The adage “leaflets three, let it be” is a good start in identification. Except in rare, five-leaflet populations in Massachusetts and Texas, poison ivy’s leaflets are arranged in threes. But the plant can otherwise be so morphologically variable that it confounds all but expert observers.

“Its plasticity is really crazy,” Ms. Pell said. And that has led to distinctive-looking phenotypes in different regions being designated as local species.

She begs to differ. As part of her long collaboration with John D. Mitchell, honorary curator at New York Botanical Garden, Ms. Pell has been sorting it out. In their upcoming Anacardiaceae chapter, to be published in 2022 as part of the massive “Flora of North America” project, they have determined that most of these are not distinct species, but fall into five varieties of T. radicans.

“We didn’t recognize some well-loved local species,” she said. And no, that is not sarcasm you hear. Taxonomists often become emotionally attached to a particular species retaining its stature in the systematic hierarchy — even if that species is poison ivy.

It can be a ground cover, or shrubby, or a woody vine (called a liana), achieving nearly treelike proportions. Its leaves can be shiny or matte (even in the same population), in various shades of green (or red-bronze, upon emergence), and they vary in size and shape, with margins from smooth to toothed or deeply lobed.

“In one area in Quebec,” Ms. Pell said, “it has straplike leaflets and looks like a fern. In the Southeast, I have seen leaflets as long and wide as my head, and elsewhere, often very close by, leaflets shorter than my thumb.”

On the East Coast, poison ivy is most often confused with box elder (Acer negundo), Virginia creeper (Parthenocissus quinquefolia) or a bramble like blackberry (Rubus). But box elder’s clusters of leaflets are arranged opposite each other, along the main stem, while poison ivy’s alternate. Mature Virginia creeper has five leaflets; its stems have tendrils with thick pads on the end, but are missing the reddish roots that often cover poison ivy vines. And Rubus has spines, unlike poison ivy, which never does.

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