Swamp Sunflower

The distal parts of Helianthus angustifolius are identical in appearance to those of H. simulans; plants of the latter are taller (200+ cm) and more robust. The leaves are not only narrower in H. angustifolius but also tend to be more strongly revolute (abaxial faces sometimes scarcely visible).

Perennials, 50–150+ cm (with crown buds, rhizomes absent or poorly developed). Stems erect, usually ± hairy. Leaves mostly cauline; opposite or alternate; sessile or subsessile; blades (3-nerved near bases) narrowly lanceolate to linear, 8–15 × 0.15–0.5(–1) cm, bases cuneate , margins entire (revolute, abaxial face sometimes obscured by rolled margins), abaxial faces strigose to hispid or hirsute, sometimes (adaxial not) gland-dotted. Heads 3–16. Peduncles 5–15 cm. Involucres shallowly hemispheric, 10–20 mm diam. Phyllaries 25–32 (loose), lanceolate, 4–9 × 1–2 mm, apices acute to slightly acuminate, abaxial faces usually scabrous, rarely glabrous, usually gland-dotted. Paleae (oblanceolate) 5.5–6.5 mm, entire or ± 3-toothed (apices purplish, mucronate, gland-dotted). Ray florets 10–20; laminae 10–20 mm (abaxial faces gland-dotted). Disc florets 75+; corollas 4–4.5 mm, lobes yellow; anthers dark brown or black, appendages dark (style branches usually yellow). Cypselae 2–3 mm, glabrate; pappi of 2 aristate scales 1.5–2.1 mm. 2n = 34.

Helianthus angustifolius var. planifolius Fernald