Eupatorium semiserratum
Eupatorium perfoliatum
smallflower eupatorium, smallflower thoroughwort
boneset thoroughwort, Chapman's thoroughwort, common boneset, eupatoire perfoliée
(from short rhizomes) single, densely branched distally, puberulent throughout.
(from short caudices) single, sparsely branched distally, puberulent throughout.
usually opposite (distal sometimes alternate, lateral buds dormant or producing 1 pair of leaves); simple, sessile or subsessile;
blades ± 3-nerved distal to bases, elliptic to lance-elliptic, (30–)50–70 × 5–25 mm (lengths mostly 2–5 times widths), bases narrowly cuneate, margins usually serrate, apices acute, faces puberulent or villous, densely gland-dotted.
usually opposite (sometimes whorled);
sessile;
blades pinnately nerved, oblong (tapering toward apices), 50–150+ × 15–40 mm, bases connate-perfoliate, margins serrate, apices acute, faces pilose, gland-dotted (abaxial), glabrate, not gland-dotted (adaxial).
5;
corollas 2.5–3 mm.
7–11;
corollas 2.5–3 mm.
7–10 in 2–3 series, elliptic, 1–3 × 0.5–1 mm, apices rounded to acute (not mucronate), abaxial faces puberulent, gland-dotted.
7–10 in 1–2 series, oblong, 2–4.5 × 0.6–1 mm, apices (whitish) acute to acuminate, abaxial faces villous or puberulent, gland-dotted.
in corymbiform arrays.
in corymbiform arrays.
1.5–2 mm;
pappi of 30–40 bristles 2.5–3 mm.
1.5–2 mm;
pappi of 20–30 bristles 3–3.5 mm.
= 20.
= 20.
Eupatorium semiserratum
Eupatorium perfoliatum
Eupatorium semiserratum has been included within E. glaucescens (E. cuneifolium); it is distinguished by its consistently smaller heads and stems that branch only within the capitulescences. It differs from the similar and sometimes sympatric E. lancifolium by its smaller heads, leaves 3-nerved distal to bases (rather than at bases), as well as preference for wetter habitats. It has been proposed that E. rotundifolium var. scabridum (E. pubescens) represents hybrids between E. semiserratum and E. rotundifolium; it also apparently hybridizes with E. hyssopifolium.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Eupatorium perfoliatum is both distinctive, with its perfoliate leaf bases, and extraordinarily widespread. Hybrids between E. perfoliatum and other species of Eupatorium have been reported. The name E. resinosum var. kentuckiense Fernald is based on hybrids between E. perfoliatum and E. serotinum (= E. ×truncatum Muhlenberg ex Willdenow). Eupatorium perfoliatum var. colpophilum refers to populations characterized by narrower, more leathery leaves found in the extreme northeastern portion of the range.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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