Stems simple or branched, 10–100 cm; branches ascending to spreading, quadrangular, sharply ridged to winged distally, glabrate, sometimes scabrous. Leaves spreading, sometimes arching, ascending, or reflexed; blade narrowly linear to linear-lanceolate, 10–70 x 0.3–6 mm, not fleshy, margins entire, adaxial surface scabrous; axillary fascicles absent or shorter than subtending leaves. Inflorescences racemes, elongate, flowers 2 per node; bracts shorter than, or longer than, or both shorter and longer than, pedicels. Pedicels ascending-spreading, some upcurved distally, 6–25 mm, glabrous. Flowers: calyx obconic to hemispheric, tube 2.3–5.5 mm, glabrous, lobes subulate to triangular-subulate, 0.3–2 mm; corolla pink to rose purple, with 2 yellow lines and red spots in abaxial throat, 7–23 mm, throat pilose externally and glabrous within across bases and sinus of adaxial lobes, lobes: abaxial projected or spreading, adaxial projected over distal anthers, 2–8 mm, abaxial pilose externally, adaxial glabrous externally or pilose proximally; proximal anthers perpendicular or oblique to filaments, distal perpendicular and vertical to filaments, pollen sacs 1–4 mm; style exserted, 6.7–18 mm. Capsules globular, 4–7 mm. Seeds tan to brown, 0.5–1.5 mm. 2n = 28.
Phenology: Flowering (late Jul–)Aug–Nov.
Habitat: Wet to dry roadsides, ditches, margins of streams and ponds, borders of woodlands, dry to moist prairies, fallow fields, railroad embankments, rocky cliff faces and bluffs.
Elevation: 0–1600 m.
Man., N.B., N.S., Ont., P.E.I., Que., Ala., Ark., Colo., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., La., Maine, Md., Mass., Mich., Miss., Mo., Nebr., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Vt., Va., W.Va., Wis., Wyo.
Populations of Agalinis tenuifolia in New Brunswick, Nova Scotia, and possibly Prince Edward Island in Canada are presumed introduced.
Agalinis tenuifolia is the most widespread and morphologically variable species of the genus in the flora area. Infraspecific taxa have been recognized based on differences in sizes of corollas, calyx lobes, anthers, capsules, and leaves; presence or absence of axillary fascicles; density of indument on stamens; branches ascending versus spreading; and even the stoutness of reticulations on seed coats. These characters intergrade within and among populations and occur in many other combinations in addition to those described, making these infraspecific taxa arbitrary and inconsistent with plants in the field. Pressed specimens of A. tenuifolia are often confused with A. gattingeri from which they differ by lacking a villous band of trichomes within the corolla at the bases of the adaxial corolla lobes present in A. gattingeri; projecting adaxial corolla lobes versus erect to recurved lobes in A. gattingeri; elongate racemes with two flowers per node versus one flower per node, often appearing to terminate branches in A. gattingeri; and low wings of tissue on the branch angles that are absent or less pronounced in A. gattingeri.