American boneset, also called common boneset or American thoroughwort, has small, white flowerheads in flat-topped clusters at the top of the plant. It 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.
American boneset is a tall perennial with noticeable, spreading hairs.
The flowers are in flat-topped clusters with 9 to 23 florets in each small flowerhead; they are dull white.
Blooms July–October.
The leaves are to 8 inches long, triangular, tapering to a sharply pointed tip, finely toothed, opposite and perfoliate (opposite pairs of leaves have their bases fused around the plant stem). The leaves are densely covered with short hairs and are roughened or felty to the touch; the undersurfaces are usually grayer than the top.
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: from 16 to 60 inches (5 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.
Status
Native Missouri wildflower.
Human Connections
This species has a long history in folk medicine. Native Americans used eupatorium species to treat many ailments.
- Many believe the name “boneset” came from the fused, opposite leaves, which had 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 so severe as to make one’s bones ache.
The species name, perfoliatum, refers to the perfoliate leaves. Perfoliate is a Botany 101 term that students learn around the same time they learn words describing the various leaf shapes.
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.
Ecosystem Connections
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.
Having perfoliate leaves can serve a plant in several ways, helping it survive and reproduce:
- The conjoined leaves can shield the upper parts of plants (including the flowers, so important for reproduction) from herbivorous insects crawling up from below.
- The conjoined leaves can form cups that catch and hold water, helpful during drought conditions and accentuating the insect shield effect.
- Missouri's native honeysuckles, such as limber, yellow, and grape honeysuckle, also have perfoliate leaves, the pair just below the flower clusters.
- Cup plant, a common rosinweed, is named for its perfoliate leaves, which join together, look like cups, and hold water.
