He also began collecting plants, which led him to write to the University of California for help with identification and preservation of specimens.
[2] This put him in contact with William Albert Setchell, who headed the botany department at UC Berkeley (UCB) and later worked closely with Gardner as a colleague and coauthor via correspondence.
[5] Upon completion of his doctoral studies in 1906, he began teaching at the Los Angeles Polytechnic High School, where he was head of biology.
[2][5] In addition to plant collecting and other kinds of field work, he developed special methods for cultivating algae in the lab.
[5] An important contribution to American botany was the multi-volume reference work Algae of Northwestern America, on which he collaborated with Setchell.