Schiedea trinervis is an erect or strongly ascending, branched shrub that typically grows between 30 to 80 cm tall, though it can reach up to 180 cm. It is generally glabrous except for the leaf margins. The stems are often swollen at the nodes and terete, with a 4-angled or -ridged appearance. Lateral branches of the primary stems usually extend several nodes below the inflorescence. The leaves of this species are simple and opposite, with elliptic-ovate or sometimes oblanceolate blades measuring 6 to 12.5 cm in length and 2.3 to 6.8 cm in width. The leaf apex is weakly mucronate, and the bases are often connate around the stem. The surfaces are green, with thick, chartaceous blades when dry. The margins are entire or serrulate, featuring teeth with antrorsely (upward) hooked hairs. The leaves typically have three principal veins and sometimes an inconspicuous looping pair of veins near the margins. Petioles are 1.5 to 3.8 cm long, pale green, and weakly to conspicuously connate around the stem. Stipules are absent. The inflorescence is pseudoaxillary, bearing 2 to 34 flowers. Bracts are much smaller than the uppermost leaves, with the lowest pair up to 1.4 cm long. Peduncles are 16 to 30 mm long, elongating to 50 mm in fruit, with secondary and tertiary internodes of lateral inflorescence branches measuring 20 to 30 mm. Pedicels are thinner, 10 to 20 mm long, and elongate uniformly during inflorescence development. The flowers are bisexual (perfect), cleistogamous, and pendent. The calyx has 4 to 5 sepals, 6 to 8 mm long, enlarging to 9 to 10 mm in fruit. The corolla (petals) is absent. There are 10 to 11 stamens with about 4.5 mm long filaments and anthers 2.2 to 2.5 mm long, pale reddish-purple at anthesis. The superior ovary has free-central ovule placentation and 4 to 7 styles. The fruit is an ovoid to sub-globose capsule, 8 to 12 mm long, enclosed by or exserted from the persistent dried calyx. Seeds are few to numerous, 1.3 to 1.6 mm long, black or brown, orbicular-reniform, reniform, or rarely orbicular, laterally compressed, with a papillose, or sometimes nearly smooth surface. It is rare and typically found in wet montane forests at elevations between 600-1,230 meters.