When Rancho Santa Ana Botanic Garden was founded in 1927 at the original location in Santa Ana Canyon, botanists J.T. Howell and later Carl B. Wolf, with the help of Ernest Johnson, set about building the holdings of the newly established herbarium. Because RSABG is a native plant garden, collections were initially made only in California, but soon expanded to include the western U.S. By the mid-1940's, the collection had grown to over 30,000 specimens.

Independently, and starting as early as 1904, the herbarium of Pomona College was established upon the large private herbarium of Charles Fuller Baker. Starting in 1917, under the guidance of Philip A. Munz, the Pomona collection blossomed. The highlight was the incorporation of another large private herbarium, that of Marcus E. Jones. Jones was a major force in botany of western America during the late 1800's and early 1900's who described many new species based on specimens in his collection.

After a full career at Pomona College, Munz became director of Rancho Santa Ana Botanic Garden in 1946. Among his visions were to bring the Garden to Claremont and to eventually integrate the Garden's herbarium with that of Pomona College to form one larger, stronger, more encompassing collection. In 1951, Rancho Santa Ana Botanic Garden moved to the Claremont site, and the Pomona College specimens were brought to the Garden to be housed nearby, but they still remained separate from those of RSA. It was not until the mid to late 1960's that his dream was fulfilled with the complete integration of the two collections.

The 1960s and 1970s were a time of considerable growth of the RSA holdings under the guidance of Curator Emeritus, Robert F. Thorne. Thorne worked toward the expansion of the worldwide emphasis to include all flowering plant families, but was also very active in building the California holdings. Growth continued in the 1980s, highlighted by the acquisition and incorporation of the seed plant collections from the Natural History Museum of Los Angeles County (LAM). Included among these were the former holdings of University of Southern California (USC) and Alan Hancock Foundation (AHFH) herbaria. These specimens were especially rich in historic specimens from Southern California and were an invaluable addition to the Garden's holdings.

The rapid growth experienced during the 1980s necessitated expanding and upgrading the facilities and space-saving "compactor" systems were installed with the assistance of grants from the National Science Foundation. Macrofungi, algae, lichens and bryophytes from RSA - POM (about 25 000 specimens) were transferred to Los Angeles Museum in the 1980’s and from where they were transferred to Berkeley (UC). Research specimens of Dr. R. K. Benjamin research (Laboulbeniales and Zygomycetes) transferred to Harvard University (FH). The herbarium maintains a small synoptic collection of Lichens donated by Kerry Knudsen (UCR), however the main collection is restricted to vascular plants.

During the last decade, the herbarium has incorporated the orphaned collections of: California State Polytechnic University of Pomona (CSPU) in 2000 (ca 4,500 vascular plant specimens); Los Angeles Museum (LAM) in 2003 (ca 9,300 vascular cryptogams); and Santa Ana College in 2004 (ca 5,500 vascular plant specimens).

Today the herbarium is increasingly becoming an electronic resource as well as a physical resource. Herbarium specimen data is available through our own online database and through the Consortium of California Herbaria. We are also working on the digitization of our type specimens (see projects tab).