A couple years ago a plant volunteered in my backyard garden in Madison, Wisconsin (zone 5). The first (leftmost) picture shows the plant (don't be fooled by the false sunflower (Heliopsis helianthoides) behind it or the dotted horsemint (Monarda punctata) in front of it). The plant appeared to have tendrils, so at first I thought it was a vine. But as the season progressed the "tendrils" formed into masses (see the second and third pictures). Around mid-summer flower buds appeared from these tendril masses, and the tendrils started to look more like leaves (see the fourth picture). So, what appeared to be tendrils are actually bracts. Eventually the buds opened into blossoms that look very much like those of white snakeroot (Ageratina altissima) or boneset (Eupatorium spp.) (see the fifth picture).
I'm usually pretty good at identifying strange plants, but this one has me baffled. And I'm not the only one who's baffled. I've tried several of the AI-assisted, plant-identification websites. From these I've received answers like white snakeroot and boneset for the flowers, tomato and late boneset for the leaves and pea and other vines for those tendril-like bracts. None of them came close to identifying those tendril-like bracts.
Does anyone have an idea what this plant could be?
