The Major Identification Challenges of Poison Ivy: Toxicodendron radicans vs. Toxicodendron rydbergii

A Big Title for a Small Analysis

By using iNaturalist frequently, I’ve noticed some recurring issues. 😀

First, Toxicodendron rydbergii is synonymous with Toxicodendron radicans subsp. rydbergii†. So it’s both a subspecies (or in Canada, a variety) of Toxicodendron radicans and a distinct species—Rydberg’s Poison Ivy, or in English, Western Poison Ivy. And that’s where it becomes important: it’s the poison ivy of the western part of North America. This already adds a layer of difficulty to identification.

On iNaturalist, the taxon Toxicodendron radicans subsp. rydbergii automatically redirects to Toxicodendron rydbergii, leaving no room for subtle identification. There’s no way to distinguish between the variety and the species. The two are very hard to tell apart, and in Quebec, Toxicodendron radicans seems completely dominant. So when in doubt—especially with photos that lack detail—I tend to identify the species without specifying the variety or subspecies, meaning I mostly opt for Toxicodendron radicans.

A perfect example is the photo above. Just from that image, many users on iNaturalist would classify it as Toxicodendron rydbergii, saying: “the pubescence isn’t strong enough for T. radicans”. However, the colony clearly has very visible aerial roots—see here: https://inaturalist.ca/observations/267338507 .

It seems that when T. radicans creeps along the ground, it doesn’t always produce aerial roots. Unless we need to dig up the woody stems to discover roots that are no longer aerial? I don’t think so. You see how complex this is?

Identification keys? Let’s talk about them! The main difference between T. radicans and T. rydbergii is the presence of aerial roots in T. radicans††! See Pierre Guertin’s beautiful photo below. T. rydbergii apparently doesn’t climb.

The problem is that T. radicans doesn’t always have those famous aerial roots! That’s right! And I prove it here: https://inaturalist.ca/observations/269708957 and here: https://inaturalist.ca/observations/268965948.

It seems that when T. radicans creeps along the ground, it doesn’t always produce aerial roots. Unless we need to dig up the woody stems to discover roots that are no longer aerial? I don’t think so. You see how complex this is?

Back to pubescence. According to Minnesota Wildflowers, in T. radicans, the leaf petioles are covered in short hairs (pubescence)††. Those of T. rydbergii are apparently glabrous. But once again, that’s not always the case—the T. radicans colony discovered by Pierre Guertin in 2015 is still there to prove it. The pubescence is very subtle: https://inaturalist.ca/observations/267338507.

Still according to Minnesota Wildflowers, the fruits of T. radicans are larger (about 1/6 inch in diameter; for T. rydbergii, it’s 1/8 inch).

Secondly, according to Go Botany, the fruits of T. radicans have sparse hairs, while those of T. rydbergii are hairless†††.

I tested this with two colonies here: https://inaturalist.ca/observations/269708957 and here: https://inaturalist.ca/observations/269716571.

Conclusion: regarding diameter, yes, in general the fruits of these colonies have a diameter greater than 1/8 inch—they’re actually closer to 3/16 inch.

As for the sparse hairs, I see absolutely nothing! It’s true that the colonies have endured the harshness of winter—wind, cold—so perhaps it would be better to verify during fruiting season.

Looking at all this, I came to the same conclusion as the prolific author Jerry Jenkins: “The two are very similar: radicans is a robust plant that climbs trees with small red rootlets and can grow as a freestanding shrub or small tree to 3 meters tall. It may have, or like these may not have, hairs on the leaves, petiole, or fruit. Rydbergii is a low creeper that doesn’t seem to produce rootlets or have hairs. Not a big difference. […] They look like great subspecies to me.”††††

Basically, what this says is: T. rydbergii is just a subspecies of Toxicodendron radicans, the differences are too minor! That’s it! 😀

One last thing! I’ve noticed that iNaturalist often suggests T. rydbergii by mistake for observations that are clearly T. radicans! Just goes to show—you have to be careful with artificial intelligence!

As for the classification of Poison Ivy, I don’t trust the suggestions offered by iNaturalist…

With that, good luck! 🤣

† Plants of the World Online https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:255789-2 , VASCAN https://data.canadensys.net/vascan/name/Toxicodendron%20rydbergii

†† Minnesota Wildflowers https://www.minnesotawildflowers.info/shrub/eastern-poison-ivy , https://www.minnesotawildflowers.info/shrub/western-poison-ivy

††† Go Botany https://gobotany.nativeplanttrust.org/species/toxicodendron/radicans/ , https://gobotany.nativeplanttrust.org/species/toxicodendron/rydbergii/

†††† Northern Forest Atlas, Digital Atlas of Woody Plants, for web site, 27 Apr 2020 https://northernforestatlas.org/