Plants sprout, vegetate, flower, and molder pervasively across nineteenth-century American literature and yet, like most roadside weeds today, are largely ignored. My dissertation demonstrates that, far from mere stylistic ornamentation, this profusion of vegetation was a means of imagining literature and humanness as inhuman: responsive to otherness outside of texts as well as at the core of a composing subject. Applied to aesthetic agents and objects, plant metaphor unsettled more rhetorical claims during the period for genius or formal convention as self-contained or individuated. The inhuman poetics I trace reveals ways in which poetry in nineteenth-century America was defined not only by genre and print conventions, but also by attempts to make literature responsive to what stands outside of texts: nature, history, and experience. I show how, by directing attention to literary texture and to the extra-literary, plant metaphors model ways of dialectically thinking through the relationship between humans and nature. Although, like many cultural forms in nineteenth-century America, this poetics drew on sources outside of the United States (particularly works of German and British Romanticism), attention to plant-life in America was necessarily localized. This common attention engaged many of the most canonical authors of the day. I undertake immersive readings of plant-life across the careers of Dickinson, Thoreau and Melville to offer new insight into some of their more under-studied works and to deepen understandings of what "poetry" meant to each. My work contextualizes these readings by demonstrating intersections between these authors and Romantic theory and biology, popular botany in ante-bellum New England, and sentimental poetry on friendship and flowers. Relating my findings to contemporary debates about poetics and crises in the humanities and in the environment, I demonstrate how historical particularity sheds light not only on the past but also on present attempts to theorize poetry's relationship to the social and ecological.