Cyanea hardyi is a branched shrub or small tree ranging from 1 to 7 meters tall. Its leaves are simple and alternate, with elliptic to oblanceolate blades measuring 18–28 cm in length and 2.5–5 cm in width. The leaf apex is acute to acuminate, and the base tapers into a winged petiole. The upper leaf surfaces are green and glabrous, while the lower surfaces are pale green and either glabrous or sparsely pubescent along the midrib. Leaf margins are callose–crenulate, and petioles measure 2.5–6 cm in length. Stipules are absent. The flowers are bisexual (perfect) and arranged in axillary, pendent inflorescences containing 15–30 flowers. The peduncles are 60–240 mm long, the rachis 20–100 mm, and the pedicels 8–12 mm long. The hypanthium is obconical, measuring 3–5 mm in length and 2–4 mm in width. The calyx has five dentiform lobes, each 0.5–1 mm long, distinct or rarely connate, with acute to acuminate apices. The calyx tube is adnate to the ovary. The zygomorphic (bilaterally symmetric) corolla is purplish to blackish purple, 20–26 mm long and 2–3 mm wide, glabrous, with a suberect tube and connate, spreading lobes. Five stamens alternate with the corolla lobes and connate into a staminal column, which is glabrous. The anthers are also glabrous, though the lower two bear apical tufts of white hairs. Anthers are dithecal, opening by longitudinal slits, and may either be coherent (separating after anthesis) or connate, forming a tube into which pollen is shed. Filaments are either distinct or connate above and are attached to the epigynous nectary disk or to the base of the corolla, occasionally adnate to the corolla tube. The ovary is inferior and two-celled with axile placentation. The stigma is two-lobed, initially appressed, and nonreceptive, while the style grows through the anther tube and then spreads apart to become receptive after pollen release. The fruit is a purple, globose berry measuring 8–10 mm in length. Seeds are numerous, small, smooth, and shiny, brown to black, and range from 0.2 to 0.5(–2) mm in length. Each seed contains a straight, short to spatulate dicotyledonous embryo embedded in oily endosperm. It occurs in mesic valleys and diverse mesic forests between 425-790 meters.