This is known mostly from Appalachian regions, Piedmont and adjacent Atlantic Coastal Plain, especially on damp to seasonally dry, medium acid soils: "a highly conservative species, without the aggressive and weedy tendencies of [calycosus and digitalis]" (F). It has often been confused with the more weedy calycosus, and some authors have combined these species (Cr, J). P. laevigatus can also appear close to alluviorum or digitalis, especially without flowers. Records from LYON, TRIG and TODD (APSC) are part of a reported disjunct northwestern section of its range in sandy or cherty hills northwest of the Nashville Basin (Ch). However, some of these plants in w. Ky. and w. Tenn. appear transitional to alluviorum or calycosus. P. laevigatus is similar to calycosus in its generally smooth anthers and pale purplish or white inflated corollas, but flowers tend to be smaller than in calycosus: corollas ca. 15-20 mm long (versus 20-35 mm); sepals ca. 3-6 mm (versus 5-12 mm); capsules ca. 5-8 mm (versus 7-8 mm). Also, leaves tend to be narrower (mostly 1.5-3 cm versus 3-6 cm), largely glabrous (versus often puberulent), somewhat bluish, and entire to obscurely denticulate (versus often clearly denticulate to serrate in most related species). Stems are usually puberulent in lines to glabrous (versus usually hirsutulous wth hairs up to 0.5 mm). The whole digitalis complex (laevigatus, calycosus, alluviorum, digitalis, deamii) differs from other species in section Gracilis as follows (Pennell 1935; F, Cr, W, FNA 17): lower lobes of the corolla about equal to upper lobes (versus projecting); corolla throat not strongly 2-ridged within (versus strongly 2-ridged on floor), the tube conspicuously dilated into the throat (versus slightly to moderately dilated); sterile filaments slightly bearded (versus densely); leaves often glabrous (versus usually pubescent); mid and upper stem below the inflorescence glabrous or with short, more or less reflexed, eglandular hairs, often distributed in patches or lines (versus pubescent throughout with short eglandular hairs plus longer, loosely spreading, glandular hairs); reported 2n = 96 (versus 16).