Lobelia monostachya is a shrubby plant characterized by woody prostrate stems that measure 15–25 cm long and are leafy in the upper half. The leaves are simple and alternate, with linear blades ranging from 7–15 cm in length to 0.4–0.7 cm in width. The leaf apex is acuminate, and the surfaces are glabrous. The margins are entire to minutely callose-denticulate and can be flat or revolute. The leaves are sessile or subsessile and lack stipules. The flowers are in an unbranched inflorescence 10–15 cm long and glabrous. The pedicels are 12–18 mm long, and each flower is subtended by filiform bracts that are 2–7 mm long. The hypanthium is obconical and measures 2–3 mm in length. The flowers are bisexual (perfect) and feature a calyx with five lobes that are subulate to linear, measuring 1–2.5 mm in length, with the sepals forming a tube adnate to the ovary. The corolla is zygomorphic (bilaterally symmetric), dorsally cleft nearly to the base, and magenta, measuring 15-18 mm long and 4-5 mm in width. The corolla is glabrous with spirally revolute lobes, and the tube is suberect. The stamens are five in number and alternate with the corolla lobes. The staminal column is glabrous, and the anthers are also glabrous, with the lower two featuring apical tufts of white hairs. The anthers are dithecal, opening by longitudinal slits, and are coherent but can separate after anthesis or remain connate, forming a tube into which pollen is shed. The filaments can be distinct or connate above and are attached to the epigynous nectary disk or the base of the corolla, rarely being adnate to the corolla tube. The ovary is inferior, 2-celled, and often topped with an annular nectary disk. The ovule placentation is axile; when the ovary is 1-celled, it has two parietal placentas. The stigmas are 2-lobed and are nonreceptive as the style grows through the anther tube and pushes out the pollen; after that, the stigmas spread apart and become receptive. It occurs on steep, sparsely vegetated cliffs in mesic shrubland between 44‐614 meters.