Plants 5-80 cm; caudices branching. Stems 1-10+, ascending to erect, proximally glabrous, strigose in arrays. Leaves: basal and proximal cauline petiolate, blades narrowly oblanceolate, (20-)50-100(-160) × (2-)5-16(-56) mm, margins serrate to crenate, apices acute to ob-tuse, glabrous; mid and distal sessile, similar, blades lanceolate to linear, 12-45 × 2-19 mm, reduced distally, margins entire to sparsely serrate, sometimes resinous. Heads 3-150, not secund, in narrowly elongate, paniculiform arrays, broadly so in robust plants (12.5-19 × 2.5-3 cm wide), consisting of short axillary and terminal racemiform clusters, proximal branches elongate in larger plants, branches glabrate to strigillose. Peduncles 3.1-10.3 mm, glabrate to sparsely strigillose; bracteoles few, linear. Involucres campanulate, 3-7 mm. Phyllaries (in 3-4 series) strongly unequal, often resinous; outer ovate, acute, inner linear-oblong, obtuse. Ray florets 7-16; laminae 2-5 × 0.7-0.9 mm. Disc florets 6-31; corollas 4-4.9 mm, lobes 0.6-1.3(-2) mm. Cypselae narrowly obconic, 1.9-3.2 mm, sometimes with dark ridges, strigillose; pappi 1.9-5.2 mm (bristles sometimes clavate). The somewhat viscid-resinous heads of Solidago simplex are its most distinctive feature, separating it from similar sympatric species. G. S. Ringius (1985) did a detailed multivariate analysis of the S. spathulata/S. simplex complex (the latter under the name S. glutinosa). The cytogeography of the species complex was presented by Ringius and J. C. Semple (1987). Neither study included data on the next three species occurring in the southeastern United States.
The species is divided into two subspecies and seven varieties following G. S Ringius (1985) and J. C. Semple et al. (1999). Three varieties occur in the diploid transcontinental subsp. simplex: var. simplex, var. nana, and var. chlorolepis. Four varieties occur in the eastern North American tetraploid-hexaploid subsp. randii: var. monticola, var. gillmanii, var. ontarioensis, and var. racemosa. Except for var. simplex, varieties are restricted to different habitats in relatively limited ranges.