Family: Anacardiaceae
Poison Ivy
Derek Anderson
Key to Wisconsin Toxicodendron
Author: John G. Zaborsky
- 1a.Plant a large shrub with pinnately compound leaves with 5 or more leaflets; infructescences drooping T. vernix
- 1b.Plant a climbing vine, upright shrub, or low shrub with trifoliolate leaves; infructescences erect 2
- 2a.Plant a climbing vine (sometimes an upright shrub), not rhizomatous, aerial roots present on climbing stems; petioles densely pubescent with long hairs; leaflets usually coarsely toothed or lobed; terminal leaflet ovate to elliptic T. radicans
- 2b.Plant a low, erect shrub from creeping rhizomes, aerial roots absent; petioles glabrous to puberulent with short, incurved hairs; leaflets usually entire or with shallow basal lobes on the lateral leaflets; terminal leaflet broadly ovate to rhombic or suborbicular T. rydbergii
This genus is infamous for the resinous oil that all of its members produce: urushiol. The oil penetrates human skin and causes an allergic reaction resulting in an itchy or painful rash. One must touch the plant to be exposed to the oil and it can also be spread via tools, clothes, or pets that have come in contact with the plants. Scratching a rash will not spread the oil to other parts of your body or to other people. Our two species of poison-ivy can look like the innocuous box-elder but differ in their alternate (versus opposite) leaves. Poison-sumac can easily be told apart from the non-poisonous sumacs by its entire leaflet margins. Species called poison-oak do not grow in Wisconsin but occur to our south and west. Anyone interested in the poison-ivies should consult the exhaustive study of Gillis (1971).
Unsurprisingly, all three of our species (especially poison-sumac) are under-collected and brave botanists should help to change this!
