1. American Herbal Products Association. 2000. 1999 tonnage survey report. Silver Spring, MD: American Herbal Products Association.
2. ———. 2003. Tonnage survey of North American wild-harvested plants. Silver Spring, MD: American Herbal Products Association.
3. ———. 2007. Tonnage survey of select North American wild-harvested plants, 2004–2005. Silver Spring, MD: American Herbal Products Association.
4. ———. 2012. Tonnage surveys of select North American wild-harvested plants, 2006–2010. Silver Spring, MD: American Herbal Products Association.
5. Baskauf, C.J., D.E. McCauley, and W.G. Eickmeier. 1994. Genetic analysis of a rare and widespread species of Echinacea (Asteraceae). Evolution 48: 180–188.