It's a real chameleon
The adage "Leaflet three, leave it" is a good place to start with identification. Except in the rare five-leaflet populations in Massachusetts and Texas, poison ivy leaflets are arranged in threes. But the plant can otherwise be so morphologically variable that it confuses everyone but expert observers.
"Its plasticity is really insane," said Ms. Pell. And that has led to distinctive looking phenotypes being designated as local species in different regions.
She asks to be different. Ms. Pell clarified this as part of her long collaboration with John D. Mitchell, Honorary Curator at the New York Botanical Garden. In their upcoming chapter on Anacardiaceae, slated to be published in 2022 as part of the massive Flora of North America project, they found that most of these are not distinct species, but rather fall into five varieties of T. radicans.
"We missed some popular local species," she said. And no, that's not sarcasm that you hear. Taxonomists are often emotionally attached to a particular species while maintaining their position in the systematic hierarchy – even if that species is poison ivy.
It can be a ground cover or a shrub or a woody vine (called a liana) that is almost tree-like in size. The leaves can be glossy or matte (even in the same population), in various shades of green (or red-bronze when emerged), and they vary in size and shape with edges ranging from smooth to serrated or deeply lobed.
“In one area of Quebec,” Ms. Pell said, “it has carrier-like leaflets and it 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 elder (Acer negundo), Virginia creeper (Parthenocissus quinquefolia) or blackberries such as blackberries (Rubus). The leaflet tufts of the box elder, however, are arranged opposite one another along the main stem, while poison ivy alternates. Mature Virginia Creeper has five leaflets; The stems have tendrils with thick pads at the end, but the reddish roots that often cover the venom-ivy vines are missing. And Rubus has spikes, unlike poison ivy, which it never does.