Subshrubs or shrubs, 2–3 m, branches ± stout, indument grayish to tawny, shaggy-tomentose, stellate hairs sessile or stalked, many-armed. Leaf blades ovate, broadly ovate, or round, 3- or 5-lobed, 4–7(–12) cm, thin or thick, surfaces: sparsely to densely grayish- to tawny-short-stellate-hairy, basal sinus open, not overlapping. Inflorescences interrupted, spicate or racemose, flower clusters sessile or subsessile, subtended by conspicuous bracts, usually densely flowered, usually 10 flowers per node; involucellar bractlets distinct or basally connate, subcordate, narrow-elliptic, or ± round, 6–15 × (1–)3–9 mm, 2/3 to exceeding calyx length. Flowers: calyx strongly plicate, angled or winged in bud, 8–17 mm, lobes subcordate, ovate, or ± round, 5–11 × 3.5–10 mm, slightly longer than broad, ca. 2 times tube length, apex abruptly acuminate, stellate-hairy, hairs many-armed; petals pale pink to rose, 1–2 cm. Mericarps 2.5–3.5 mm. 2n = 34.
Phenology: Flowering Apr–Jun, Aug–Oct.
Elevation: 100–800(–1700) m
Malacothamnus aboriginum occurs principally in the central inner Coast Ranges and in the Laguna Mountains in San Diego County. There, plants have involucellar bractlets about 1 mm wide but otherwise appear to be M. aboriginum; these have sometimes been assigned to M. densiflorus. The character of the often connate, wide involucellar bractlets is distinctive of M. aboriginum.