Boundary plants lie at the intersections of landscape ecology, social structure, and cultural meaning-making. They typically relate resource rights to social groups and cultural identities, and make these connections meaningful and legitimate. Landscape boundaries such as hedges and fence lines are often repositories for social identities and cultural meanings, and tools for the negotiations and struggles that comprise them. This article surveys botanical boundaries in classic ethnography, outlines social science approaches to boundary objects, and describes new theoretical work on space, place, and agency. It also introduces the concepts of monomarcation and polymarcation to delineate the contrast between technologically simple and socially complex forms of marking land. Three case studies, concerning the social lives of Dracaena in sub- Saharan Africa and Cordyline in the Caribbean, illustrate how boundary plants have a particular sort of vegetative agency to turn space into place in culture-specific ways.