Sticky Rosinweed

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.

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.