Tharp blue-star

Stems usually erect (rare long stems always ascending), 10–20(–50) cm, moderately (sparsely) pubescent; branches usually borne on distal portion, seldom to near base of stem, equaling or slightly exceeding infructes­cence. Leaves: petiole 0.5–2 mm, sparsely to moder­ately pubescent (glabrous or densely pubescent); blades heteromorphic; stem leaf blades narrowly oblong-elliptic, lanceolate, narrowly lanceolate, or narrowly elliptic to ligulate (markedly narrower towards distal part of stem), 2.5–5(–7.5) cm × (2–)4–10(–13) mm, margins entire, not revolute, sometimes ciliate, apex acute, surfaces sparsely pubescent abaxially primarily on midrib (or glabrous); branch leaf blades very narrowly lanceolate to very narrowly oblong or linear, 2–5.5 cm × 1–3 mm. Flowers: sepals subulate to nar­rowly deltate, (2–)3–5(–6) mm, usually ciliate; corolla tube purplish, (11–)12–17 mm, lobes white to greenish white or pale blue, (5–)6–9(–12) mm, outer surface of corolla glabrous. Follicles erect, 2–7(–12) cm × 2.5–5 mm, apex acuminate, glabrous. Seeds 7–11 × 2–3 mm.

Flowering spring; fruiting late spring–summer. Rocky limestone hills and ridges; of conservation concern; 900–1400 m; N.Mex., Tex.

The U.S. Fish & Wildlife Service concluded in 2009 that Amsonia tharpii should be considered for listing as federally threatened or endangered due to its limited range and threats from grazing and development, although no formal action has been taken. It is listed as endangered in New Mexico. Collections were seen only from Eddy County in southeastern New Mexico and Pecos County in western Texas. This species is notable for its usually small size and the frequent appearance of pubescence at the base of the adaxial surface of the corolla lobes, which is an extension of the indument inside the corolla tube.