Plants caulescent 80-150(-180) cm; fibrous rooted. Stems terete, scabrous, stipitate-glandular. Leaves: basal caducous; cauline usually opposite, rarely alternate, petiolate or sessile; blades lanceolate to ovate, 3-22 × 1-12 cm, bases attenuate to rounded, margins toothed or entire, apices acute to acuminate, faces scabrous, stipitate-glandular. Phyllaries 19-23 in 3-4 series, outer appressed to reflexed, apices acute to obtuse, abaxial faces scabrous, stipitate-glandular. Ray florets 8-16; corollas pale yellow. Disc florets 50-70; corollas pale yellow. Cypselae 6-8.5 × 4-7 mm; pappi 0.5-2 mm. 2n = 14. Flowering summer-early fall. Limestone glades, along rocky streams; 100-300 m; Ala. James Allison (in protologue) suggested that Silphium perplexum is a hybrid between S. glutinosum and S. integrifolium. It has glandular hairs on its stems, leaves, and phyllaries as in S. glutinosum; it is also found on chalk, has taller stems, larger leaf blades, and deeper yellow corollas. Further study is needed to determine its status.