Perennial herb 0.7 - 1.5 m tall Stem: branched. Leaves: opposite, stalkless, 4 - 10 cm long, lance-shaped to elliptic with a blunt to pointed tip, non-toothed, sometimes clasping around the stem. Flowers: few, usually borne solitary at the ends of branches, 4 - 6 cm wide, with five yellow petals 12 - 22 mm long, numerous stamens attached at the base into five sets, five non-persistent styles attached only near base, and dense head-like clusters of stigmas. Fruit: a five-chambered capsule, 1.5 - 3 cm long, egg-shaped, unbeaked, many-seeded.
Similar species: Hypericum adpressum, Hypericum ascyron, Hypericum perforatum, Hypericum punctatum, and Hypericum sphaerocarpum have more than 30 stamens per flower. Hypericum adpressum is distinguished because it is 30 cm - 0.8 m tall, the stems are usually unbranched below the inflorescence, the leaves are linear-oblong to narrow elliptic and curl under slightly, the flowers are less than 3 cm wide with 45 to 85 stamens and three styles that are attached for most of their length, and the capsules are less than 1 cm long and are beaked at the tip. Hypericum perforatum has highly branched stems, usually less than 1 cm wide leaves with the midrib continuing down the stem as a sharp ridge, sepals lacking or with few black glands, petals having black glands concentrated along the margins, 50 to 80 stamens, and rough seeds that are 1 - 1.3 mm long. Hypericum punctatum differs because it has non-angled and black-spotted stems with little branching below the inflorescence, its leaves are over 1 cm wide and covered with black dots, the flowers have sepals and petals covered with black dots and 30 to 60 stamens, and the seeds are nearly or entirely smooth and less than 1 mm long. Hypericum sphaerocarpum is 30 cm - 0.7 m tall, the stems are often clustered and become woody toward the base, the flowers are less than 3 cm across and have three united styles that separate near the tip and 45 to 85 stamens, and the one-chambered capsules are 5 - 7 mm long and narrow-beaked.
Flowering: mid July to early August
Habitat and ecology: This species has become rare in the Chicago Region. It grows in partial shade along stream banks and in shrubby prairies. It is also found in calcareous fens, usually along borders between shrub thickets and fens.
Occurence in the Chicago region: native
Etymology: Hypericum is the Greek name for St. John's Wort, which blooms around St. John's Day (June 24). Ascyron is the name for a type of Hypericum.