George Engelmann (1809-1884) founded the St. Louis Academy of Science and was the principal scientific advisor to Henry Shaw. Engelmann was a German-born physician and botanist and a noted authority on plants, particularly the cacti and conifers of the middle and southwestern territories that later became part of the United States.

William Trelease (1857-1945) was the first Engelmann Professor of Botany at Washington University and the first director of the Garden after Shaw. As director from 1889 to 1912, Trelease immediately established programs in exploration, scientific publications, herbarium building, and graduate education, while maintaining and enlarging Shaw's horticultural displays.

Jesse More Greenman (1867-1951) came to the Garden in 1913 as the first curator of the herbarium. He arranged the collection, which grew from 600,000 specimens into a well-organized research tool of about 1.5 million, and supervised a large number of graduate students. Many, including the late Mildred Mathias and Julian Steyermark, went on to distinguished careers in botany.

Edgar Anderson (1897-1969) was an early student of the importance of hybridization in the evolution of plants. A gifted teacher, he popularized botany for non-scientists through his many delightful articles for the Garden's Bulletin and his book Plants, Man and Life (1952). Anderson served briefly as director of the Garden from 1952 to 1956 but always preferred the simple title, Botanist.

Robert E. Woodson Jr. (1909-1963), a native St. Louisan, earned his Ph.D. in 1929 under Greenman and was curator of the Garden herbarium until his death. After several collecting and exploring expeditions to Panama, Woodson initiated the Garden's first long-term tropical floristic study, Flora of Panama, which was completed in 1982.

Julian A. Steyermark (1909-1988) was St. Louis born and bred, earning his Ph.D. through Washington University in 1933. Although he spent most of his career in Chicago and Caracas, his Flora of Missouri (1963) is a classic. Steyermark also produced numerous tropical American floras and returned to the Garden in 1984 to work on Flora of the Venezuelan Guayana. He was credited in the Guinness Book of World Records for collecting 138,000 plant specimens during his lifetime.

Alwyn H. Gentry (1945-1993) became interested in plants of the tropics as a student of tropical ecology in Costa Rica in 1967. After receiving his degree from Washington University in 1972, he continued as a Garden curator until his death in a plane crash while exploring the forests of Ecuador. Gentry specialized in Bignoniaceae, the trumpet creeper or catalpa family, and was renowned for his extraordinary expertise in identifying all woody groups of tropical plants.