Stems erect, 15–71 cm, tomen­tose; branches usually borne on most of stem (occasionally confined to distal portion), often exceeding main stem even in flower, always exceeding infruc­tescence. Leaves: petiole 0–1.5(–3) mm, tomentose; leaf blades slightly heteromorphic; stem leaf blades linear (ligulate or very narrowly elliptic), (2–)3–6(–7) cm × 1.6–3.5(–5.5) mm, margins entire, not revolute, not ciliate, apex acute (round-tipped), surfaces sparsely to moderately tomentose; branch leaf blades linear (ligulate, very narrowly elliptic), (2–)2.9–4.5(–6) cm × 1–2(–3.1) mm. Flowers: sepals subulate (to fili­form, aberrantly narrowly deltate), (2–)3–4.5(–5.3) mm; corolla tube 9–11(–11.5) mm, lobes purple to magenta, maroon, or lavender (blue), (5–)6–8.3(–9.4) mm, outer surface of corolla glabrous. Follicles erect, 2.9–10.5 cm × 4.8–6.3 mm, apex acuminate, usually somewhat tomentose on one side and near base (gla­brous). Seeds 14–19(–23.5) × 3.6–5 mm.

Flowering spring; fruiting summer. Dunes, sand hills, deserts, gravelly plains; 1100–1500 m; N.Mex., Tex.; Mexico (Chihuahua).

S. P. McLaughlin (1982) included Amsonia arenaria within A. tomentosa var. stenophylla, along with narrow-leaved populations of A. tomentosa from Arizona and Utah. The geographic range of the latter group is almost contiguous with that of the broader-leaved A. tomentosa var. tomentosa, which is native to southern California and Nevada and western Arizona, and both of those groups share features such as an intermingling of glabrous and densely tomentose individuals within single populations. By contrast, the populations of southern New Mexico, western Texas, and northern Mexico formerly termed A. arenaria are geographically disjunct. They are also morphologically distinctive, often having very long distal branches at flowering; consistent moderate leaf pubescence, rather than dense or no pubescence; more pronounced narrowing of leaves than in var. stenophylla; a usually different pattern of calyx indument; and, according to available label notes and photographs, usually different flower colors. Single-seeded fruits are relatively common in A. tomentosa but rare in A. arenaria, and the fruits of A. arenaria are usually somewhat tomentose only on one side and near the base, a state that is rare although not unknown in A. tomentosa. The seeds of A. arenaria are somewhat longer on average. These differences are at least equal in magnitude to those differentiating other pairs of commonly recognized species in this problematic genus. Thus, the continued recognition of A. arenaria at the species level is preferable.