Bidens bipinnata L.
Family - Asteraceae
Stems - To +70cm tall, herbaceous, erect, glabrous, 4-angled, dichotomously branching, from taproot.
Leaves - Opposite, petiolate, deeply bipinnatifid, glabrous or with a few sparse hairs, to +/-20cm long, +/-12cm broad. Petiole to 10cm long, slightly winged. Ultimate divisions acute to acuminate.
Inflorescence - Single axillary pedunculate flower head. Peduncles elongating in fruit to -10cm long, glabrous or sparse puberulent.
Involucre - 3mm in diameter, -5mm tall (long). Phyllaries deep green, spatulate to oblanceolate or subulate, to 6mm long, acute, unequal, sparse appressed pubescent externally, glabrous internally, with strigillose margins. Some phyllaries with slightly scarious or lighter-colored margins.
Ray flowers - Sterile. Ligule yellow, 2-5 in number (sometimes absent), 4.5mm long, 2mm broad, spatulate, rounded at apex, glabrous. Achene compressed, 1mm long, glabrous. Pappus absent.
Disk flowers - Typically +/-10 in number, fertile. Corolla tube 3mm long, 5-lobed, glabrous, orange, near and at apex. Lobes acute, .5mm long, glabrous. Stamens 5, adnate near base of corolla tube. Anthers connate around style, included, reddish, 1.1mm long. Style bifurcate, barely exserted, yellow. Achene (in flower) 2mm long, glabrous, 4-angled. Pappus 4 retrorse barbellate awns to 2.5mm long. Achenes in fruit to 1.3cm long, blackish-green, typically unequal, with persistent pappus. Receptacle flat. Chaff to 4mm long, 1mm broad, scarious, greenish, glabrous.
Flowering - August - October.
Habitat - Open woods, glades, pastures, open rocky ground, thickets, waste ground, roadsides, railroads.
Origin - Native to U.S., and tropical regions around the globe.
Other info. - The genus name Bidens means "two teeth", referring to the awns
of the pappus and fruit (other species only have two awns). Because of
the awns, the fruits will cling to most anything, thus distributing the
plant quickly to new locations. The plant is not stout and wilts when hit
with direct hot sun. Ray ligules are not always present in the flower heads.
Photographs taken at the Parkville Nature Sanctuary, Platte County, MO., 8-12-00, and in the Ozark Scenic Riverways, Shannon County, MO., 8-17-03 (DETenaglia); also along the Katy Trail near Dutzow, Warren County, MO, 8-3-2015 and 7-10-2018 (SRTurner).