Trematolobelia macrostachys grows 1–4 m tall. The stem is 0.5–2 cm in diameter, glabrous, with cicatrices that are shallowly obtriangular, widely obtrullate, narrowly transversely rhombic, or narrowly transversely elliptic, measuring 2–5 mm tall and 3–6.5 mm wide. The leaves are simple and alternate, forming a dense spherical, oblate, or cylindrical apical rosette. The blades are oblanceolate, narrowly oblong, narrowly elliptic, or linear, 15–36 cm long, 0.8–3.1 cm wide, with a length-to-width ratio of 8.5–26. The apex is acuminate, and the base gradually narrows to the petiole. The surfaces are glabrous, and the margins are crenulate or entire, sometimes undulate. The leaves are petiolate, and stipules are absent. The flowers are arranged in racemes of 4–10, each with 15–60 flowers, and are glabrous. The peduncle is 10–25 cm long. Sterile bracts are lanceolate, oblanceolate, narrowly oblong, narrowly elliptic, or linear, 15–170 mm long, 3–20 mm wide, with crenulate or entire (rarely denticulate) margins and an acute or acuminate apex. The rachis is 10–34 cm long. Floral bracts are lanceolate, elliptic, narrowly elliptic, oblong, narrowly oblong, or linear, 12–50 mm long, 2–6.5 mm wide, with crenulate or entire margins and an acuminate, acute, obtuse, or rounded apex. Pedicels are 16–36 mm long, and bracteoles are ovate, oblong, narrowly oblong, or elliptic, 0.7–4 mm long. The hypanthium is campanulate, hemispheric, obovoid, or oblate, 4–9 mm long, 4–8.5 mm in diameter, 1/14–1/9 (up to 1/6) as long as the corolla. These flowers are tetracyclic, bisexual (perfect), protandrous, zygomorphic (bilaterally symmetric), epigynous, pedicellate, resupinate, ornithophilous, and large. The calyx lobes are triangular, narrowly triangular, linear-triangular, oblong, narrowly oblong, or ovate, erect or ascending, 3–10 (up to 17) mm long, 1–3 mm wide, with entire or rarely denticulate margins and an obtuse, acute, or acuminate apex. The corolla is sub-bilabiate, in various shades of pink, 43–73 mm long, with a tube that is gentle to strongly curved, 21–37 mm long, 3–6.5 mm tall at the middle, 2–5 mm tall at the mouth, and 3.8–7.7 times longer than the height at the middle. The dorsal lobes are 26–37 mm long, 1–2.5 mm wide, equal to or half again as long as the tube, with an acuminate apex. The ventral lip is elliptic, narrowly oblong, or obovate, 22–32 mm long, 4–7 mm wide. There are five stamens, antisepalous, connate distally for most of their length. The staminal column is strongly exserted, emerging well above the lobes. The filament tube is 36–59 mm long, 3.2–5.6 times longer than the dorsal anthers, sparsely pubescent toward the apex (rarely glabrous). The anther tube is 3–3.8 mm in diameter, pubescent at the base or over the basal half. The dorsal anthers are 10–14 mm long, and the ventral anthers are 7–12 mm long, with tufts of white trichomes 1–2.2 mm long at the apex. The ovary is inferior, 2-loculed, adnate to the hypanthium, flat at the apex, with axile placentae and numerous ovules. The style is slender and cylindrical, with a ring of stiff white hairs near the apex. The stigma is 2-lobed, with the lobes appressed and non-receptive as the style grows through the anther tube, pushing out pollen, after which the stigmas spread and become receptive. The fruits are globose or oblate capsules, 7–12 mm long, 6–15 mm wide, 5–10 mm thick, with numerous pores scattered over the entire lateral surface. The seeds are 0.8–1 mm long, 0.4–0.5 mm wide. It is found in lowland wet mixed, lowland wet shrublands, and montane wet shrublands between 640–1730 meters.