Bidens tripartita L.
MOC = 62
Family - Asteraceae/Heliantheae
Habit - Taprooted annual forb.
Stem - Erect, to about 1 m, glabrous. Plants taprooted annuals.
Leaf - Leaves all similar, opposite, sessile or petiolate. Blades usually unlobed, lanceolate to elliptic, to 8 cm, sharply toothed, glabrous to moderately pubescent. Deeply 3-lobed leaves occasional present.
Inflorescence - Solitary terminal heads or in loose, open clusters of heads.
Heads - Usually discoid. Involucral bracts strongly dimorphic. Outer bracts 4-9, to 35 mm, loosely ascending to spreading, oblanceolate to linear, green, leafy, the margins entire or finely toothed, often also with spreading hairs, the outer surface glabrous or sparsely short-hairy. Inner bracts 7-8, to 12 mm long, narrowly ovate to ovate, usually yellow, glabrous. Chaffy bracts present, narrowly lanceolate, usually purplish-tinged at the tip.
Flowers - Ray florets when present 1-5, the corolla inconspicuous, 3-8 mm long, yellow. Disc florets 20-40, the corollas 3-4 mm long, sometimes only 4-lobed, yellow. Pappus absent or more commonly of 3 or 4 awns mostly 2-3 mm long, these barbed, erect or somewhat spreading at fruiting.
Fruits - Fruits 3-11 mm long, awned, linear to narrowly wedge-shaped, flattened and somewhat 3- or 4-angled in cross-section, the faces each with a longitudinal nerve, dark brown to purplish black, glabrous or finely pubescent, sometimes also with minute tubercles.
Flowering - July - October.
Habitat - Streambanks, pond margins, sloughs, swamps, fens, bottomland forests.
Origin - Native to the U.S.
Lookalikes - Bidens frondosa, B. vulgata, and others. Bidens tripartita is distinguished by having main stem leaves which are simple and undivided.
Other info. - This is one of three Missouri species of Bidens which (usually) has simple rather than compound leaves. This feature in combination with discoid heads is a strong indicator of the species. It is unusual among plants of its family, and genus, in having disk florets which are often 4-lobed rather than 5-lobed. Ray florets are occasionally present but never showy. The plant is found throughout most of Missouri but is uncommon or absent in the southwestern third of the state. Beyond Missouri its range includes much of the northeastern 2/3 of the continental U.S., also extending into Canada. It is a plant of wet areas.
Photographs taken at Catawissa Conservation Area, Franklin County, MO, 9-14-2017, Riverfront Park, Washington, Franklin County, MO, 9-20-2019, and Shaw Nature Reserve, Franklin County, MO, 9-28-2018, and near Creve Coeur Lake Park, St. Louis County, MO, 9-26-2019 (SRTurner).