More often these plants have no rays on their flower heads, but forms with rays do occur, and this patch growing in Bird Park (Mount Lebanon) had unusually abundant rays on some flower heads (none on others). The “bur” in the name is the seed, which has a pair of little spikes that attach it to fur or clothing, spreading the plant very efficiently. It is distinguished from Spanish Needles (Bidens bipinnata) by larger flower heads with more florets, and by its leaves with three or five leaflets. The stems are often reddish.
Gray describes the genus and the species:
BÌDENS L. BUR MARIGOLD. Heads many-flowered; the rays when present 3-8, neutral. Involucre double, the outer commonly large and foliaceous. Receptacle flattish; chaff deciduous with the fruit. Achenes flattened parallel to the bracts of the involucre, or slender and 4-sided (rarely terete), crowned with awns or short teeth (these rarely naked). — Annual or perennial herbs, with opposite various leaves, and mostly yellow flowers. (Latin, bidens, two-toothed.)
N. B. — In this genus the measurements of the fruit relate to the inner mature achenes. The outer are often shorter and uncharacteristic.
B. frondòsa L. (BEGGAR-TICKS.) Stems tall (7 dm. or less in height), paniculate-branched; leaves 3–5-divided, glabrous, the terminal leaflet long-stalked, acuminate, often again divided, lateral ones shorter, less acuminate, all sharply serrate; heads 1.5 cm. long or less, on slender peduncles; outer involucre of 5–8 ciliate bracts; rays small, yellow; achenes narrowly cuneate, 7-10 mm. long, black, strongly 1-nerved on each face, often slightly hairy, the retrorsely barbed slightly divergent slender awns barely half as long, exceeding the 5-toothed orange corolla. (B. melanocarpa Wiegand.) — Common in damp ground, throughout. Aug., Sept. Fig. 994. Var. ANOMALA Porter. Awns upwardly barbed. — Local, N. S. to Pa.