1858 (March) Iowa Governor Ralph Lowe signs a bill providing for establishment of a state Agricultural College and Farm. Instruction would be offered in 19 subjects, including botany.
1868 (October) Iowa State Agricultural College opens. Dr. Norton S. Townshend teaches the first botany courses.
1870 (February) Dr. Charles E. Bessey arrives at Iowa State Agricultural College as instructor in botany, horticulture, and zoology. The herbarium is founded.
1871 (December) Dr. Bessey states in the Fourth Biennial Report of the college that the Herbarium is officially established and contains "representatives of perhaps about 2500 species." Bessey and his student J.C. Arthur list 588 species of Iowa flowering plants and ferns, the list is added as an appendix to the report.
1876 Dr. Bessey and Mr. Arthur prepare an exhibit of Iowa plants and woods for the nation's Centennial Exposition in Philadelphia.
1877 Mr. Arthur receives the first graduate degree (M.S.) from Iowa State Agricultural College upon completion of his studies in botany and zoology.
1884 (August) Dr. Bessey leaves Iowa State to become Dean at the University of Nebraska; he is replaced by Dr. Byron D. Halsted, who "did not add much to the Herbarium in the way of flowering plants, but he added a considerable number of fungi." Bessey reports that the herbarium contained approximately 15,000 specimens at the time he left.
1889 (spring) Dr. Louis H. Pammel arrives at Iowa State College, following Halsted's move to Rutgers University.
1894 Dr. Pammel arranges for the purchase of the C.C. Parry Collection, which contains approximately 15,000 specimens collected in Iowa, the American west, Mexico, and other areas. The purchase price was $5,000.
1894 George Washington Carver receives a B.S. degree, and is the first African American to do so at Iowa State.
1895 G.W. Carver is appointed "first assistant" in Botany to assist Pammel in botanical instruction.
1900 (December 7) Fire destroys a wing of the building (Old Main) housing the herbarium, but most specimens are saved or salvaged by remounting plant material from charred sheets.
1903 Harriette Kellogg was hired as curator of the herbarium and the botanical library.
1916 Harriette Kellogg dies of pneumonia. In the years preceding her death, she published papers on dye plants and medicinal weeds of Iowa.
1918 Ada Hayden receives a Ph.D.; she is the first woman to receive a Ph.D. at Iowa State College.
1918 Mr. Robert I. Cratty is appointed by Pammel as curator of the herbarium. Cratty brings his collection of more than 7,000 specimens collected primarily in Emmet County, Iowa. These are later donated to the herbarium. The herbarium contains approximately 120,000 specimens when Cratty arrives.
1920 Ada Hayden is appointed Assistant Professor of Botany at Iowa State.
1927 A Report on the Herbarium of Iowa State College, 1927, states that the herbarium consists of 180,000 specimens.
1929 L. H. Pammel resigns as head of the Botany Department and is replaced by Irving E. Melhus.
1933 Mr. Cratty prepares a checklist of Iowa's flora based on specimens in the herbarium.
1934 Ada Hayden becomes functional curator of the herbarium, continuing until her death in 1950.
1936 Dr. George J. Goodman becomes official curator of the herbarium, following Cratty's retirement. His personal herbarium, consisting of approximately 1300 specimens, is added to the herbarium.
1941 Mr. Jacob P. Anderson brings his collection of approximately 30,000 Alaskan plants collected over a forty year span to Iowa State college. They are later given to the herbarium and form the basis for Anderson's Flora of Alaska.
1944 Dr. Goodman leaves Iowa State to become a member of the botany faculty at the University of Oklahoma. Dr. Hayden assumes the official title of Curator of the herbarium.
1944 Dr. Duane Isely is hired as Seed Technologist. He donates his personal herbarium of over 4000 specimens, mostly legumes collected in the eastern U.S., Wyoming, and Colorado, to the Iowa State Herbarium.
1946 Dr. Richard Pohl is appointed to the botany faculty. He brings more than 1000 plants collected on the Texas Plains to add to the herbarium.
1948 Dr. Hayden reports that the herbarium has grown to 250,000 specimens. In addition to the Anderson Collection, Hayden also reports the gift to the herbarium of the Oleson collection containing approximately 23,000 specimens assembled by the Fort Dodge Botanical Club.
1950 Dr. Hayden dies of cancer, Dr. Pohl is appointed curator of the herbarium.
1955 The annual report of the activities of the herbarium for 1954 states that "since 1950, 24,302 new plant specimens have been mounted and added to the herbarium."
1966 Dr. Pohl travels to Costa Rica to collect tropical grasses; this and numerous subsequent trips by Pohl and his graduate students, and more recently by Dr. Lynn Clark, establish a large collection of Central and South American grasses.
1968 The Herbarium and the Department of Botany and Plant Pathology move from "Old Botany" (now Catt Hall) to their current home in Bessey Hall.
1980 The Annual report of the Iowa State Herbarium, 1980, states that the herbarium has grown to contain approximately 350,000 specimens
1984 Ms. Deborah Qualls (now Deborah Lewis) is appointed as a full-time curator of the herbarium. Previously the herbarium was curated by a member of the faculty, with student assistance for day-to-day operations.
1986 Dr. Pohl retires; Dr. Isely is named Director of the herbarium.
1988 The herbarium is named in honor of Ada Hayden; the 400,000th specimen is accessioned into the Ada Hayden Herbarium.
1989 Dr. Lynn G. Clark becomes Director of the Herbarium after Dr. Isely retires.
2003 The herbarium becomes a facility in the newly formed Ecology, Evolution and Organismal Biology Department
2004 The University of Iowa herbarium (IA) is transferred to the Ada Hayden Herbarium.