Perfoliated Boneset (Eupatorium perfoliatum) of the Sunflower (Asteraceae) family can be distinguished from other members of the genus by its distinctive leaves. Genus name recognizes Mithridates Eupator (132-63 B.C.) King of Pontus, who invented a “universal antidote” against poisoning and is said to have used a member of the genus in medicine. Specific epithet derives from Latin word “perfoliatus” translating to “through the foliage”; a reference to the plant’s stems that grows through the middle of the leaf. The common name “Boneset” comes from use of the plant to treat dengue fever, also known as “break-bone fever”. In the US, it occurs in all states from Texas to North Dakota thence eastward to the Atlantic Coast and southward to the Gulf Coast. In Arkansas, occurrence is statewide. Habitat preference is wet to wet-mesic soils of poorly drained sites that are in full to partial sun, such as, various wetlands, wet prairies, low-lying open woods, stream margins and roadside ditches. It is usually found in sites where standing water is temporary but can occasionally be found in drier sites. Also knows as Common Boneset and Thoroughwort.
This rhizomatous, clump-forming perennial has a few to many, terete, closely spaced, erect stems to 4 feet tall. New stem (cauline) leaves appear in early spring from a spreading, fibrous, rhizomatous, colony-forming rootstock. The upper half of stems bears opposite, decussate (rotated 90⁰), axillary pairs of primary branches set at 45⁰. Primary branches bear axillary secondary branches with the same growth habit – branches typically in matched pairs. The straight, stout, terete stems and primary branches and secondary branches terminate with closely spaced, spreading flower-bearing branches. Near and within the inflorescence, branches become sub-opposite to alternate. Lengths of primary branches (to 2 feet) and secondary branches (to 9 inches) decrease distally to and into the inflorescence. Internode lengths decrease distally from stem-base into the inflorescence. Stems and branches, from plant base and into the inflorescence, are noticeably densely pubescent with weakly spreading to descending fine (puberulent) hairs. White pubescence, retained throughout the growing season, provides a whitish cast to the pale green stems and branches.
In the lower two-thirds of stems and extending onto lower portions of branches, narrowly elliptic to lanceolate (4-8 inches long and to 2 inches across) leaves are in opposite, decussate pairs that unite around the stem/branch. These individual connate-perfoliate leaves are widest below mid-leaf with a short taper toward the stem before blades widen ear-like (auriculate) around the stem/branch. A more gentle taper extends from mid-leaf to acute apices with blunt tips. Leaf margins are serrate with equally spaced, rounded teeth. In the upper two-thirds of stems (and upper portion of branches), more distal opposite leaf pairs become simple and sessile (without stalks) with the leaves near and within the inflorescence becoming sub-opposite and alternate (see Photo 13). The medium-green leaves are a darker green than stems/branches. Leaves, blade elevated to either side of the midrib, have a horizontal to smoothly down-arched orientation. Veins are recessed above (adaxially) and strongly expressed below (abaxially). Venation of individual leaves is weakly pinnate while venation at the auriculate portion of connate pairs is palmate. Secondary and tertiary veins form a rhombic, reticulated network. Upper and lower surfaces have pilose (fine and soft) pubescence with that of lower surface being longer especially along veins. Blades have a wrinkled, felty surface. Through the growing season, margins of the elongating leaf blades become wavy.
The inflorescence, August-October, consists of small compound flowerheads on very short pedicels (stalks) in tight clusters. Flowerheads are in cymes in that the terminal flowerhead of each ultimate cluster develops before adjoining lateral flowerheads. The inflorescence of any particular stem consist of many cymes on multiple stout, ascending, spreading floral branches wherein lengths of branches decrease toward the apex of the inflorescence so that the floral array of any particular stem/branch has a flat to convex top (corymb structure). Bloom period extends for a month or more. Floral branches, peduncles (stalks bearing several flowerheads), pedicels and phyllaries (bracts of involucres) are covered with dense puberulent pubescence.
Flowerheads consist of 9-13 elongate disc florets (¼ inch long) tightly enclosed by a pale green, ⅛-inch-long, cylindrical involucre. Involucres (⅛ long) have 7-10 narrow-oblong, rib-like, appressed, white-tipped phyllaries in a single, slightly overlapped series. The white florets have a tubular corolla, 5 stamens (white filament + brown anthers) and a white pistil (ovary + 2 styles + 2 stigmas). Corollas (⅛-inch-long) have a campanulate (bell-shaped) throat, with 5 triangular lobes, and a narrow-tubular lower portion. Lobes of corolla are wide spreading. At anthesis, anthers are fused into a tube-like ring throat through which the long styles extend so that pollen is carried out of the corolla. The two ⅛-inch-long thread-like arms extend, in a wispy fashion, well above the corolla. Stigmas are the blunt tips of the flattened, conspicuous arms. Corollas are set atop elongate ovaries rimmed by long hairs. Floral branches, pedicels and involucres are equally, densely pubescent.
With fertilization, the inferior ovaries enlarge slightly as they become dark brown, 1-seeded cypsela (fruits of the Sunflower family) as the now-brown corollas drop-off and phyllaries spread wide. Twenty to thirty crinkled hairs (pappus) atop the ovary become wide-spreading. The 4-sided cypsela are about ⅛-inch long with pappus more than twice as long. The whitish, frizzy infructescence is retained for a month or more as the cypselae are dispersed by wind.
Nine additional species of the genus and 2 recognized hybrids occur in Arkansas. Many of the other species and hybrids have flowerheads and floral arrays that are similar to that of Perfoliated Boneset (E. perfoliatum); however, leaf shapes vary by species/hybrid. E. x cordigerum, a hybrid between E. perfoliatum and E. rotundifolium has leaf texture and shape somewhat similar to that of E. perfoliatum but leaf pairs of the hybrid are broadly ovate with bases that are clasping to narrowly connate-perfoliate and, too, the hybrid has fewer florets per flowerhead.
Perfoliated Boneset can be an excellent choice as a specimen plant for various garden styles and for natural areas that have various wet to mesic-wet soils in full to mostly sunny sites. This herbaceous, medium-tall plant remains mostly erect into winter whereafter stem may need to be removed to better expose spring growth. Plants are attractive from spring into fall. Foliage is interesting and its late inflorescence has a pleasant scent. The numerous flowerheads, bearing nectar and pollen, is a magnet for various bees, flies, wasps, moths and butterflies. Easily propagated by seed and root divisions.
Late Boneset (E. serotinum) has been addressed in a previous article in this series.
Article and photographs by ANPS member Sid Vogelpohl
