This project seeks to identify a heuristic epistemology that lies between practical knowledge and theoretical explanation in early modern European agriculture. It focuses on the “doctrine of signatures”—an early modern idea that plants that look like human body parts were able to cure their corresponding body parts—and its undiscussed role in agriculture. Drawing on seventeenth-century German and English archives on chymistry, natural history, and agricultural techniques, it asks how the “doctrine of signatures” functioned as a heuristic tool for constructing the medicinal and economical value of crops, bridging academic and vernacular knowledge of cultivation, and bringing together local and colonial plantations into the nationalist vision of development. This approach goes beyond the dichotomous framework of theory versus practice and sheds light on the conjectural, casual, hermeneutically-inspired discourse that permeates various aspects of early modern agriculture. It also contributes to existing scholarship on early modern chymistry, medicine, agricultural sciences, and natural knowledge in the rise of nationalism.