Linum medium is a perennial herb 20-70 cm tall, unbranched up to inflorescence, plant waxy, pale green.
Leaves stalkless (sessile), opposite below, 1-2.5 cm long, 0.2-0.5 cm wide, smaller above, narrowly lance-shaped or inversely lance-shaped (oblanceolate), tips pointed, veins obscure.
Flowers yellow, radially symmetrical, petals 5, 0.4-0.8 cm long, heart-shaped, widest at rounded to notched tip, narrowed at base (clawed), soon deciduous, sepals 5, overlapping, lance-shaped, 0.2-0.3 cm long, persistent in fruit, stamens 5, ovary superior, 5-parted; inflorescence branches stiffly ascending; blooms June-Aug.
Fruit dry, a rounded capsule, flattened and often purple above, 0.2 cm wide, partly enclosed in old purplish sepals; splitting into 10 segments, each releasing 1 flattened seed; fruits July-Sept. (Hough 1983).
Wetland status: FACU.
Frequency in NYC: Rare (NYS S2 T). Possibly extirpated in NYC.
Habitat: Dry, sterile soil, open habitats or woodlands.