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Key to Silphium

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1 Leaves basally disposed, the basal leaves large and persistent, the stem with very few to many leaves, but these definitely reduced upward in size; leaves entire to toothed, to deeply cut; plants with definite taproots (except S. brachiatum, S. confertifolium, S. gracile, S. mohrii, S. simpsonii, and S. wasiotense).

2 Stem relatively leafy, with 4-5 nodes or more, the stem leaves smaller than the basal, but not merely bracteal.

3 Leaves deeply pinnatifid to bipinnatifid

4 Rays white; stems less than 1 m tall  Silphium albiflorum

4 Rays yellow; stems usually 1-3 m tall  Silphium laciniatum

3 Leaves nearly entire to coarsely toothed (but not pinnatifid).

5 Basal and lower stem leaves cordate, subcordate, or truncate-sagittate; [Interior provinces: KY, TN, n. AL, and nw. GA].

6 Stem glabrous; peduncles glabrous; phyllaries acute  Silphium brachiatum

6 Stem hispid; peduncles hispidulous with hairs ca. 1 mm long; phyllaries obtuse.  Silphium wasiotense

5 Basal and lower stem leaves rounded or cuneate at the base.

7 Stem, inflorescence, and phyllaries glabrous or very nearly so; [AL and perhaps MS]  Silphium confertifolium

7 Stem, inflorescence, and phyllaries obviously scabrous, hirsute, or hispid; [collectively more widespread].

8 Stem, inflorescence, and phyllaries densely shaggy-hirsute, hairs > 5 mm long; inflorescence corymbose with many heads (around 3-15); pales eglandular; [TN, north AL, and northwest GA]  Silphium mohrii

8 Stem, inflorescence and phyllaries scabrous to hispid, hairs < 2.5 mm long; inflorescence consisting of a 1-5 heads subtended by a long, wand-like scape; pales glandular or eglandular.

9 Pales eglandular; [LA and TX]  Silphium gracile

9 Pales glandular; [Coastal Plain, SC, GA, FL, AL, and MS]  Silphium simpsonii

2 Stem nearly naked, bearing only a few bracteal (very reduced) leaves.

10 Heads relatively large (involucre 13-25 mm high, disk 15-25 mm wide), with 14-40 ray flowers; [calcareous or mafic glades or woodlands].

11 Principal leaves shallowly to deeply pinnatifid; leaf blade base cuneate, rounded or shallowly cordate; leaf blade often > 2× as long as wide  Silphium pinnatifidum

11 Principal leaves only toothed (or subentire); leaf blade base cordate or truncate at the base (rarely abruptly narrowed); leaf blade < 1.5× as long as wide  Silphium terebinthinaceum

10 Heads relatively small (involucre 6-11 mm high, disk 8-15 mm wide), with 6-12 ray flowers; [a wide range of mostly dry, often acidic habitats]; ["Silphium compositum group"].

12 Blades of basal leaves unlobed (or with a single obscure basal lobe on one or both sides), reniform, usually wider than long, often > 25 cm wide; leaves usually puberulent beneath; achenes shorter than the phyllaries at maturity; [upper Piedmont and Mountains]  Silphium reniforme

12 Blades of basal leaves divided or shallowly to deeply lobed, with several lobes on each side, about as wide as long, or longer than wide, < 25 cm wide; leaves usually glabrous (or sparsely scabrous) beneath; achenes longer than (or as long as) the phyllaries at maturity; [collectively widespread].

13 Involucre mostly 1.0-1.5 cm wide; achenes 6-9 mm long at maturity; achene wings < 1 mm wide, the wing tips long acute to acuminate, the sinus between the wing tips V-shaped; [VA south to GA, and west to AL]  Silphium compositum var. compositum

13 Involucre mostly 1.5-3.0 cm wide; achenes 8-14 mm long at maturity; achene wings 1-2 mm wide, the wing tips either acute to acuminate or obtuse, the sinus between the wing tips either V-shaped or narrowly U-shaped.

14 Achene wing tip obtuse, the sinus between the wing tips narrowly U-shaped; leaf blade usually longer than wide; petiole short, as long as or shorter than the leaf blade (midrib); [se. SC south to c. peninsular FL and FL Panhandle]  Silphium compositum var. ovatifolium

14 Achene wing tip acute to acuminate, the sinus between the wing tips V-shaped; leaf blade usually as long as wide; petiole long, as long as or longer than the leaf blade (midrib); [se. NC south to se. GA and FL Panhandle]  Silphium compositum var. venosum

1 Leaves primarily on the stem, basal leaves usually absent or soon withering, the stem with many leaves, these similar in size; leaves entire or toothed; plants fibrous-rooted from a crown, rhizome, or caudex.

15 Stem square in ×-section; upper leaves connate, fused basally, the stem thus perfoliate.

16 Stem spreading-hispid (rarely nearly glabrous); heads with usually ca. 8 or ca. 13 rays; hairs on lower leaf surface or veins 1-2 mm long  Silphium connatum

16 Stem glabrous or glabrescent; heads with usually ca. 21 or ca. 34 rays; hairs on lower leaf surface absent or < 1 mm long  Silphium perfoliatum

15 Stem terete; leaves not connate.

17 Basal and lower cauline leaf blades cordate, sagittate, or truncate at the base, and on well-developed petioles  Silphium brachiatum

17 Basal and lower cauline leaf blades either rounded or cuneate at the base, or sessile.

18 Ray flowers 20-30 per head (or more); pales eglandular; [west of the Mississippi River].

19 Stem, inflorescence branches, and phyllaries glabrous  Silphium speciosum

19 Stem, inflorescence branches, and phyllaries hispid or puberulent.

20 Mid-stem leaves predominately opposite; outer phyllaries not notably foliaceous, closely reflexed  Silphium integrifolium var. integrifolium

20 Mid-stem leaves alternate or opposite; outer phyllaries foliaceous, spreading to loosely reflexed  Silphium radula

18 Ray flowers 12-22 per head; pales eglandular or stipitate-glandular (glands often mixed with other pubescence, use 20× magnification); [east of the Mississippi River, except for S. asperrimum]

21 Mid-stem leaves predominately alternate (lower leaves often opposite); inflorescence eglandular except for inconspicuous dull yellow-brown stipitate-glands on pales; stem glabrous, hispid, or hirsute with hairs >1 mm.

22 Stem glabrous or short-hispid; mid-stem leaves petiolate or sessile (not clasping); inflorescence open and widely branching  Silphium dentatum

22 Stem hispid or hirsute, with hairs often > 1mm long (rarely nearly glabrous); mid-stem leaves clasping, sessile, or short-petiolate; inflorescence open or congested.

23 Phyllary abaxial surface hispid; inflorescence congested, with peduncles usually less than 4 cm long; basal leaves withered by flowering  Silphium asperrimum

23 Phyllary abaxial surface scabrous or glabrous; inflorescence open, with peduncles up to 13 cm long; basal leaves sometimes persisting at flowering; [Coastal Plain, SC, GA, FL, AL, and MS]  Silphium simpsonii

21 Mid-stem leaves predominately opposite or whorled (occasionally alternate in damaged re-growth); inflorescence eglandular, or if glandular then glands conspicuous and not restricted to pales; stem various, but never hirsute with hairs >1 mm.

24 Mid-stem leaves petiolate or subsessile (not clasping), with a cuneate or rounded base; leaves typically lanceolate, gradually tapering towards the tip; stem and inflorescence branches eglandular.

25 Mid and lower stem leaves opposite; stem conspicuously hispid (or at least scabrous); inflorescence congested, with less numerous heads  Silphium asteriscus var. asteriscus

25 Mid and lower stem leaves opposite or in whorls of 3; stem glabrous or nearly so; inflorescence open, with more numerous heads (up to 50 per inflorescence, although often much less).

26 Mid and lower stem leaves opposite, broadly ovate, 5-8 cm wide when 10-16 cm long  Silphium asteriscus var. latifolium

26 Mid and lower stem leaves whorled or opposite, lance-ovate, 2-4 cm wide when 10-16 cm long  Silphium asteriscus var. trifoliatum

24 Mid-stem leaves clasping to short-petiolate, with a subcordate to rounded base; leaves typically ovate, widest at base and abruptly tapering to an acute tip; stem and inflorescence branches eglandular or stipitate-glandular.

27 Stems, leaves, and phyllaries eglandular, or if glands present then dark and dull, and restricted to phyllaries and inflorescence bracts.

28 Stem and inflorescence branches glabrous  Silphium integrifolium var. gattingeri

28 Stem and inflorescence branches finely puberulent or hispid  Silphium integrifolium var. integrifolium

27 Stems, leaves, and phyllaries with yellow-brown and shining stipitate glands, usually well-evident on leaves and stem in addition to phyllaries (in addition to the eglandular pubescence).

Show caption*© Scott Ward                                                             29 Plants mostly 8-15 dm tall, with usually 6 or 7 nodes below the inflorescence; glandular hairs of the stems and leaves longer than the eglandular hairs; rays (8-) 12-14 (-16) per head; [dolomite or limestone in Bibb County, c. AL]  Silphium glutinosum

29 Plants mostly 15-20 dm tall, with usually 9-12 nodes below the inflorescence; glandular hairs of the stems and leaves about as long as the eglandular hairs; rays (17-) 19-23 (-33) per head; [chalk in c. AL]  Silphium perplexum