References
For Heliopsis
Cronquist, A. 1980. Asteraceae, Volume I, Vascular flora of the Southeastern United States. University of North Carolina Press, Chapel Hill, N.C. Google Scholar
Fisher, T.R. 1957. Taxonomy of the genus Heliopsis (Compositae). Ohio J. of Sci. 57: 171-191. Google Scholar
Smith, A.R. 2006a. Heliopsis. In Flora of North America Editorial Committee. 2006c. Flora of North America north of Mexico. Volume 21, Magnoliophyta: Asteridae, part 6: Asteraceae, part 3. Oxford Univ. Press, New York, NY. 616 pp. Google Scholar