Stems simple or branched, 7–100 cm; branches ascending, terete, ridged, scabridulous proximally, angular, ridged, scabrous distally, sometimes sparingly. Leaves ascending; blade linear to filiform, 15–50(–60) x 0.4–1.5(–2) mm, margins entire, siliceous, adaxial surface scabrous; axillary fascicles well developed, shorter to longer than subtending leaves. Inflorescences racemes, flowers 1 or 2 per node; bracts longer than pedicels. Pedicels ascending-spreading, 4–20 mm, glabrous or proximally scabridulous. Flowers: calyx campanulate, tube (3–)3.5–5 mm, glabrous, lobes triangular-lanceolate, (1.5–)2–4 mm; corolla pink, with 2 yellow lines and dark red spots in abaxial throat, 15–28 mm, throat pilose externally and glabrous within across bases and sinus of adaxial lobes, lobes: abaxial projecting to spreading, adaxial spreading to erect, 3–7 mm, glabrous externally; proximal anthers parallel to filaments, distal perpendicular to filaments, pollen sacs 1.8–3 mm; style included or exserted, 8–15 mm. Capsules ovoid-oblong, 6.5–11 mm. Seeds dark brown, 0.9–2 mm. 2n = 26.
Phenology: Flowering Jul–Sep.
Habitat: Tallgrass or loess hills or upland prairies, dry rocky or sandy soils over limestone, limestone bluffs, gravelly moraines.
Elevation: 200–400 m.
Man., Ill., Iowa, Kans., Minn., Miss., Mo., Nebr., N.Dak., Okla., S.Dak., Wis.
Plants of Agalinis aspera have strongly siliceous hairs that are rough to the touch and appear marbled. The style is usually pilose.