Eupatorium album
Eupatorium ×cordigerum
white thoroughwort
(from short caudices or stout rhizomes) single, sparsely branched distally, pubescent throughout.
(from short rhizomes) single, branched distally, densely pilose.
usually opposite (distal sometimes alternate);
sessile;
blades ± 3-nerved or pinnately nerved, elliptic to oblanceolate, 30–120 × 10–40 mm (lengths mostly 3–4 times widths), bases narrowly cuneate (sometimes oblique), margins subentire, serrate, or serrulate, apices rounded to acute, faces setulose (denser on midribs and veinlets) to glabrate, usually gland-dotted.
usually opposite (distal sometimes alternate);
sessile;
blades ± 3-nerved or pinnately nerved, ovate, 3–9.8 × 1.6–5.7 cm, bases clasping or narrowly perfoliate, margins serrate, apices acute, faces (rugose) sparsely pilose (adaxial) or densely pilose (abaxial) and densely gland-dotted.
(4–)5;
corollas 4–4.5 mm.
5–8;
corollas 2.5–3 mm.
9–15 in 2–4 series, linear, 1.5–9 × 0.6–1 mm, apices acuminate to attenuate, sometimes mucronate, abaxial faces pubescent throughout, gland-dotted.
7–10 in 2–3 series, 4–12 × 1–4 mm, apices (usually white) acuminate to acute, abaxial faces pilose, gland-dotted.
in corymbiform arrays.
in corymbiform arrays.
2.5–3.5 mm;
pappi of 40–50 bristles 3.5–4.5 mm.
2–3 mm;
pappi of 30–40 bristles 3.5–4.5 mm.
= 30, 40.
Eupatorium album
Eupatorium ×cordigerum
Varieties 3 (3 in the flora).
Eupatorium album is represented by widespread diploid populations and by polyploid, apomictic populations, segregated as varieties, that may have arisen by hybridization with other species. Molecular data suggest that var. vaseyi arose via hybridization between var. album and E. sessilifolium. They also suggest that E. album var. subvenosum arose from hybridization between E. album var. album and E. serotinum.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Eupatorium ×cordigerum is an apomictic, polyploid hybrid of E. rotundifolium and E. perfoliatum that occurs in disturbed areas where those two species occur together. It is not uncommonly encountered; it is unclear how long it persists.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Leaf blades: margins coarsely serrate, apices obtuse to rounded, faces pubescent | var. album |
1. Leaf blades: margins subentire, serrulate, or serrate, apices acute, faces pubescent to glabrate | → 2 |
2. Leaf blades 3-nerved from bases, 40–70 × 10–20 mm, margins subentire to serrulate | var. subvenosum |
2. Leaf blades 3-nerved distal to bases, 50–110 × 20–40 mm, margins evenly serrate | var. vaseyi |
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