Willa Cather is well known for her work’s attention to flowers and botanical specificity. This essay explores her early interest in plants and the intersection of her college career with that of Frederic Clements, the dominant figure in American plant ecology of the first half of the 20th century. O Pioneers! (and to some extent all of Cather’s prairie novels) understands the dynamic of human community in terms much like those through which Clements was defining plant community structure; later in her career, however, Cather adopted a more statically “taxonomic” approach to both botanical and human worlds.