (1).
Synonymy: Toxicodendron radicans var. rydbergii (Small ex Rydb.) Erskine (1).
Etymology: Toxicoden'dron: means "poison tree." Ryd'bergii: named after Per Axel Rydberg (1860-1931), a curator at the New York Botanical Garden who wrote the first book on the flora of the Rockies from Canada to Mexico and several other floras. Rad'icans: with rooting stems (6).

Identification
Growth form: Shrub, subshrub,or rarely vine-like (1); different morphology depending on its location. Can be vine-like ivy, especially when it grows over rocks. Sometimes erect with few-branched stems up to a few feet tall with trifoliate, shiny, dentate leaves. Large stands with this morphology occur along creeks and rivers (4). up to 2m (6ft) tall (8).
Stem: Dull brown with conspicuous sloped scars where previous leaves were attached (2).
Leaves: Alternate, compound; shiny; leaflets 3, ovate-lanceolate, dentate, the terminal leaflet with a longer stalk than the lateral leaflets (4).
Inflorescence/flowers: Inconspicuous,in small auxiliary clusters (5).
Fruit: Berries are clustered and off-white (2).
Similar species: Toxicodendron radicans, the eastern species, has aerial roots and is more viney with hairy petioles and a smaller terminal leaflet. Toxicodendron radicans ssp. divaricatum occurs in Cochise and Santa Cruz counties.

Ecology
Life history: Perennial
Native/introduced: Native (1).
Distribution: Broadly distributed throughout the western and central states including (AZ, CO, CT, IA, ID, IL, IN, KS, MA, MD, ME, MI, MN, MT, NC, ND, NE, NH, NM, NV, NY, OH, OK, OR, PA, RI, SD, TX, UT, VA, VT, WA, WI, WV,WY) and Canadian Providences (AB, BC, MB, NB, NS, ON, PE, QC, SK, YT). In Arizona it has been reported from all counties except Greenlee, La Paz , Maricopa, Pinal, and Yuma (1).

Uses : All species of Toxicodendron are equally toxic. Juices and all parts of the plants are toxic year round (2). Most people have an allergic reaction to poison ivy, becoming sensitized by repeated exposure. The reaction is a contact dermatitis, with a severe form consisting of weeping open blisters. The more severe reactions require treatment by a physician, while milder ones can be treated with over the counter medications. The offending agent in the plant is the chemical Urushiol. The appearance of the rash may be delayed by a few days after exposure in some people so that it may be difficult to figure out the cause of the reaction (4)

References
1. USDA, NRCS. (2007). The PLANTS Database . Retrieved Sept. 21, 2007 , from http://plants.usda.gov .
2. Southwest Colorado Wildflowers (2001). Retrieved October 7, 2009 from http://www.swcoloradowildflowers.com/White%20Enlarged%20Photo%20Pages/toxicodendron%20rydbergii.htm 3. Lady Bird Johnson Flower Center (2009). Native Plant Database. Retrieved Sept 9, 2009 from http://www.wildflower.org/plants/result.php?id_plant=LOAR
4. Vascular Plants of the Gila Wilderness. Western New Mexico University Department of Natural Sciences. (2009). Retrieved Sept 9, 2009 from
http://www.wnmu.edu/academic/nspages2/gilaflora/lonicera_arizonica.html
5. Epple, Anne Orth. (1995). A Field Guide to the Plants of Arizona . LewAnn Publishing Company, Mesa , AZ in cooperation with SkyHouse Publishers.
6. Charters, Michael L. California Plant Names: Latin and Greek Meanings and Derivations. Retrieved Sept 21, 2007 from http://www.calflora.net/botanicalnames/index.html
7. Moerman, D. 2003. Native American Ethnobotany Database ( http://herb.umd.umich.edu/ ). University of Michigan , Dearborn , MI 48198 U
8. Oaklahoma Biological Survey. (1999) Retrieved October 5, 2009 from http://www.biosurvey.ou.edu/shrub/tory.htm