He was one of the most prolific western collectors from 1892 until 1940. While living in Los Gatos, California, south of San Francisco from 1904 to 1908, Heller collected extensively in central California. He also obtained an impressive collection from Puerto Rico. In 1913, Heller moved to Chico, California, and taught at the local high school, but continued to collect botanical specimens.
His first herbarium of over 10,000 sheets is at the Brooklyn Botanical Garden and his second herbarium and library is at the University of Washington, in Seattle. At the University of Washington, Heller's Puerto Rico plant collecting itineraries of 1900 and 1902–1903 and their utility for the historical study of endangered plants are housed.