After he graduated, Rydberg received a commission from the United States Department of Agriculture to undertake a botanical exploration of western Nebraska. He received another one in 1892 to explore the Black Hills of South Dakota, and in 1893 he was in the Sand Hills, again in western Nebraska. During this time he continued to teach at the Luther Academy.
In 1900 Rydberg conducted field work in southeast Colorado. In 1901 he visited Kew Gardens in England and made a return trip to Sweden as well. In 1905 he was collecting in Utah with visits to the University of Wyoming, Los Angeles, and San Francisco. In 1911 he undertook an exploration of southeast Utah and in 1925, the Allegheny Mountains. A trip in 1926 took him to Minnesota, Iowa, Nebraska, Kansas, and the Dakotas. His final field expedition was in 1929 to Kansas and Minnesota but it was cut short due to illness and only included work in Kansas.
He was a prolific research publisher, he described around 1700 new species in the course of his career. His expertise was principally in the flora of the Great Plains and Rocky Mountains. From 1899, Rydberg was on the staff of the New York Botanical Garden, and he later became the first curator of the Garden's Herbarium.[4]
^Rydberg, P.A. 1895. Flora of the sand hills of Nebraska. Contributions from the United States National Herbarium 3:133-203.
^Nelson, Aven (1900). "Review of Catalogue of the Flora of Montana and Yellowstone National Park by Per Axel Rydberg". Botanical Gazette. 30: 61–64. doi:10.1086/328013. S2CID83556923.
Tiehm, Arnold; Frans Antonie Stafleu (1990) Per Axel Rydberg : a biography, bibliography, and list of his taxa (Bronx, N.Y.: New York Botanical Garden) ISBN9780893273514
Related reading
Benson, Adolph B.; Naboth Hedin (1969) Swedes In America (New York: Haskel House Publishers)