1 Stem and lower leaf-surfaces grey strigose-canescent; annuals, 2-10 dm tall, with taproots

1 Stem and lower leaf surfaces glabrous or pubescent, but not grey strigose-canescent; perennials, 4-40 dm tall, with fibrous or fleshy-fibrous roots.

  2 Leaves primarily opposite (the uppermost sometimes alternate).

    3 Internodes winged; [collectively widespread].

      4 Plants 4-5 (-10) dm tall, perennating from short horizontal rhizomes; ray flowers (5-) 8; disc flowers 20-60+; [endemic to ne. FL and se. GA]

    3 Internodes not winged; [collectively of sw. GA, s. AL, FL Panhandle, and TX].

        5 Ray flowers 0; heads 1 (-3)

        5 Ray flowers (5-) 11-13+, yellow; heads 1-20.

          6 Phyllaries 2-3+ mm long; disk flowers 40-80+; [AL, FL, GA]

          6 Phyllaries 5-12+ mm long; disk flowers 80-120; [TX]

  2 Leaves primarily alternate (the lowermost sometimes opposite).

             7 Heads few, 1-15 (-20) in a compact inflorescence; disc 7-16 mm wide at anthesis; ray flowers (5-) 7-15, yellow; plants 5-12 dm tall

             7 Heads numerous, 10-200+ in a dense to open inflorescence; disc 3-15 mm wide at anthesis; ray flowers either absent, or 1-5 and white, or 2-10 and yellow; plants 10-40 dm tall.

               8 Ray florets absent; disk flowers white

               8 Ray florets present; disk flowers white or yellow.

                 9 Ray flowers and disc flowers yellow

                 9 Ray flowers and disc flowers white.

                   10 Ray flowers 9-12+; disk flowers 20-25+; [TX only]

                   10 Ray flowers 1-7; disk flowers 8-15; [collectively widespread].

                     11 Lower and middle leaves pinnately lobed or dissected; achenes of ray florets glabrous; [of the outer Coastal Plain from SC southward]

                     11 Lower and middle leaves entire, serrate, or slightly undulate; achenes of ray florets papillose or short-pubescent; [more widespread in our area]

1 Pappus present, of 5-60 barbellate bristles; receptacle naked (without paleae or well-developed bristles); [tribe Heliantheae; subtribe Eupatoriae].

  2 Plant a twining herb, phyllaries and disk florets 4 per head

  2 Plant stiffly erect to sprawling but never twining, phyllaries and disk florets usually > 4 per head.

    3 Leaves in whorls of 3-7, > 2 cm wide

    3 Leaves opposite, rarely alternate or whorled, if whorled, < 2 cm wide.

      4 Cypselae rostrate (with long beaks); the achene portion not bearing ribs

      4 Cypselae not beaked, the achene variously ribbed.

        5 Achenes (and ovaries) 8-10-ribbed; outer phyllaries longitudinally striate

        5 Achenes (and ovaries) (3-) 4-5-ribbed; outer phyllaries not noticeably longitudinally striate.

          6 Heads pink to blue.

             7 Phyllaries in 4-6+ series, deciduous

             7 Phyllaries in 2-4 series, persistent (or deciduous in Praxelis).

               8 Heads pink (rarely bluish); receptacles flat; florets 18-25 per head; leaves sometimes gland-dotted

               8 Heads blue; receptacles conic; florets 25-70 per head; leaves gland-dotted.

                 9 Bristles ca. 30; florets 35-70+; phyllaries ca. 25, subequal, persistent

                 9 Bristles ca. 40; florets 25-30; phyllaries 15-25, unequal, deciduous (absence apparent in fruit)

          6 Heads white to cream or, rarely, pale lilac.

                   10 Florets 3-7 per head

                   10 Florets at least 9 per head.

                     11 Phyllaries not strongly imbricate, with the principal ones subequal and sub-biseriate; petioles 0.5-10 cm

                     11 Phyllaries clearly imbricate, in 3+ series, the margins usually glandular; some species epetiolate

1 Pappus either absent, or of scales, setae, or awns; receptacle naked, paleate, or bearing bristles.

                       12 Leaves whorled, linear, < 2 mm wide; head solitary; [aquatic herb growing in shallow stagnant water]

                       12 Leaves opposite (or alternate in part), broader in shape and > 5 mm wide; heads typically not solitary; [terrestrial or wetland plants].

                          13 Receptacles naked (epaleate).

                            14 Pappus of 2-12 scales; heads white, pink, or blue (yellow in Schkuhria).

                              15 Heads with 20-125 florets; leaves serrate or ± dentate (at least distally); [rarely naturalized exotics or s. TX/FL natives]; [tribe Eupatorieae].

                                16 Leaves petiolate; corolla tube glabrous, typically blue or lavender colored (sometimes white tinged); heads with 20-125 florets; [naturalized exotics or s. FL native]

                                16 Leaves sessile; corolla tube glandular-pubescent, typically white colored (sometimes pink-purple tinged); heads with 75-125 florets; [s. TX native]

                              15 Heads with 10-30 florets (or fewer in Schkuhria); leaves entire; [natives, of FL, s. GA, MS, and TX in our area]; [tribe Heliantheae; subtribe Chaenactidinae].

                                  17 Leaves 3-5 lobed (or foliolate); cypselae with curled hairs (Florestina tripteris) or villous on angles (Schkuhria pinnata); [s. TX in our area or ne. waif].

                                    18 Heads discoid, corolla white; cypselae with curled hairs; [s. TX in our area]

                                    18 Heads discoid, sometimes with 1 (-2) diminuitive rays, corolla (of discs) yellow; cypselae villous, at least at bases or on angles; [waif of ne US, native in sw. US]

                                  17 Leaves simple or unlobed; cypselae with straight hairs; [variously distributed in s. GA and FL w. to MO and s. TX]

                            14 Pappus none, or of a few bristles or irregular coroniform lobes; heads green to yellow.

                                       19 Heads bright yellow, closely aggregated into corymbiform arrays of flat-topped to dome-shaped glomerules

                                       19 Heads greenish or greenish yellow, borne in spiciform, racemiform, or paniculiform arrays.

                                         20 Heads in paniculiform arrays, all or most of the heads not subtended by bracts

                                         20 Heads in spiciform or racemiform arrays, each 1 (-2) heads subtended by a bract

                          13 Receptacles with paleae or well-developed bristles (the pales sometimes modified into specialized perigynia or burs surrounding the achene).

                                           21 Involucre of four decussate, deltoid bracts (the outer two large and longer than and often enclosing the flowering heads); pappus absent; paleae tightly enclosing the cypselae; plants rooting at the nodes, sometimes free-floating in water; [uncommon non-native, c. FL]

                                           21 Involucre not of four decussate, deltoid bracts; plants not with the above combination of characters; [widespread natives and non-natives]

                                             22 Heads small, less than 1 cm in diameter at anthesis (the female heads enlarging in Xanthium); disc florets dull white or suffused with green or purple; florets mainly unisexual (either in the same heads and then males central and females peripheral, or in separate female and male heads); female florets 0-8 per head; [tribe Heliantheae; subtribe Ambrosiinae].

                                               23 Heads unisexual; cypselas shed within an indurated bur or “nut” with hooked or straight spines developed from the phyllaries and/or paleae.

                                                 24 Involucre of the female heads with tubercles or straight spines developing from the phyllaries; burs 1-8 mm long

                                                 24 Involucre of the female heads with hooked spines developing from the phyllaries/paleae; burs 10-35 mm long

                                               23 Heads bisexual, with functionally male and female flowers in the same head; cypselas shed individually, not enclosed.

                                                    25 Heads in paniculiform arrays, all or most of the heads not subtended by bracts

                                                    25 Heads in spiciform or racemiform arrays, each 1 (-2) heads subtended by a bract

                                             22 Heads larger, mostly > 1 cm in diameter at anthesis; disc florets conspicuously white, yellow, pale yellow, or purple; florets mainly bisexual; female florets > 12 per head (except 2-8 in Polymnia and 8-15 in Verbesina occidentalis).

                                                      26 Leaves strongly basally disposed, the basal-most leaves sitting almost flat on the ground and overlapping; disc flowers maroon-purple

                                                      26 Leaves mainly or strictly cauline (not basally disposed); disc flowers white, yellow, or reddish-brown.

                                                        27 Disc flowers bright yellow or reddish-brown.

                                                          28 Involucre of phyllaries subtended by a calyculus of bracts obviously different in color, texture, and shape than the phyllaries.

                                                            29 Phyllaries usually distinct, rarely connate proximally; disk florets bright yellow

                                                            29 Phyllaries connate (both proximally-distally connate); disk florets yellow to reddish-brown

                                                          28 Involucre of phyllaries not subtended by a calyculus (phyllaries sometimes in 2+ series, but without obvious calyculi below).

                                                              30 Plants erect; heads without ray florets; [wet pine savannas and bogs; FL panhandle]

                                                              30 Plants primarily prostrate or trailing, sometimes erect; heads with or without ray florets; [habitats various, widespread natives and non-natives]

                                                        27 Disc flowers bright white or pale yellow.

                                                                 31 Stems not winged; plants erect or spreading.

                                                                   32 Cypselae 3-4 angled; pappus falling (of 2-12 awns or bristles); leaf margins irregularly crenate to serrate, the surfaces sometimes roughened but lacking glands; [e. of TX]

                                                                   32 Cypselae 5-ribbed; pappus absent; leaf margins entire or serrulate, the surfaces gland-dotted; [s. TX]

                                                                 31 Stems winged; plants erect.

                                                                     33 Disc flowers creamy or pale yellow; leaves prominently pinnately lobed, with 5-many lobes