Stems simple or branched, 6–60 cm; branches ascending to spreading, quadrangular-ridged, wingless distally, glabrous or sparsely scabridulous. Leaves ascending to spreading; blade linear-filiform to linear, (6–)8–25 x 0.4–1.5 mm, margins entire, often siliceous, adaxial surface finely scabrous; axillary fascicles absent. Inflorescences racemes, short to elongate, interrupted, flowers 1 or 2 per node; bracts usually shorter than, sometimes both shorter and longer than, pedicels. Pedicels ascending to +/- spreading, (3–)6–26 mm, glabrous. Flowers: calyx obconic to hemispheric, tube (1.3–)2–4.5(–5.5) mm, glabrous, lobes subulate to triangular, (0.2–)0.5–1.5(–2) mm; corollas pale pink to pink, usually with 2 yellow lines and red spots in abaxial throat, (5–)6–14.5 mm, throat pilose externally and villous within across bases and sinus of adaxial lobes, lobes: abaxial spreading, adaxial erect to recurved, (2–)3–6(–7) mm, glabrous externally; proximal anthers parallel to filaments, distal perpendicular to filaments, pollen sacs 0.8–2.2 mm; style exserted, (5–)7–12 mm. Capsules ovoid, 3.5–5 mm. Seeds yellowish tan, 0.3–1.2 mm. 2n = 26.
Phenology: Flowering Aug–mid Oct.
Habitat: Sand plain grasslands, dry prairie remnants, dry roadsides, cemeteries, margins and openings in mesic to dry mixed woodlands, serpentine grasslands.
Elevation: 0–700 m.
Ala., Conn., D.C., Ky., Md., Mass., N.Y., N.C., Pa., R.I., S.C., Tenn., Va.
J. B. Pettengill and M. C. Neel (2011) provided morphological and molecular evidence that showed Agalinis decemloba and A. acuta are conspecific. Their data also indicated that A. tenella is most closely related to A. decemloba (including A. acuta) and may merit infraspecific status within A. decemloba. The authors maintain A. tenella based on morphological characters that include: larger corollas with proportionally larger recurved lobes; larger anthers; longer styles and stigma; and larger plants with many laxly spreading branches than A. decemloba. Agalinis decemloba is rare, and populations show extreme variation in numbers of plants produced per year. The authors agree with Pettengill and Neel that A. decemloba, as circumscribed here to include the federally listed endangered A. acuta, is threatened and deserves protection. Study of a population of A. decemloba (as A. acuta) in Massachusetts showed these plants were self-compatible (Neel 2002). Agalinis decemloba is distinguished from A. obtusifolia by the following characteristics: stems and branches flexible and drying green versus stiff and drying stramineous, leaves linear to linear-filiform versus linear-elliptic to spatulate, bracts shorter than to slightly longer than pedicels versus much shorter than pedicels, calyx lobes subulate to triangular versus deltate, and corollas usually with two yellow lines and red spots in abaxial throat versus two yellow lines and pink spots absent or pale in abaxial throat.
Agalinis decemloba is in the Center for Plant Conservation’s National Collection of Endangered Plants.