According to the Flora of North America, "Hybrids with other species differ from H. divaricatus in having short but distinct petioles, hairy stems, leaves with more rounded bases, and primary lateral leaf veins diverging in a subopposite manner distal to bases, rather than being strictly opposite and basal." Notice hairy stem, petioled leaves, and "additional" subopposite lateral veins in distal part of blade (visible in leaf closest to the heads).