Flora of the Southeastern United States
2022 Edition

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Key to Eupatorium, Key B: leaves simple, flowers usually 5 per head

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1 Phyllaries acuminate to attenuate.
..2 Larger leaves 0.2-1.3 cm wide; stems puberulent; involucre 3.5-7 mm high.
....3 Rhizome absent to < 2 cm long; leaves usually reflexed-spreading to spreading-ascending, the larger (5-) 6-13 mm wide; leaf margins and surfaces moderately to densely strigose; involucre 5-8 mm long; pappus (3.3-) 3.9-5.0 mm long; corolla:pappus length ratio 0.63-0.89; mature achenes 2.2-3.5 mm long
....3 Rhizome 2-20 cm long; leaves usually ascending to erect-recurved, the larger 2-4.5 mm wide; leaf margins and adaxial surface glabrous to sparsely strigose; involucre 3.5-5.5 mm long; pappus 2.7-4.1 mm long; corolla:pappus length ratio 0.83-1.00; mature achenes 1.6-2.3 mm long
..2 Larger leaves 1.5-3 (-4) cm wide; stems villous to puberulent; involucre 8-11 mm high.
......4 Larger leaves < 6 cm long; leaves with few or no resin glands.
........5 Phyllaries lacking resin glands; leaf venation pinnate, with at least 2 pairs of lateral veins; [of the Coastal Plain of GA and n. FL]
........5 Phyllaries (at least the outer) with resin glands; leaf venation 3-nerved from the base; of the Coastal Plain of DC, DE, NJ, and northward]
......4 Larger leaves > 6 cm long (and usually > 8 cm long); leaves with sparse to abundant resin glands.
..........6 Leaves sparsely pubescent; lower stem pubescence typically appressed, the hairs < 1 mm long; phyllaries acute-acuminate to mucronate.
............7 Leaves with abundant resin glands
............7 Leaves with sparse resin glands
..........6 Leaves moderately pubescent; lower stem pubescence typically spreading, the hairs 0.5-1 mm long; phyllaries (at least the inner) long-attenuate.
..............8 Leaves lanceolate, > 3× as long as wide; inner phyllaries glandular only in the lower half; [widespread in our area]
..............8 Leaves lance-ovate to ovate, < 3× as long as wide; inner phyllaries glandular to near the apex; [from s. MS westward in our area]
1 Phyllaries acute to obtuse.
................9 Leaf bases broadly cuneate, truncate, or subcordate, the leaves generally distinctly broadest near the base.
..................10 Leaves (2.5-) 3-6 (-7)× as long as wide; plants glabrous below the inflorescence.
....................11 Leaves subcoriaceous, the larger ones 8-18 cm long, 3-6 cm wide, averaging about 3× as long as wide
....................11 Leaves membranaceous, the larger ones 9-18 cm long, 2-4 cm wide, averaging about 5× as long as wide
..................10 Leaves 1-3 (-3.5)× as long as wide; plants pubescent below the inflorescence.
......................12 Leaves pinnately veined
......................12 Leaves prominently 3-veined from the base or just above it.
........................13 Leaves averaging (1.5) 2-2.5× as long as wide, usually with a purple border (or not, in E. species 1); upper leaves and main inflorescence branches often alternate (opposite in E. species 1).
..........................14 Leaves prominently 3-veined, the 2 lateral veins diverging (3-) ca. 5 (-7) mm above the leaf base; leaves usually purple-bordered; inflorescence branches (and upper leaf nodes) often alternate
..........................14 Leaves prominently 3 (-5) veined, the 2 (-4) lateral veins diverging at or < 2 mm above the leaf base; leaves green-bordered; inflorescence branches (and upper leaf nodes) opposite
........................13 Leaves averaging 1-2× as long as wide, usually lacking a purple border; upper leaves and main inflorescence branches usually all opposite.
............................15 Leaf base broadly rounded, cordate-clasping; leaves very densely pubescent, the pubescence often harsh; larger leaves usually 4-10 cm long; principal pair of lateral veins diverging from the midrib 2-10 mm above the base of the leaf; toothing of leaf often irregular and coarse
............................15 Leaf base cuneate, broadly cuneate, rounded, or cordate (but not clasping); leaves densely to sparsely pubescent; larger leaves usually 2-6 cm long; principal pair of lateral veins diverging at the base or 2-10 mm above the base of the leaf; toothing of leaf regular and relatively fine.
..............................16 Leaf blades mostly 1-1.5 (-1.7)× as long as wide, tending to be obtuse (the apex usually 90º or more), the teeth generally rounded (the 2 sides of each tooth usually distinctly convex-curved, the end of the tooth therefore rounded), the principal pair of lateral veins diverging directly from the base of the midrib
..............................16 Leaf blades mostly (1.2-) 1.5-2× as long as wide, tending to be acute (the apex usually 90º or less), the teeth generally rather sharp (the 2 sides of each tooth straight to gently curved, the end of the tooth therefore triangular), the principal pair of lateral veins diverging 2-10 mm above the base of the midrib.
................................17 Leaves broadly cuneate to broadly rounded, thin in texture, the pubescence rather soft and long (and also often sparse), the leaf blade not twisted at base, borne in a horizontal plane, up to 10 cm long and 6.5 cm wide; axillary fascicles lacking
................................17 Leaves cuneate to broadly cuneate, firm in texture, the pubescence rather harsh and short, the leaf blade twisted at the base, thus borne in a partially or fully vertical plane, up to 5.5 cm long and 3 cm wide; axillary fascicles regularly present
................9 Leaf bases narrowly cuneate, the leaves generally broadest near the middle or toward the tip.
..................................18 Plants from conspicuously tuberous-thickened (ca. 1 cm in diameter) horizontal rhizomes; leaves deflexed, spreading, or ascending.
....................................19 Leaves 15-30 mm wide, spreading or ascending.
......................................20 Leaves sessile, with the major lateral veins diverging basally; tuberous rhizomes whitish; all heads with 5 flowers; [maritime and inland habitats, NC (?) southwards and westwards to FL and AL]
......................................20 Leaves usually short-petiolate (petioles > 5 mm long), with the major lateral veins diverging distal to the base; tuberous rhizomes often pinkish; some heads with more than 5 flowers; [maritime habitats in ne. NC and e. VA]
....................................19 Leaves 2-12 mm wide, deflexed to erect-ascending.
........................................21 Stems 3-6 (-7) dm tall, often erectly branching from near the base; involucres 3-4 mm high, the bracts with rounded apices
........................................21 Stems (6-) 10-15 dm tall, not branching near the base; involucres 5-7 mm high, at least some of the inner bracts with acute apices
..................................18 Plants from crowns or caudices; leaves usually spreading or ascending (not deflexed).
..........................................22 Plants generally with numerous branches from at or near the base, the axillary shoots of the lower internodes elongating; leaves 2-5 cm long, oblanceolate.
............................................23 Leaves broadly oblanceolate, 5-15 mm wide, crenate or serrate in the upper half
............................................23 Leaves narrowly oblanceolate, 3-8 mm wide, entire or remotely serrate apically
..........................................22 Plants generally simple below the middle, the axillary shoots of the lower nodes not elongating (except in response to injury of the main stem); leaves 3-12 cm long, lanceolate or linear.
..............................................24 Leaves mostly 6-40× as long as wide, the larger ones usually < 10 mm wide, ranging from 1-12 mm wide, whorled or opposite (rarely alternate above).
................................................25 Leaves linear to narrowly lanceolate, the principal leaves 2-7 cm long, 1-5 mm wide, 10-40× as long as wide, entire to obscurely toothed, the leaves mostly in whorls of 3 or 4
................................................25 Leaves lanceolate, the principal leaves 5-12 cm long, 5-10 (-12) mm wide, 6-15× as long as wide, conspicuously and divergently toothed, the leaves mostly opposite or in whorls of 3
..............................................24 Leaves mostly 2.5-7× as long as wide, the larger ones > 10 mm wide, ranging from 8-30 mm wide, opposite, alternate, or whorled.
..................................................26 Involucre 2.5-4 mm high; leaves obtuse to acute, elliptic to elliptic-oblanceolate, the 2 main lateral veins separating from the midrib about 1 cm above the base; leaves commonly 3 per node
..................................................26 Involucre 4.5-7 mm high; leaves acute to attenuate-acuminate, lanceolate, the 2 main lateral veins separating from the midvein at the base; leaves rarely 3 per node.
....................................................27 Leaves 3-5 cm long, 5-13 mm wide; leaf surfaces generally glabrous; [of AL westward]
....................................................27 Leaves 5-12 cm long, 5-20 mm wide; leaf surfaces short or long puberulent; [widespread].
......................................................28 Leaf surfaces glandular-punctate, densely puberulent on the surfaces and veins, the hairs fairly long and curling or twisted (as seen with at least 10× magnification); stem densely puberulent; leaves entire to serrate, the teeth varying from obscure to sharp, generally about 1 mm long (measured on the side toward the leaf apex), rarely to 3 mm long, generally forward-pointing; leaves acuminate to acute, the terminal portion not strongly attenuated, and about as likely to have teeth as the rest of the margin
......................................................28 Leaf surfaces densely glandular-punctate, sparsely puberulent (mainly on the veins), the hairs short; stem sparsely puberulent; leaves serrate to pinnatifid, the teeth often 1-5 mm long (measured on the side toward the leaf apex), often salient or divergent; leaves attenuate-acuminate, the terminal 1/3 extended and generally entire