This is a conspicuous genus in moist habitats in the fall, several species often growing together on shores and recently exposed mudflats (our species all annual except Bidens beckii). The abundant heads of the radiate species may offer a sea of color in such habitats, and the abundant fruits with usually retrorse-barbed awns offer abundant nuisance as they embed themselves in socks and other garments. Central and outer achenes in the heads may differ considerably, and their position should be checked closely when specified

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1. Leaves all or mostly aquatic (submersed), apparently whorled, fully dissected with filiform segments (usually at least 1 pair of simple, opposite, toothed leaves on emersed stem beneath head); pappus awns elongating to 2 (-4) cm in fruit.

B. beckii

1. Leaves not aquatic (if finely divided, nevertheless terrestrial); pappus awns, if any, less than 1 cm long.

2. Leaves of the main stem below the inflorescence simple, the middle and lowest sometimes cleft but without a distinctly petiolulate terminal leaflet (except in a rare variety of B. connata); achenes normally 3-4-awned.

3. Leaves sessile, at least the larger ones somewhat connate basally; rays (rarely absent) large and showy, larger than the disk; heads ± nodding at maturity; achenes 4-angled and 4-awned, retrorsely barbed on angles and awns.

B. cernua

3. Leaves mostly narrowed to distinct (sometimes winged) petioles; rays (very rarely present) shorter than disk; heads erect at maturity; achenes various.

4. Achenes retrorsely barbed on the margins their entire length (smooth and glabrous on the faces), strongly flattened with at most 1 distinct midrib, 3- (rarely 2- or 4-) awned at summit.

B. comosa

4. Achenes antrorsely barbed (often sparsely so) at the very base (in one form, up to and including awns), ± flattened, usually rather warty, but at least the central ones in a head 4-angled (i.e., ribbed on 2 faces) and 4-awned at summit when mature (the median awns generally shorter).

B. connata

2. Leaves of the main stem below the inflorescence definitely compound or so deeply lobed that the terminal leaflet has a distinct petiolule; achenes normally 2-awned.

5. Heads with well developed rays; outer phyllaries small, scarcely if ever exceeding the rays (usually shorter than the disk); margins (and usually awns) of achenes ± antrorsely barbed or ciliate.

6. Achenes narrowly cuneate (± straight-sided), ca. 1-2.5 mm wide, not thin-margined; native wetland species.

B. trichosperma

6. Achenes strongly convex-sided, ca. 2.6-4 mm or more wide, with distinct thin margin; aliens of disturbed sites, sometimes dry or saline.

7. Outer (green) phyllaries 10 or fewer, scarcely if at all exceeding the inner (brownish) ones; sparsely to densely ciliate with slender hairs.

B. aristosa

7. Outer phyllaries more than 10, much exceeding the inner ones, densely ciliate with coarse broad-based hairs.

B. polylepis

5. Heads discoid (rarely with rays under 4 mm long); outer phyllaries mostly large and foliose, much exceeding the disk; achenes various.

8. Outer phyllaries mostly (3-) 4 (-5), not ciliate; achene faces, margins, and awns antrorsely pubescent.

B. discoidea

8. Outer phyllaries mostly 6-10 or more, very sparsely to densely ciliate, at least basally; awns of achenes (except in an uncommon form) retrorsely barbed (their margins and faces mostly antrorsely pubescent or ciliate, or glabrate).

9. Outer phyllaries mostly 6-8 (very rarely 10); at least one face of achene antrorsely pubescent.

B. frondosa

9. Outer phyllaries 10 or more; faces of achenes glabrous or nearly so.

B. vulgata