Eastern Poison Ivy vs Western Poison Ivy
This guide is under construction and has not been published yet. It may have errors. When in doubt, double-check other sources for definitive ID.These two species are visually similar and have significant range overlap in eastern Canada and the north and central US. Vining plants are easily identified as T. radicans, which averages hairier and with slightly larger fruit. T. radicans ranges farther southeast, to lower elevations, and into shadier habitats, and is found in a wide range of moisture conditions and soil pH. T. rydbergii ranges farther west and north, is more restricted to higher elevations, and is more restricted to open habitat. In the zone of overlap,
Eastern Poison Ivy (Toxicodendron radicans) | Western Poison Ivy (Toxicodendron rydbergii) |
A perennial woody vine that can cause an unpleasant and dangerous allergic reaction following contact with its sap. Widespread and abundant in much of its range, and highly variable in appearance. | A rhizomatous shrub native to the interior West and North of North America that can cause a dangerous allergic reaction upon contact with its sap. |
 Although it can grow as a free-standing shrub, often grows as a vine that climbs by aerial rootlets that attach to surfaces, or by twining around other vegetation. Photo © botanygirl (iNaturalist), CC BY 4.0. |  Only grows as a free-standing shrub. Does not climb other vegetation and does not form aerial rootlets, although it may lean on or prop itself up against rock faces or other flat objects. Photo © colinpena, CC BY 4.0. |
 Fruit may be hairy, especially when unripe. Note: hair on mature fruit is sparse, if visible at all. Fruits average smaller (2.5-5.5mm) although with much overlap. Some populations tend to have larger fruit. Photo © Alex Abair, CC BY 4.0. |  Fruit is hairless, even when unripe. Fruits average larger (4-7mm, rarely as small as 3mm) although with much overlap. Some populations tend to have smaller fruit. Photo © Alex Abair, CC BY 4.0. |
 Especially on T. radicans var. negundo, the variety most likely to overlap with T. rydbergii, the petiole (stem connecting the leaf to the twig) is pubescent, although hairs may be very fine and require a close view to see. Foliage also averages hairier. Photo © Thomas Koffel, CC BY 4.0. |  Petioles are either completely hairless or nearly so. Foliage also averages less hairy and often looks hairless or nearly so. Photo © Jesse, CC BY 4.0. |
 Climbing plants can reach canopy height (to 45m or 150ft). Free-standing plants reach similar heights of 1m (3ft), rarely taller. Photo © Jenn E., CC BY 4.0. |  Plants usually grow only up 1m (~3ft) tall, rarely to 3m (~10 ft). Photo © Z, CC BY 4.0. |
 Although many leaflets have only shallow lobing or even none, leaflets are more likely to have deep lobing. Photo © Ryan Donnelly, CC BY 4.0. |  Although lobing is still variable, lobing averages shallower and the deepest-lobed leaflets are still more shallowly-lobed. Photo © Bob Nieman, CC BY 4.0. |
Additional Notes
The names eastern and western can be misleading, as T. radicans ranges farther west than most "eastern" species, well into the drier portions of the Great Plains, West Texas, and even southern Arizona. Similarly, T. rydbergii ranges farther east. In parts of the range overlap, it is useful to think of T. radicans as the more southerly species and T. rydbergii as the more northerly one. As one would expect with this labeling, T. radicans favors lower elevations than T. rydbergii.There is an asymmetry in these species in that it is easier to identify a plant definitively as T. radicans than as T. rydbergii. However, in most of the range overlap, the variety T. radicans var. negundo is most likely to overlap with T. rydbergii, and this species both is more likely to have deep lobing and pubescent petioles.
Although T. rydbergii is rare near the southeastern limits of its range, it has been reported in the mountains of WV and VA, and the PA Piedmont, and it is possible that it is under-reported due to the difficulty of identifying it.
References & External Resources
These short lists show only links helpful for ID. For a complete list of references and resources also covering other aspects of ecology, visit the links section of the full article on each plant, which is the first entry here.
