Herbs annual, usually erect, sometimes shortly decumbent and rooting, basal branches none, rarely with 1–3(–6) pairs of narrowly ascending branches distally, 0.6–7 dm. Stems: internodes 4-angled. Leaves spreading, sessile or amplexicaul; blade usually ovate-triangular to broadly ovate, rarely oblong (mid and distal blades lanceolate-deltate), 5–25 × 3–12 mm, papery to membranous, margins plane, apex usually subacute, basal veins (3–)5, midrib usually with 1–2 pairs of branches. Inflorescences laxly corymbiform to cylindric, (1–)5–65-flowered, branching mostly dichasial. Flowers 4.5–7 mm diam.; sepals lanceolate to narrowly ovate, equal, 3–5 × 0.8–1.2 mm, margins sometimes ciliate, not setulose-ciliate, apex acute to acuminate; petals bright yellow, oblanceolate, 2–4 mm; stamens 10–14, scarcely grouped; styles 0.5–0.7 mm; stigmas broadly capitate. Capsules narrowly conic-ellipsoid, 3–5 × 1.5–2 mm, usually broadest at or near middle. Seeds 0.5–0.6 mm; testa finely linear-scalariform. 2n = 16.
Phenology: Flowering summer (Jun–Sep).
Habitat: Bogs, ditches, open and cleared woods, damp habitats
Elevation: 0–400 m
Ala., Ark., Del., Fla., Ga., Ill., Ind., La., Md., Miss., Mo., N.J., N.C., Ohio, Pa., S.C., Tenn., Tex., Va., Central America (Guatemala), introduced Atlantic Islands (Azores).
Hypericum gymnanthum was introduced into Poland; it is now extinct there. It is closely related to H. mutilum; it differs from that species in the broader, usually deltate leaves; fewer, stricter branches; no condensed apical stem internode; and larger flowers with lanceolate to ovate sepals.
Hybrids of Hypericum gymnanthum with H. mutilum have been reported from Maryland, Mississippi, North Carolina, Tennessee, and West Virginia, and, perhaps erroneously, with H. canadense from Virginia.
Hypericum gymnanthum has clearly been introduced (recently?) into the Azores, not necessarily by man. Seeds may well have been carried there by birds.