Tall thoroughwort is a stout native perennial whose stems are hairy in the top half of the plant. The flowers are clustered loosely at the tops of stems in narrow, flat-topped configurations; there are only 5 florets in each head, and the flowers are dull white. The bracts beneath the flowerheads are broad and rounded. Blooms August–October.
The leaves are mostly opposite, widely spaced, to 5 inches long, strongly 3-nerved, very short-stemmed or sessile, slender lance-shaped and tapering nearly to the plant stalk, with short hairs and dotted with glands, with teeth above the middle of leaves only.
Similar species: Nine species of Eupatorium have been recorded in Missouri. Leaf characteristics are usually the best way to distinguish among the species:
- Late boneset (E. serotinum) is the most common or widespread lookalike, and it’s the one most similar to tall thoroughwort. Late boneset, however, has wider and more coarsely serrated leaves that are less pubescent (hairy) than tall thoroughwort’s. Also, late boneset’s flowerheads contain 9–15 florets (as opposed to just 5). Note that late boneset can be just as tall as tall thoroughwort.
- Upland boneset (E. sessilifolium) is scattered in the Ozark and Ozark border regions and in the eastern half of northern Missouri. Its leaves are stalkless and lance-shaped to narrowly ovate, tapering to a sharply pointed tip, with margins sharply toothed. The leaf bases, however, are quite different than tall thoroughwort’s: ovate, rounded, or truncate; in the pairs of opposite leaves, the bases often overlap slightly. Also there is only 1 main vein.
- Smallflower thoroughwort (E. semiserratum) is also similar, but it is uncommon in Missouri, known only in the Bootheel lowlands. Like tall thoroughwort, its leaves are serrated mostly above the midpoint and it has three obvious veins in the leaves. But the two side veins branch out from the midvein well above the base of the leaf blade, instead of at the base of the leaf blade.
Learn more about Missouri’s thoroughworts, or bonesets, on their group page.
Height: generally 3–4 feet, but often more than 6 feet.
Statewide but uncommon in Southeast Lowlands.
Habitat and Conservation
Occurs in prairies, fields, glades, open woods, banks of streams and rivers, ledges and tops of bluffs, ditches, quarries, roadsides, and other open, disturbed, or waste places.
Folk medicine has given this plant its enduring name of “boneset.” The first pharmacists and doctors were botanists, since humans have for millennia recognized the medicinal properties of plants. People are still researching the chemicals in plants to find cures for human illness.
Many types 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.