Shrubs, erect, forming slender to rounded or flat-topped bush, (1.4–)2–6(–10) dm. Stems: internodes 4-lined at first, then terete. Leaf blades narrowly oblong to oblanceolate or linear, (15–)20–45 × 3–7(–10) mm, base articulated, narrowly cuneate to subattenuate, margins subrecurved to revolute, apex rounded to obtuse, midrib with 9–14 pairs of branches. Inflorescences usually (1–)3–7(+)-flowered from apical node, rarely with flowers from 1–2 proximal nodes. Flowers 20–35 mm diam.; sepals deciduous, not enclosing capsule, (4–)5, elliptic or oblong to obovate, subequal, 4–9 × 1.5–5 mm; petals (4–)5, golden yellow, obovate to oblong, 8–15 mm; stamens deciduous, 150–200; ovary (3–)5(–6)-merous. Capsules narrowly ovoid-conic, 7–11 × 4–7 mm. Seeds narrowly carinate, 0.7–1.1 mm; testa subscalariform. 2n = 18.
Phenology: Flowering summer (Jul–Aug).
Habitat: Sandy or calcareous dune slacks or swales, rocky shores, plains and low prairies, along streams, sphagnum-sedge swamps
Elevation: 10–400 m
Ont., Que., Ill., Ind., Mich., N.Y., Ohio, Wis.
Hypericum kalmianum is a northern derivative of H. prolificum with shorter stems, narrower leaves, fewer and larger flowers, and, usually, five styles and placentae. Natural hybrids with H. prolificum have been reported from Wisconsin.