In 1891 Tidestrom went to the
University of California, Berkeley intending to study engineering, but after becoming an assistant to botany professor
Edward Lee Greene he switched to botany. When Greene moved to the
Catholic University of America in 1895, Tidestrom went with him and in 1897 earned the first
PhB given out by the university, in botany. Tidestrom entered the military once more with the onset of the
Spanish-American War in 1898, taking part in the
Battle of San Juan Hill (not as part of the
Rough Riders, although Tidestrom admired
Teddy Roosevelt greatly). By 1903 Tidestrom was out of the military and had a job at the
Bureau of Plant Industry under botanist
Frederick Vernon Coville. From 1906 to 1910 Tidestrom self-published
Elysium Marianum, an attempt at a flora of
Maryland. Starting in 1915, Tidestrom became an assistant to Edward Lee Greene at the Forest Service, who was identifying species collected by rangers; after Greene's death later that year, he took over the job. In 1925 he published his best-known work,
Flora of Utah and Nevada. In 1934 Tidestrom retired from government service and shortly afterwards accepted a position at the
Catholic University of America, retiring from there in 1939 and afterwards moving to Florida. With his student Sister
Mary Teresita Kittell he published
Flora of Arizona and New Mexico in 1941. Tidestrom continued to collect specimens, his last major trip being a trip to Sweden in 1954 at the age of 90 for a family reunion, on which he collected about 300 plants with the aid of a Swedish niece to add to the 14,000 sheets which he had already added to the Smithsonian collections. ==Personal life==