New Fl. 2: 64. 1837
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.