Eupatorium

Perennials, 30–200 cm. Stems erect, usually not branched proximal to arrays of heads (from caudices or rhizomes). Leaves mostly cauline; usually opposite (rarely whorled, distal sometimes alternate); petiolate or sessile; blades usually 3-nerved from or distal to bases, or pinnately nerved, mostly deltate or ovate to lanceolate or linear (and intermediate shapes, sometimes elliptic, oblong, rhombic, or suborbiculate, sometimes pinnatifid, 1–2-pinnately, ternately, or palmately lobed), ultimate margins entire or toothed, faces glabrous or puberulent, pubescent, scabrous, or setulose, usually gland-dotted. Heads discoid, in corymbiform or diffuse to dense, paniculiform arrays. Involucres obconic to ellipsoid, 1–3(–5+) mm diam. Phyllaries persistent, 7–15+ in 2–3(–4+) series, (usually green) 2–3-nerved, or not notably nerved, or pinnately nerved, elliptic, lanceolate, oblong, or obovate, usually unequal, sometimes ± equal (margins scarious, hyaline, apices rounded to acute or acuminate sometimes mucronate, faces usually puberulent or villous, usually gland-dotted, rarely glabrous). Receptacles flat or convex, epaleate. Florets (3–)5(–15+); corollas usually white, rarely pinkish, throats funnelform to campanulate, lobes 5, triangular; styles: bases sometimes enlarged, usually puberulent (glabrous in E. capillifolium), branches mostly filiform. Cypselae (brownish to black) prismatic, 5-ribbed, usually glabrous, usually gland-dotted; pappi persistent, of 20–50 (whitish) barbellulate bristles in 1 series. x = 10.

Distribution

e North America, Europe, e Asia.

Discussion

Species 41+ (24 species, including 2 hybrids, in the flora).

Eupatorium is treated here in a restricted circumscription, following R. M. King and H. Robinson (1987) in excluding genera that traditionally have been included in a broad Eupatorium (e.g., Ageratina, Chromolaena, Critonia, Conoclinium, Fleischmannia, Koanophyllon, Tamaulipa); Eutrochium (Eupatorium sect. Verticillatum) is also excluded here.

Species identification within Eupatorium is sometimes complicated; polyploidy and apomixis have contributed to the complications. Some species include both sexual diploid and apomictic polyploid plants or populations. V. I. Sullivan (1972) made important contributions to understanding Eupatorium in North America by showing that some fairly distinct, sexual diploid species may include apomictic polyploid plants or populations that do not differ greatly from the diploids. Other apomictic polyploids appear to be intermediate morphologically between pairs of diploid or diploid/polyploid species and were proposed by Sullivan to have originated from interspecific hybridization. Distinction and level of recognition of hybrid apomictic taxa have a large arbitrary component, in part because some apomicts appear to be ephemeral and others may be relatively stable and in part because differences in the relative genomic contributions of the progenitors through dosage effects or backcrossing may affect whether an apomict is morphologically distinctive or part of a continuous series of variation.

Selected References

Sullivan, V. I. 1972. Investigations of the Breeding Systems, Formation of Auto- and Alloploids and the Reticulate Pattern of Hybridization in North American Eupatorium (Compositae). Ph.D. dissertation. Florida State University.

Lower Taxa

Eupatorium album, Eupatorium altissimum, Eupatorium anomalum, Eupatorium cannabinum, Eupatorium capillifolium, Eupatorium compositifolium, Eupatorium godfreyanum, Eupatorium hyssopifolium, Eupatorium lancifolium, Eupatorium leptophyllum, Eupatorium leucolepis, Eupatorium linearifolium, Eupatorium mikanioides, Eupatorium mohrii, Eupatorium perfoliatum, Eupatorium petaloideum, Eupatorium pilosum, Eupatorium resinosum, Eupatorium rotundifolium, Eupatorium semiserratum, Eupatorium serotinum, Eupatorium sessilifolium, Eupatorium ×cordigerum, Eupatorium ×pinnatifidum

Key

1	Leaves (at least the principal) pinnatifid, or pinnately or ternately lobed, or palmately 3(–5)-lobed	> 2
1	Leaves simple (not lobed, margins crenate, entire, laciniate, serrate, or serrulate)	> 6
2	Leaves (at least larger proximal) palmately 3(–5)-lobed (lobes relatively broad, margins serrate; corollas usually pinkish)	Eupatorium cannabinum
2	Leaves pinnatifid or pinnately or ternately lobed (lobes relatively narrow; corollas usually white)	> 3
3	Leaves (mostly ternately or 1–2-pinnately lobed): blades or lobes linear; heads in subcorymbiform to subpaniculiform arrays; florets 7–9	Eupatorium ×pinnatifidum
3	Leaves (usually pinnately or ternately lobed or pinnatifid): blades or lobes linear; heads in paniculiform arrays; florets usually 5	> 4
4	Stems glabrous (gland-dotted, branches supporting heads recurved and secund)	Eupatorium leptophyllum
4	Stems puberulent (branches supporting heads not recurved or secund)	> 5
5	Leaf blades or lobes 0.2–0.5(–1) mm wide (margins strongly revolute); phyllaries usually glabrate or glabrous, usually not gland-dotted	Eupatorium capillifolium
5	Leaf blades or lobes 0.5–2.5(–4) mm wide; phyllaries usually puberulent(mostly on midveins), usually gland-dotted	Eupatorium compositifolium
6	Leaves petiolate (petioles 10–30 mm)	> 7
6	Leaves sessile or subsessile (petioles 0 or 1–10 mm)	> 8
7	Leaf blades deltate to rhombic (held vertically; stems green); florets 5	Eupatorium mikanioides
7	Leaf blades lanceolate (held horizontally; stems sometimes reddish to purplish);florets 9–15	Eupatorium serotinum
8	Florets 7–11 (leaf bases connate-perfoliate)	Eupatorium perfoliatum
8	Florets (4–)5–8 or 9–14 (leaf bases not connate-perfoliate)	> 9
9	Florets usually 9–14	Eupatorium resinosum
9	Florets (4–)5–8	> 10
10	Phyllary apices (usually white) acuminate to attenuate	> 11
10	Phyllary apices (usually with narrow white margins, sometimes pigmented) acute, obtuse, or rounded	> 15
11	Leaf bases (especially larger leaves) clasping to narrowly connate-perfoliate (leaf faces rugose); florets 5–8	Eupatorium ×cordigerum
11	Leaf bases attenuate, cuneate, or rounded (leaf faces relatively smooth); florets (4–)5	> 12
12	Leaves 4–10(–15) mm wide	Eupatorium leucolepis
12	Leaves mostly 10–45 mm wide	> 13
13	Leaves little, if at all, gland-dotted; phyllaries glabrous, not gland-dotted (the larger 8–10 mm)	Eupatorium petaloideum
13	Leaves usually gland-dotted; phyllaries puberulent to villous (at least toward bases and on midveins), gland-dotted (the larger 5–10 mm)	> 14
14	Leaf blades elliptic to oblanceolate (lengths mostly 3–4 times widths), bases narrowly cuneate (sometimesoblique); phyllaries linear (larger 6–9 mm)	Eupatorium album
14	Leaf blades elliptic, lanceolate, or lance-ovate (lengths mostly 1–2 times widths), bases rounded to rounded-cuneate; phyllaries oblong to lance-oblong (larger 5–7mm)	Eupatorium pilosum
15	Stems glabrous or glabrate proximally (and leaves pinnately nerved, 70–150 mm); leaves: bases truncate to somewhat rounded, facesglabrate (scattered, fine hairs)	Eupatorium sessilifolium
15	Stems usually pilose, puberulent, or pubescent proximally or throughout (if glabrous, leaves 3-nerved from bases, 30–50 × 5–13 mm, bases narrowly cuneate); leaves (3-nerved from or distal to bases or pinnately nerved): bases usually narrowly to broadly cuneate, rounded, subcordate, or truncate, sometimes clasping to perfoliate, faces glabrous, puberulent, scabrous, or villous.	> 16
16	Leaf blades deltate, elliptic, lanceolate, lance-ovate, ovate, or suborbiculate (usually broadest proximal to middles), bases usually broadly cuneate, rounded, subcordate, or truncate, sometimes clasping to perfoliate	> 17
16	Leaf blades elliptic, lance-elliptic, lanceolate, lance-oblong, linear, oblanceolate, or oblong (usually broadest near or distal to middles), bases cuneate	> 20
17	Leaf bases (especially larger leaves) clasping to narrowly connate-perfoliate (faces conspicuously rugose); florets 5–8	Eupatorium ×cordigerum
17	Leaf bases broadly cuneate, rounded, subcordate, or truncate (faces smooth to somewhat rugose); florets (4–)5	> 18
18	Leaves pinnately nerved, blades usually 50–100 mm	Eupatorium godfreyanum
18	Leaves 3-nerved, blades 15–50(–90) mm	> 19
19	Leaf blades elliptic, lanceolate, or lance-ovate (lengths mostly 2–2.5 times widths, usually with purple borders, sometimes visible only in live plants; distal leaves sometimes alternate)	Eupatorium pilosum
19	Leaf blades usually deltate to suborbiculate, sometimes ovate (lengths mostly1–2 times widths; distal leaves sometimes alternate)	Eupatorium rotundifolium
20	Leaves strongly to obscurely 3-nerved from bases	> 21
20	Leaves 3-nerved distal to bases, or pinnately nerved, or relatively narrow and obscurely 3-nerved from bases	> 23
21	Stems usually glabrous or glabrate (at least proximally), sometimes pilose; leaves 30–50 mm, faces glabrous abaxially	Eupatorium lancifolium
21	Stems pubescent or pilose proximally or throughout; leaves 20–60(–120, the larger usually 50+) mm, faces puberulent, scabrous, or villous abaxially	> 22
22	Leaves: margins entire proximally, serrate distally (teeth sharp, antrorse; mid-ribs and 2 major lateral veins prominent)	Eupatorium altissimum
22	Leaves: margins usually laciniate-serrate (teeth ± divergent; 2 major lateral veins usually obscure)	Eupatorium hyssopifolium
23	Leaves usually in whorls (3s or 4s), sometimes opposite, blades lance-linear to linear (lengths 6–40 times widths)	Eupatorium hyssopifolium
23	Leaves usually opposite (sometimes whorled in E. semiserratum), blades elliptic, lance-elliptic, lance-oblong, oblanceolate, or oblong (lengths 2.5–7 times widths)	> 24
24	Leaves (spreading): blades adaxially finely puberulent or villous (stems from short caudices or rhizomes)	> 25
24	Leaves (ascending, recurved, or spreading): blades adaxially glabrous or glabrate (stems from tuberous-thickened rhizomes, not usually apparent until flowering is well under way)	> 26
25	Stems densely branched distally; leaf blades (30–)50–70 mm, margins usually serrate; phyllaries elliptic (larger 2.5–3 mm); corollas 2.5–3 mm	Eupatorium semiserratum
25	Stems usually branched at or near bases; leaf blades mostly 20–35(–45) mm, margins weakly serrate to subentire; phyllaries lanceolate (larger usually 4–5 mm); corollas 3–3.5 mm	Eupatorium linearifolium
26	Leaves (usually spreading or ascending): blades 15–50 × (5–)10–20 mm	Eupatorium anomalum
26	Leaves (usually recurved): blades 20–80 × 5–10(–20) mm	Eupatorium mohrii