The Hoover Herbarium houses 85,000+ specimens of vascular plants, algae, lichens, and bryophytes. The geographic focus is San Luis Obispo County, California. The collection also includes many specimens from other areas of California, other states of the US, particularly Arizona, and some from other regions of the world, especially Mexico. Emphasis areas in the collection include Asteraceae, Lupinus, and cultivated Eucalyptus. Major collections include Robert F. Hoover (1946–1969), David J. Keil (1966–present), Rhonda Riggins (1970s–2000), Tracy Call (mostly Apiaceae—late 1940s–1960s), and Robert J. Rodin (1940s–1977). The collection is used extensively in undergraduate teaching and training.

Important Collections: Robert F. Hoover (1946–1969), David J. Keil (1966–present), Rhonda Riggins (1970s–2000), Tracy Call (mostly Apiaceae—late 1940s–1960s), and Robert J. Rodin (1940s–1977)