Douglas Provincial Park
Flora of Alberta keys genus Helianthus as having "at least the lower leaves opposite". Yet it keys Helianthus petiolaris (=H. couplandii) as having "leaves mostly alternate".
I have found that neither flora easily distinguishes between Helianthus and Rudbeckia (Rudbeckia hirta is Black-eyed Susan) in their keys. However, they do note that the leaves of Rudbeckia are always alternate.