Upon finishing his degree, he went to Stanford University for a 2-year postdoc, working with William C. Steere to study moss specimens from Canada, Alaska, and Puerto Rico.
[2] In 1953, Crum left California and accepted a position in the Department of Biology at the University of Kentucky at Louisville.
[2] He began to work closely with Lewis E. Anderson on a compendium of the mosses of eastern North America.
[3] Crum began to teach summer bryology classes at the University of Michigan Biological Station.
Realizing that no adequate textbook was available, Crum wrote his own, entitled Mosses of the Great Lakes Forest.
[2] Crum is the namesake of three genera: Bryocrumia, Crumia, and Crumuscus; and eight species: Bellia crumii, Encalypta brevicollis subsp.