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Key to Helianthus

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1 Leaves basally disposed, the plants scapose to subscapose, the stem leaves relatively few (with 2-8 nodes below the inflorescence), those on the upper stem opposite or alternate, strongly reduced upward in size as compared to the persistent basal leaves; [section Atrorubentes]
1 Leaves cauline, plants leafy the length of the stem, the stem leaves many (with 10 or more nodes below the inflorescence), basal leaves lacking (at least at anthesis).
..2 Plant a tap-rooted annual (rarely surviving a second year)
..2 Plant a perennial from crown buds or rhizomes, the roots sometimes tuberous-thickened; [section Atrorubentes].

Key to Helianthus, Key A: sunflowers with basally disposed leaves

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1 Disk flowers yellow.
..2 Basal leaves 6-15 cm long, 2-8 cm wide; leaves 1.5-5× as long as wide, scabrous or hirsute (rarely glabrous)
....3 Leaves rough to touch, scabrous or hirsute; leaves entire or nearly so; [widely distributed]
....3 Leaves smooth to touch, strigose or glabrous; leaves usually serrate; [AR, LA, TX]
..2 Basal leaves 10-30 cm long, 0.5-2.0 cm wide; leaves 10-20× as long as wide, glabrous.
......4 Head 1 (-3); phyllaries 8-16 mm long, 3-5 mm wide; [of wet flatwoods and wet prairies of ne. FL]
......4 Heads 3-12; phyllaries 5-11 mm long, 1-2.5 mm wide; flowering Aug-Oct; [of sandstone and granite glades and woodlands and loamy to xeric longleaf pine sandhills; rarely escaped elsewhere]
1 Disk flowers red or purple (at least in part).
........5 Basal leaves 6-20 cm long; lower several pairs of stem leaves up to 1/2 as long and wide as the basal leaves; [collectively widespread in our region from VA, KY, IL, MO, and OK southwards, in the inland provinces and the Coastal Plain].
..........6 Trichomes on the leaf abaxial midrib > 1 mm long; lower stem with a few pairs of leaves (< 8 nodes below the capitulescence), these strongly reduced upward; leaf blades (1.3-) 1.7-2.5 (-3)× as long as wide; petiole often > 1/3 as long as the blade, broadly winged toward the blade; plants to 2 m tall; nonflowering stems usually absent; [widespread in our area]
..........6 Trichomes on the leaf abaxial midrib < 1 mm long; lower stem leafy, often to above the middle (> 8 nodes below the capitulescence); leaf blades 1-1.7 (-2)× as long as wide; petiole usually < 1/3 as long as the blade, narrowly winged toward the blade; plants to 3 m tall; nonflowering stems usually present; [west of our area]
........5 Basal leaves 4-15 cm long; lower several pairs of stem leaves often < 1/2 as long and wide as the basal leaves; [Coastal Plain, NC to FL to e. LA].
............ 7 Basal leaves (1.6-) 2-5× as long as wide; ray flowers present, 12-18, typically 1.5-3.5 cm long; [wet savannas and bogs]
............ 7 Basal leaves 1-1.5× as long as wide; ray flowers none, or present, 2-8, but < 1 cm long; [dry savannas and sandhills]

Key to Helianthus, Key B: annual sunflowers

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1 Disk flowers yellow.
..2 Leaves ovate, 10-40 cm long, 5-25 cm wide, toothed, the base often cordate or subcordate; disc corollas 5-8 mm long; stems 10-30 dm tall; [section Helianthus]
..2 Leaves lanceolate, 5-10 cm long, 0.2-1.0 cm wide, entire or nearly so, the base cuneate; disc corollas 2.8-3.5 mm long; stems 4-10 dm tall; [section Porteri]
1 Disk flowers red or purple (at least in part).
....3 Leaves, stems, and phyllaries densely covered with soft, silvery-white pubescence; [section Helianthus]
....3 Leaves, stems, and phyllaries nearly glabrous to scabrous or hirsute.
......4 Style branches yellow; [section Agrestes]
......4 Style branches red; [section Helianthus].
........5 Phyllaries ovate to ovate-oblong, > 4 mm wide; leaves 5-25 cm wide; disk (2-) 3-30 cm wide; plants (0.5-) 1-3 m tall
........5 Phyllaries lanceolate, 1-4 (-5) mm wide; leaves 1.5-9 cm wide; disk 1-2.5 cm wide; plants 0.4-1.5 m tall.
..........6 Tips of the receptacular bracts (paleae) in the center of the head conspicuously white-bearded.
............ 7 Leaves mostly alternate; petioles 5-9 cm long; ray flowers 10-30; [widely distributed in our region, native and adventive]
............ 7 Leaves mostly opposite; petioles 2-4 cm long; ray flowers 11-13; [TX coastal prairies and s. TX]
..........6 Tips of the receptacular bracts (paleae) in the center of the head not bearded.
............ ..8 Leaves mostly opposite; phyllaries 30-39, short acuminate at tip; [TX]
............ ..8 Leaves mostly alternate; phyllaries 20-30, acute to long-attenuate at tip; [collectively widespread in our region].
............ ....9 Stems decumbent, with ascending or erect flowering branches; peduncles 9-20 (-22) cm long; blades of the ray flowers 11-16 mm long; [of FL peninsula].
............ ......10 Stems glabrous or puberulent; leaves serrulate to evenly serrate; [of east coast of FL peninsula, from St. Johns County south to Monroe County)]
............ ......10 Stems hirsute-villous; leaves deeply and irregularly coarsely serrate; [of west coast of FL peninsula, from Hernando County south to Lee County]
............ ....9 Stems erect; peduncles 10-50 cm long; blades of the ray flowers 14-30 mm long; [collectively widespread in our region].
............ ........11 Leaves 8-14 cm long; peduncles 20-40 cm long; discs 10-15 (-17) mm in diameter; [w. LA and e. TX]
............ ........11 Leaves 2.5-9 cm long; peduncles 10-50 cm long; discs 14-20 mm diam; [collectively widespread in our region].
............ ..........12 Peduncles 25-50 cm long; leaves usually shallowly but regularly serrate; ligules usually > 2 cm long; blades of the ray flowers (15-) 20-30 mm long
............ ..........12 Peduncles usually 10-25 (-30) cm long; leaf usually deeply and irregularly coarsely serrate; ligules usually < 2 cm long; blades of the ray flowers 12-20 (-22) mm long

Key to Helianthus, Key C: perennial sunflowers with leafy stems and red disk flowers

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1 Stems glabrous or glabrate and often glaucous; leaf margins not revolute.
..2 Plant 4-7 dm tall; blades of ray flowers 8-9 mm long; leaf blades 3-7.5 cm long
..2 Plant 15-25 or more dm tall; blades of ray flowers 28-35 mm long; leaf blades 8-21 cm long
1 Stems pubescent, not glaucous; leaf margins revolute or not.
....3 Leaf blades long and narrow, linear or lanceolate and usually > 10× as long as wide.
......4 Plants < 1.5 m tall; leaves < 1 cm wide; rhizomes lacking or poorly developed
......4 Plants > 1.5 m tall; leaves > 1 cm wide; rhizomes well developed
....3 Leaf blades shorter and broader, lanceolate, lance-ovate, deltoid, deltoid-ovate and usually < 5× as long as wide.
........5 Phyllaries 3-5 mm broad, oblong, ovate, or obovate.
..........6 Abaxial surfaces of leaves and ligules lacking subsessile glandular trichomes; leaves usually broadly ovate to orbicular and with a petiole > 1 cm long
..........6 Abaxial surfaces of leaves and ligules with subsessile glandular trichomes; leaves usually lanceolate to lance-ovate or rhombic-ovate and with a petiole usually < 1 cm long.
............ ..8 Plants 8-20 dm tall; upper leaves usually alternate; leaf blades oblong-lanceolate to lance-ovate, 8-27 cm long, acuminate at tip
............ ..8 Plants 5-12 dm tall; leaves opposite; leaf blades rhombic-ovate to lance-linear, 5-12 cm long, acute or obtuse at tip

Key to Helianthus, Key D: perennial sunflowers with leafy stems and yellow disk flowers

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1 Stems below the capitulescence glabrous or nearly so, sometimes glaucous.
..2 Leaves whorled at principal nodes, either alternate or opposite at other nodes
..2 Leaves either alternate or opposite (or both), never whorled.
....3 Leaves grayish-green or bluish green in color, sessile, and glabrous and glaucous on the undersurface.
......4 Rays 10-14; leaves strumose adaxially, rough to the touch; phyllaries 3.5-4.5 mm wide
......4 Rays 5-10; leaves glabrous or glabrate adaxially, smooth or only slightly rough to the touch; phyllaries 2-3 mm wide
....3 Leaves light to dark green, sometimes whitish abaxially, but not grayish or bluish green in color; leaves sessile or petiolate, glabrous or pubescent.
........5 Leaves linear-lanceolate, with only a single main vein
..........6 Rays few, usually 5 or 8; heads small, the involucres 9 mm broad or less.
............ 7 Leaves abaxially whitish in color and glabrous and glaucous, lacking subsessile glandular trichomes (‘resin dots’)
............ 7 Leaves abaxially greenish in color, usually tomentulose (sometimes glabrate), with abundant subsessile glandular trichomes
..........6 Rays usually 10 or more in larger heads; heads larger, the involucres usually > 9 mm broad.
............ ..8 Leaves sessile, rounded to cordate at base, and trinerved, with the 2 lateral veins diverging from the midrib at the very base of the leaf
............ ..8 Leaves sessile to petiolate, but narrowing gradually to base and triplinerved, the 2 lateral veins diverging from the midrib above the base of the blade.
............ ....9 Anther appendages yellow.
............ ......10 Leave blade lanceolate to lance-ovate, sessile to petiolate but the petiole usually < ¼ as long as the blade; phyllaries not conspicuously graduated and imbricate, usually loose and spreading
............ ......10 Leaf blade ovate to elliptic, with a distinct petiole usually > 2 cm long and ½ as long as blade or longer; phyllaries conspicuously graduated and imbricate, usually appressed, not exceeding disk
............ ....9 Anther appendages dark or reddish-brown.
............ ........11 Plants producing abundant tubers; leaves subsessile, the petioles < 1 cm long; [endemic to the Piedmont of NC and SC]
............ ........11 Plants rhizomatous, but not producing tubers; leaves petiolate, the petioles 1-5 cm long; [collectively widespread in our area].
............ ..........12 Longer phyllaries usually exceeding disk by ½ their length or more, apex acuminate; larger leaves moderately to conspicuously serrate, with a petiole 2-5 cm long, and abaxially with usually relatively few subsessile glandular trichomes
............ ..........12 Phyllaries equal to or slightly exceeding disk, apex acute; leaves moderately serrate to entire, with a petiole 1-3 cm long, and abaxially with usually abundant subsessile glandular trichomes (‘resin dots’)
1 Stems pubescent throughout, not glaucous.
............ ............ 13 Leaves sessile and cordate, mostly or all opposite
............ ............ 13 Leaves petiolate or sessile, but not cordate, and alternate or opposite.
............ ............ ..14 Phyllaries attenuate, conspicuously exceeding the disk in length and reflexed, apically with numerous subsessile glandular trichomes (‘resin dots’); leaf bases often convex, the basically ovate or lance-ovate blade joined to a broadly winged and gradually narrowed petiole
............ ............ ..14 Phyllaries acute to attenuate, but not reflexed, subsessile glandular trichomes present or absent; leaf bases usually attenuate to truncate or rounded, the blade lance-linear or lanceolate, or if ovate or lance-ovate either sessile or with a petiole that is at most narrowly winged.
............ ............ ....15 Leaves conduplicate (strongly folded along the midvein, thus V-shaped) and entire, usually with only a single prominent main vein; inflorescence when well developed spiciform or racemose
............ ............ ....15 Leaves not conduplicate, entire or serrate, triplinerved (with a prominent lateral pair of veins near the base); inflorescence not spiciform or racemose.
............ ............ ......16 Phyllaries conspicuously graduated and imbricate, usually appressed.
............ ............ ........17 Leaf blades lanceolate to ovate, petiole 1-5 cm long and usually < ½ as long as blade; anther appendages with dark pigment; cypselas 4-5 mm, usually sterile
............ ............ ........17 Leaf blades ovate to elliptic, petiole distinct, > 2 cm and usually > ½ as long as the blade; anther appendages yellow; cypselas 3-4 mm long, fertile
............ ............ ..........18 Leaves rough to touch, scabrous or hirsute; leaves entire or nearly so; [widely distributed]
............ ............ ......16 Phyllaries not conspicuously graduated and imbricate, usually loose or spreading.
............ ............ ............ 19 Leaves with a prominent petiole > 2 cm long, blades lance-ovate to ovate and > 5 cm broad; cypselas 5-7 mm long; tubers produced late in growing season
............ ............ ............ 19 Leaves sessile or with a short petiole usually < 2 cm long; blades linear to lanceolate, < 4.5 cm broad; cypselas 3-5 cm long; tubers present or absent.
............ ............ ............ ..20 Leaves truncate to broadly rounded at base, shortly but distinctly petiolate
............ ............ ............ ....21 Ligules lacking subsessile glandular trichomes; leaves not strongly revolute
............ ............ ............ ....21 Ligules abaxially with subsessile glandular trichomes (‘resin dots’); leaves usually revolute.
............ ............ ............ ......22 Heads relatively small, the discs usually < 15 mm across; tubers present
............ ............ ............ ......22 Heads larger, the discs (at least the larger) > 15 mm across; tubers absent.
............ ............ ............ ........23 Leaves conspicuously undulate; ovate to elliptical to lanceolate, occasionally linear (if so, usually < 10 cm long), usually < 5× as long as wide; heads 1-6 per plant; outer phyllaries acute to obtuse
............ ............ ............ ........23 Leaves not conspicuously undulate; linear to lanceolate, > 5× as long as broad (and also 8-16 cm long); heads 3-16 per plant; outer phyllaries acuminate to acute.
............ ............ ............ ..........24 Plants short, < 1.5 m tall; leaves < 1 cm wide; rhizomes lacking or poorly developed
............ ............ ............ ..........24 Plants robust, > 1.5 m tall; leaves > 1 cm wide; rhizomes well developed
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