Boerhavia herbstii is a prostrate perennial herb with pink, decumbent stems extending up to 60 cm or more. The stems are sparsely and finely puberulent, with internodes measuring 4–15 cm long. The plant is very sparsely branched, except in the inflorescence, and features nodes with a transverse ridge bearing a line of pubescence. The branching angle is narrow. The leaves are simple and opposite, with triangular-ovate blades reaching up to 7 cm long and 4.5 cm wide. The leaf apex is bluntly acute to obtuse, and the base is obtuse to subtruncate. The upper leaf surface is subglabrous and pale green, while the lower surface is very pale. Margins are slightly scabrous. The petioles are 0.4–1 cm long and puberulent. Stipules are absent. The flowers are bisexual (perfect) and are borne in glomerules of 2–4 flowers. These glomerules are umbellately arranged on long, slender peduncles 6–7.5 cm long, arising alternately in the leaf axils on the distal half or more of the plant. The inflorescence is very openly paniculate, with glabrous branches that ascend, and one branch of the primary and secondary axis is always much larger. The panicle branches are ramified up to 6 times, with penultimate branches usually umbellate. Each ray or branchlet bears a glomerule of 1–4 or more flowers. Each branch of the inflorescence is subtended by a bract, which becomes successively smaller and more narrowly lanceolate. The penultimate bracts are lanceolate–subulate, while those subtending the glomerules and flowers are ovate, subulate-acuminate, and straw-colored–hyaline with a dark purple acumen, glandular ciliolate. The calyx is shallowly 5–ribbed, 2–2.5 mm long, and constricted near the middle. The lower half is green, obovoid, and glabrous or sparsely puberulent in the grooves. The limb is pink, drying yellowish pink, very broadly cylindrical–campanulate, ventricose at the base, and glabrous except for sparse puberulence distally. There is no corolla (petals). The stamens are 3, with exserted anthers that have opened orbicular cells. The ovary is superior, sessile, and 1-celled, with a single basal ovule, a filiform unbranched style, and a capitate or somewhat disk-shaped, exserted stigma. The fruit is an ellipsoid to clavate anthocarp, 3–3.5 mm long, somewhat constricted at the base, with a rounded–subtruncate apex. The fruit has five prominent ribs, is glabrous with minutely glandular furrows, and appears glabrous with a few very narrow raphide bundles longitudinally placed. Each fruit contains one seed that is closely enclosed by the thin ovary wall. It occurs on beaches, rocky shores, dry slopes, and ridges below 50 m.