Toxicodendron rydbergii the western poison ivy, is a species of Toxicodendron in the cashew family. It is native to most of Canada from the Maritimes to British Columbia, and most of the contiguous United States except the southeastern states, New Jersey, Delaware, and California. It can be found growing in forests, and other wooded areas, usually near streams and rivers.
Unlike Toxicodendron radicans (eastern poison ivy), which often appears as a trailing or climbing vine, Toxicodendron rydbergii is a shrub that can grow to 1 m (3 ft) tall, rarely up to 3 m (10 ft). The leaves are trifoliate and alternate. The leaflets are variable in size and shape, and are usually 15 cm (6 in) long, turning yellow or orange in autumn. On the compound trifoliate leaves, the two leaflets opposite each other are typically asymmetrical, in contrast to the terminal leaflet which always shows bilateral symmetry. The fruits are small, round, and yellowish.
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- "Toxicodendron rydbergii". County-level distribution map from the North American Plant Atlas (NAPA). Biota of North America Program (BONAP). 2014.
- Innes, Robin J. (2012). "Toxicodendron radicans, T. rydbergii". Fire Effects Information System (FEIS). US Department of Agriculture (USDA), Forest Service (USFS), Rocky Mountain Research Station, Fire Sciences Laboratory – via https://www.feis-crs.org/feis/.
- Rydberg, Per Axel 1900. Memoirs of The New York Botanical Garden 1: 268–269 as Rhus rydbergii