Pittonia 4: 268. 1901.
Annuals, (30–) 50–120 cm. Leaves sessile or petioles 10–50 mm (winged); blades lance-elliptic to lance-oblong, 80–120 (–250) × 30–100 mm (proximal usually with 1–2 lobes near bases), bases cuneate, margins laciniate to serrate, ciliate, apices acuminate, faces glabrous. Heads in ± corymbiform arrays. Peduncles (10–) 30–70+ mm. Calyculi of 8–10 usually spreading, lance-linear to lanceolate, sometimes ± foliaceous bractlets or bracts 10–20 (–75) mm, margins entire or incised, ciliate, abaxial faces ± hispid at bases. Involucres ± hemispheric or broader, 6–10 × 18–25 mm. Phyllaries 13–16+, oblong to lanceovate, 6–10 mm. Ray-florets 6–11; laminae yellow, 4–8 mm. Disc-florets 50–100+; corollas yellow, 2.5–3.5 mm. Cypselae olivaceous green, ± flattened, unequally 3–4-angled, cuneate, outer 5–6 mm, inner 6–7 mm, margins antrorsely to retrorsely barbed, apices truncate to concave, faces weakly striate, glabrous or sparsely puberulent; pappi 0, or of (1–) 3 (–4) suberect, retrorsely barbed awns 2–4 mm.
Phenology: Flowering Aug–Oct.
Habitat: Seasonal wetlands, borders of ponds, lakes, streams, estuaries
Elevation: 0–100 m
Of conservation concern.
For decades, Bidens amplissima was known only from Vancouver Island; it is now known from southwestern British Columbia and northwestern Washington (F. R. Ganders et al. 2003). Occurrences in southern Manitoba and northwestern Nebraska may represent ephemeral introductions. The restricted natural range of B. amplissima has stimulated interest in it, the addition of it to rare plant lists, and the consideration of need for protected sites where it grows.