Flora of North America:
"Some taxonomists include this species within the European S. valerandi; that species has larger flowers and capsules, fewer racemes, and staminodes occurring in clusters of one to three. No specimens have been found of true European S. valerandi in the flora area; previous specimens labeled as S. valerandi are native species, usually S. parviflorus."

link:
http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242417215