Lepidium arbuscula is a gnarled shrub reaching 60–120 centimeters tall and pubescent with simple hairs. The leaves are simple, alternate, or rarely opposite, crowded at the branches’ ends. The blades are spatulate to oblong–elliptic or elliptic, measuring 2–6 cm in length and approximately 0.8–1.8 cm in width. The surfaces are glabrous, and the blades are coriaceous. The margins are serrate nearly to the base. The leaves are petiolate, and stipules are absent. The flowers are usually bractless, arranged in 1–3 erect, simple racemes that reach 7–15 cm in length at maturity and are moderately puberulent. The flowers are bisexual (perfect), actinomorphic (radially symmetric), or rarely slightly irregular. The petals are white, pale yellow, or greenish, usually 2–3 mm long, and sometimes absent. The calyx consists of four deciduous sepals, which are erect, usually oblong, and sometimes have inner sepals with gibbous bases that hold nectar. The sepals are about 1–1.2 mm long, puberulent, and have white margins. The corolla comprises four petals, which may occasionally be absent, are yellow, white, or lavender. The petals are entire to emarginate, rarely lobed or fimbriate, and usually have an elongate claw measuring 2–2.5 mm long. The stamens are six in number, as long or nearly as long as the petals. The inner four stamens are usually in pairs, sometimes connate at the base in pairs. The anthers are dithecal and open by longitudinal slits. The ovary is superior, 2(4?)-carpellate, and usually 2-celled by a false but usually complete septum, though it is rarely 1-celled. It is sessile or rarely stipitate and contains one to numerous ovules borne on parietal placentas on the replum margin at the periphery of the ovary wall. The ovules are campylotropous or occasionally anatropous. The styles are short or essentially absent, and the stigma is capitate. The fruit is a capsule divided into two cells by a usually thin and membranous septum. The capsule is elongate (at least three times as long as wide) and referred to as a silique. It is flattened at right angles to the septum, broadly ovate to suborbicular, and measures 3.5–4 mm in length and width. The apex is notched. The seeds are reddish brown, narrowly obovoid, compressed, and measure 2.5–3 mm long and 1–2 mm wide. They often become mucilaginous when wet, and the endosperm is essentially absent. It occurs in open, dry ridges from scattered localities nearly throughout the leeward side of the Waianae Mountains on O‘ahu between 330-520 meters.