American boneset, also called common boneset or American thoroughwort, is easily identified by its opposite, stalkless leaves that are fused together across the stem, making them look like a single leaf that has been pierced by the stem.
It is a tall perennial with noticeable, spreading hairs. The flowerheads in flat-topped clusters with 9 to 23 florets in a head, dull white. Blooms July–October. Leaves to 8 inches long, triangular, tapering to a sharply pointed tip, finely toothed, opposite and perfoliate (opposite pairs with bases fused around the stem).
Similar species: Nine species of Eupatorium have been recorded in Missouri. This is one of the most common and widespread species. It is easily identified by the perfoliate leaves.
Learn more about Missouri’s thoroughworts, or bonesets, on their group page.
Height: usually 4, but sometimes up to 6 feet.
Scattered nearly statewide, but apparently absent from much of the western part of the Glaciated Plains of northwestern Missouri.
Habitat and Conservation
Generally occurs in moist situations: banks of streams and rivers, margins of ponds, lakes, and sinkhole ponds, bases and ledges of bluffs, fens, borders of sloughs, bottomland prairies, moist depressions of upland prairies, marshes, bottomland forests, and rarely openings of rich upland forests; also margins of fields and moist roadsides.
Native Americans used eupatorium species to treat many ailments. Many say the fused, opposite leaves led to the notion that this plant could help heal and strengthen bones. Others say that pioneers called these plants “boneset” because they used them to treat fevers that made one’s bones ache.
Why are eupatoriums called “thoroughworts”? Apparently, it comes from this species, with its perfoliate leaves. With each pair of leaves joined at the bases, the stem appears to go through the “leaf.” In the past, the words “through” and “thorough” were not so sharply different in meaning. (Think of the word “thoroughfare,” for example.) None of our other Missouri eupatoriums have perfoliate leaves.
Many kinds of insects drink nectar from the flowers, and plants like this, that bloom late in the season, are crucial for insects that mature at that time. Because of the foliage’s bitterness, mammals don’t usually eat this plant. Thus it can become increasingly abundant in pastures.